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Howard Stone

Howard Stone

Donald R. Dixon '69 and Elizabeth W. Dixon Professor of Mechanical and Aerospace Engineering, Princeton University

By David Zierler, Director of the Caltech Heritage Project
January 6, 2023

DAVID ZIERLER: This is David Zierler, Director of the Caltech Heritage Project. It's Friday, January 6th, 2023. I am delighted to be here with Professor Howard A. Stone. Howard, it's wonderful to be with you. Thank you for joining me this morning.

HOWARD STONE: Thank you, David. I'm glad to be here, and am looking forward to talking to you.

ZIERLER: Wonderful. Howard, to start, would you please tell me your title and institutional affiliation?

STONE: I'm a Professor at Princeton University in mechanical and aerospace engineering, where my professorial title is the Donald R. Dixon '69 and Elizabeth W. Dixon Professor of Mechanical and Aerospace Engineering. I moved here in 2009 after spending 20 years at Harvard in engineering.

ZIERLER: Howard, are you the inaugural holder of the Dixon chair? Do you know the Dixons? Were you ever able to meet them?

STONE: I have met Donald Dixon. I believe I am the inaugural holder of the chair. Donald Dixon is a very technology investor, and has been very dedicated to Princeton, both in time and some resources. I have had the opportunity to meet him several times.

ZIERLER: Howard, we'll start at a very high level. In looking over your research agenda, there's everything there. There's biology. There's physics. There's math. There's chemistry. There's engineering. There's science. There's translational research. There's basic science. What would you say is your home discipline, either by training or by motivation?

STONE: I'm originally trained in chemical engineering, and within chemical engineering, I'm trained in the area of fluid dynamics. I generally view myself as someone that does fluid dynamics, I think you can say, broadly interpreted. My generic interest, slightly outside of that, I guess you could say, would be systems out of equilibrium. I can tell you more about that. My approach to problems is often from the viewpoint of the training you get in fluid dynamics.

ZIERLER: Because your research extends into so many fields, when you get involved in things that have chemical or biological or physical applications, is it because collaborators reach out to you, and they say, "Here's a particular area of expertise that you have that I want to orient for this project," or how does that all work out that gives you such a wide array of interest that that you've worked on?

STONE: I guess I can say that, as an undergraduate, I always liked physical chemistry. In fact, that was my route into engineering. Within fluid dynamics, there's a branch of fluid dynamics that is the interface between systems that have flow—that's sort of what you mean by fluid dynamics—and problems that are influenced by physical chemistry. The term for that in the field is physicochemical hydrodynamics. One aspect early in my career—my graduate work was at Caltech, and then I did a postdoc in Cambridge, England, and then started at Harvard—some of my early work was in this area, learning about and understanding how physical chemistry impacts fluid motions. There's a very beautiful common example that many people have seen, and that is if you pour a glass of wine or some hard liquor, and you swirl the glass of wine to get a film on the side of the glass, what you'll often see is a rim that forms at the top of the glass, and then rivulets, which are called legs in a glass of wine, that flow downwards. The formation of this pattern, if you like, is an interplay between fluid mechanics because there's a flow of a liquid, and physical chemistry because you have a solution that has alcohol and water. That gives the system some distinctive characteristics such as surface tension and differences in surface tension that create the flow. I think that was my background. Then over the years, what I found that I liked most in research wasn't any specific problem. Over the years, what I discovered is I enjoy problem solving. I'd like to think that I think of many problems, but I don't think that's really quite true. I discovered as a young professor that the best thing I could possibly do—this is partly a joke, but it's true—is I would leave my door open, and every day I'd walk into my office, and I would say, "I hope someone interesting walks in today."


STONE: Over time, that happened. A very early thing that happened for me was I was a young professor at Harvard, and there was a great biologist, biophysicist: Howard Berg trained at Caltech with Max Delbrück, I think, was a professor in biology, and he, I think, was aware that I was on campus, and I was studying so-called low-Reynolds-number flow problems where you have flows involving small objects. That's true with bacteria. He invited me to give a talk in his research group. That was a wonderful experience, because then I got to know him. Then a few years later, because I had my door open, one of his students, Aravi Samuel, who happened to have been an undergraduate at Harvard, and taken a math class with me, came into my office, and told me about this beautiful recent paper that had come out about swimming microorganisms. It had math in it, and physics, and an odd result, and he was interested in it. That led to a paper that he and I then wrote together. Some part of all of this is I like people. I like meeting people. I like going to seminars. I've discovered over the years that lots of different things look like fluid mechanics to me. I could tell you lots of them. Knowledge of fluid mechanics is not a bad starting point for talking to people in other fields, because living systems have flow, whether it's blood flow or airflow or perspiration. Things swim. They fly, whatever. It's not a bad starting point.

ZIERLER: Howard, the research strategy of leaving your door open, and happenstance meetings that lead to great science just makes me think how difficult COVID and the mandates of social isolation are for science, where you're denied those happenstance meetings.

STONE: It's very hard in general, even with Zoom, to focus well enough to do things. It's possible, but you really have to focus. You have to turn off all email. You have to really focus on the conversation and topic that is being discussed. I think for many scientists, it's easier with a blackboard or a whiteboard, sitting around a table with a cup of coffee, not having these other distractions.

ZIERLER: Howard, in terms of your motivations, when is it about translating a problem into some societal benefit, and when is it just about the basic science, just figuring out how nature works?

STONE: I'm probably not the best person when it comes to translation. We've tried a few times. I won't claim that I'm terribly successful. I think there's great benefit in it. I wish I knew how to do it better. It's not the thing I think I start with. In my own case, I like to find problems that I think are interesting. That's an unclear term, because I could find lots of things interesting. For me, "interesting" often means that there are one or two other people around me who find it interesting and want to talk to me about it. I don't like to work too much in isolation on things because I'm not sure that's the best path. I can go down many paths that just don't lead anywhere, I guess. But sometimes these things you're working on, you start to see links to applications, and sometimes that happens because you're working with someone in industry. In my own case, where I've been successful that way is not because I necessarily knew something about their application that they didn't know. In fact, many of the people in industry, they know their application area much better than I would ever know of it. But they need help in other things that are related to science and engineering: how big effects are going to be; how to think about the design. What are the fiscal trade-offs they have to worry about? That's where I think I've been able to have also a little impact. But that's, again, I think part of a collaborative effort where you team your expertise with someone else. In a couple cases, we stumbled on problems where we think there is a societal benefit. We had one happen now, but it happened not because of me, per se, but it happened because of one of my postdocs who had a certain expertise. We started to realize that, really, he started to realize that things we were doing had a bigger impact—could have a bigger impact—in some technological area.

ZIERLER: Howard, in terms of your—

STONE: I should say there are people who are very good. They see an area. They think they know what the industrial challenge is, the societal challenge is, and they can marshal their resources and energy in that direction. I admire that. I don't believe I have quite that skill. I try to get there a different way, working with people that might have a little bit more expertise than I have, and I try to add mine to make it better, if possible.

ZIERLER: As a fluid dynamicist, where do you see yourself at the interface of experiment and theory?

STONE: Most people, I think, would call me a theorist. When I was at Caltech, my advisor was Gary Leal, who eventually moved to UC Santa Barbara. With Gary, I inherited a magnificent experiment, effectively a robot in the early 1980s designed by a man named Barry Bentley. Half my thesis was experimental. That turned out to be good for me; not that I was a good experimentalist, I think, probably far from it, but it gave me a lot more physical intuition for the world. My research group is probably fifty-fifty experimental versus theory. There are some people that work with me that are primarily experimental, and there's some people that work with me that are primarily theorist or numerical simulators, and there's some people that enjoy both. I find the intersection of the two very rewarding. Lots of theories and simulations have assumptions built into them. It's good to know that they might agree sometimes with real observations, and I have a Caltech story about that, by the way. But I just find it more rewarding. In fluid mechanics, because there's so many different fluid systems, and there're so many opportunities to visualize things, and now so many numerical tools to test things, there are lots of places in the field of fluid dynamics where you have comparisons of theory and experiment, and often it's sometimes surprising.

ZIERLER: What can be read into your departmental appointment in mechanical and aerospace engineering? Did that indicate some research shift, circa 2009?

STONE: No. I can tell you. I'm not sure this is appropriate for recording, but it's a true story. When I was hired as an assistant professor, I was in the then division of applied sciences, which then became the division of engineering and applied sciences at Harvard. There it was a fairly small program at the time, and the sort of intersection between engineering and applied physics that was there, which was very strong, was through materials-related problems. Lots of the fluid dynamics problems that I worked on, which are these so-called small-scale problems, where viscous effects, frictional effects are important, come up in different materials like processing, creating a flow to coat a surface, looking at how materials rearrange when they're melted or molded or whatever. For whatever reason, after 17 or so years at Harvard, I was asked to apply to Princeton, first as department head, but that didn't work out. The story I have is I'm a chemical engineer by training, and when I came to Princeton, I was employed in mechanical and aerospace engineering. The chemical engineering department, of whom I knew almost all of the faculty, asked me if I would consider a courtesy appointment, which is called an associated faculty appointment. I said yes, and so they said, "In that case, you should come give a 20-minute talk in our faculty meeting." I went to their faculty meeting. I give a 20-minute talk. At the end of the talk, a very famous chemical engineer named Jimmy Wei, who I got to know over the years—a very nice, kind man; very impactful in chemical engineering—raised his hand after my talk. He has a habit of pointing. He pointed at me, and he said, "Aren't you a chemical engineer by training?" This is a true story. Every word is true. I said, "Yeah, both my degrees: bachelor's degree, PhD." Then he said, pointing, "Why are you in that department, and not in this department?" I said, "I don't know, but I assume it's because I got a job offer in that department, and that's because I wasn't good enough for this department." At which point, he said, "Oh, yeah, that's right. That's right. That's right. That's right."

All his colleagues put their head in their hands. No one's ever said anything to me. But I think academics are an interesting set of people and conversations. I think it's fair to say I was probably discussed in that department. For either reasons of fit or other reasons of intellectual depth or quality, they did what they did, and my job offer was in mechanical and aerospace engineering because they were interested in the kinds of fluid mechanics I do. I'm very excited about it. I think it's a wonderful department. I remember going home, and telling this story to my wife, and I said, "This isn't my problem. This is their (the chemical engineers) problem. It is what it is. I can't change who I am or the way I work." Then the interesting thing, of course, that happened, I was then a chemical engineer by training in the mechanical engineering department. At each university, this is true everywhere—you can decide what to do with all these stories—but these two very highly rated departments each offer a graduate fluid mechanics class in the same semester. But now I was the fluid dynamicist from chemical engineering in the mechanical engineering department. The two chairs—and they exchanged an email with me, so I have all the data I need that tells this true story—they exchanged an email where they proposed to compare the syllabi for the two classes to see if it was appropriate that I teach the class for both departments. Now, historically, the reason they would never do this—and I'm sure this is true at Caltech too—is the chemical engineers think the mechanical engineers, to a first approximation, are not capable of teaching their students, and they would never let—

ZIERLER: [laugh]

STONE: —them teach a fluid mechanics class, and vice versa for the mechanical engineers thinking about the chemical engineers. It's probably like quantum mechanics between physics and chemistry. Anyway, they compared the syllabi and, not surprisingly, they were 80% to 90% identical. For 12 or so years, I taught a joint class, which was a little more work because there were more students, maybe twice as many students. But it was roughly the same class, so why not save the effort, and let one person teach it? Recently, the chemical engineering program changed their rules so now I know longer teach an official joint class. In general, I'm probably not a bad teacher. I certainly reflect my Caltech training, and the training I received from Gary Leal and colleagues. That's a true story. That happened word-for-word, event-for-event.

ZIERLER: [laugh]

STONE: For me, that's the way some of these things are. I don't think it's ideal, but people are people.

ZIERLER: Howard, just as a snapshot in time, what are you currently working on?

STONE: I have a couple of problems I'm working on. We're working on some problems with so-called soft materials in the area of fluid mechanics, and the crossover to biophysics. The subject is now called soft matter. Sometimes, on the fluid mechanics side of it, it'll be called complex fluids. This is because if you squeeze your tissue, it's soft. If you look at blood, it has blood cells in it that flow and rearrange. We have a couple problems involving soft materials where we're trying to build in more of the understanding from elasticity into the fluid mechanics problem. I have a couple problems like that.

This past week, I've been reading a lot. It's not something that I do a lot, but I've been working with a German scientist named Patrick Huber, who comes from condensed matter physics. He's very interested in how liquids infiltrate into nanoporous materials. Those are porous materials with nanoscale pores. There's an interesting interplay of fluid mechanics and elasticity. I was trying to read some of that literature, which was very foreign to me, but it has been enjoyable to learn. Then I do a lot of work, or at least trying to learn, about how electrical effects affect fluids. This is this area of physicochemical hydrodynamics. If you put salt in water, the salt ions dissolve, so you get sodium ions and chlorine ions, or potassium and chlorine ions. Those ions affect how particles move in the fluid, and so we have a number of problems where we work on that. That's an area sometimes called colloid science, but all those words come up in biology, and so we've also been talking to people here in molecular biology for how some of the things we've been learning might apply to a biological problem.

ZIERLER: Have there been technological advances over the course of your career, either in instrumentation or computation, that have really been game-changers for the kinds of things that you can work on?

STONE: I think they are probably two on the experimental side that I can think of right away. One is just the huge evolution, and I'm not sure "revolution" is the right word but certainly evolution in common laboratory tools using optical methods for visualizing things, and then also high-speed imaging. There are some fluid mechanical phenomena that we see but we see at the resolution that we pick up with our eyes, whereas there are crazy things happening on much shorter timescales, and you need high-speed imaging to do that. It used to be that you could do that already, I don't know, 80-120 years ago, Harold Edgerton at MIT. But now the tools are much more common. You can actually do things now with your iPhone, apparently—I'm not good at it, but people do that—that help you see some phenomena that happens quickly. I would say those are at least two things in experimental sciences that give you a lot more quantitative information. Then on the simulation side, just the sheer increase in speed of computers and open-source tools—again, I've lost my personal ability to code—but just the enormous increase in computational power, and access to algorithms and open-source tools, I think, have made things a lot better. Then the intersection of the two is image processing tools.

ZIERLER: What about machine learning and AI? Have you embraced those technologies?

STONE: I have colleagues who certainly have. I've tried to learn. I haven't succeeded, unfortunately. I did create with one of my students a couple years ago, who wanted to leave fluid mechanics, and go into areas of finance where AI and machine learning were being used—her last project in her PhD we created a project for her, one, to learn some of those tools and, two, to do so in a fluid mechanics context so that I might learn. She did much better than I did, by the way. But I certainly have colleagues that are using these tools, and it's providing many new insights. I haven't gone down that path. There might be another reason why I haven't. I tend to—and this might be part of my personality—in a certain way, I'm not the most competitive person. In sports, I am, I suppose. But I don't like in my professional life to feel like I'm competing, so if I feel like there are lots of people working in one area, I often try to work on something different, just to lower my stress level.

ZIERLER: Let's take it back to the beginning. Let's start first with your parents. I assume you're a California boy, but what about your parents?


ZIERLER: Where are they from?

STONE: No, I grew up in Upstate New York, in a city called Schenectady.

ZIERLER: Oh, so you went really cross country for college?

STONE: My father got transferred just before I entered high school. Schenectady is where General Electric was based, and he worked at a part of it called Knolls Atomic Power Laboratory, the part of the then organization that built nuclear reactors for the ships in the Nuclear Navy. But he got transferred when I was about to start high school, and so I went to high school in San Jose, California, and then I was an undergraduate at UC Davis. There, my first semester I was pre-law and history because I liked that kind of thing. I had watched too much TV, probably, in high school.

ZIERLER: [laugh]

STONE: But I took a chemistry class because everyone in the class in the dormitory was pre-med and taking chemistry. I took a chemistry class that changed my trajectory. My father was an engineer, and so kept encouraging me to take an engineering class. My second semester, I tried to put the two together by taking an engineering class, continuing chemistry, and ended up then changing majors to chemical engineering, not realizing they didn't do as much chemistry there. But it was really good for me. I even have a story about my first semester chemistry teacher because, really, as I said, he sort of changed my life or my outlook, I should say, by teaching in a way that, for the first time in my life, let me understand things. I was always pretty good with math. It was for the first time I really understood how things got connected.

The chemistry professor's name was Dino Tinti. The odd thing was—I always believed this—I thought this was the guy that made my trajectory change because he was able to lecture in a way that I could understand the connection of math and science. After being at Caltech one time, I went to see a friend back at Davis, so I drove up to Davis. When I stopped for gas, Professor Tinti was pumping gas at the next pump. I was so shy that I didn't say anything to him. This was five years after his class. When he drove away, I kicked myself. I thought, how could I have let this opportunity pass? I wrote him a letter. Then a few years later when I was back on campus, I gave a talk, and he came to the talk, and I got to spend time with him. When he passed away a few years ago, his family asked me to write a little memory of their father as a teacher. Those were my first few semesters on campus, and it was very formative because I found a lot of the teaching at Davis, really, for the person I was at 18, it was really great.

ZIERLER: Now, focusing on chemical engineering with all of the industrial aspects to that, did you think you would be going into industry like your dad, or did you always have academic aspirations?

STONE: I think in my subconsciousness, I had academic aspirations, but my first steps were towards industry. I think my second or third summer, I had an internship at Union Oil, which had a facility in Bakersfield. I spent the summer at an on-site oil pumping operation in the Bakersfield area. It gets pretty hot in the summer, by the way. Then the following summer, I worked in a research lab at a position at Union Oil in Fullerton, CA. But when I started at Caltech, what I discovered was that I enjoyed the intellectual features, including the teaching, the problem-solving, talking to other people about problem-solving. Although I applied to Caltech probably originally for a master's degree, they accepted me in the PhD program. I think then I just recognized that that was probably the path I was going to take, even if I didn't quite articulate that.

ZIERLER: Did you really excel as an undergraduate? Was there a professor who encouraged you that a place like Caltech was within range?

STONE: UC Davis had a really good chemical engineering department. There was one man named Ruben Carbonell, who had taught a graduate math class there that, by mistake of advising, I took very early in my career at Davis. He and then several other professors there—Steve Whitaker, Alan Jackman, the late Dick Bell, and Ben McCoy—I think, encouraged a number of us to consider graduate school. In my year, I think, three or four of us went to leading chemical engineering departments, all trained to do things related to fluid mechanics and transport phenomena, because that was a real strength of the Davis program.

ZIERLER: Tell me your first impressions when you arrived on campus in Pasadena.

STONE: My first impression was actually before even applying, I had very good friends from Davis who lived in La Cañada, a couple named Tom and Kelly Williams. I visited them, I think, in January, during the application process—no, in the summer before. Sorry, I'm getting confused. The summer before senior year, I was in Fullerton. Remember, I said I worked at Union Oil. One of the Davis professors, I think, suggested I visit Caltech. That's a direct answer to your question. I went over, and John Seinfeld—I don't know if you know that name—

ZIERLER: Of course.

STONE: —a wonderful professor. John agreed to meet with me over the summer, and so I met several of the professors. John was wonderful, and I met several people from his research group. I really liked the fact that it was a small environment. To me, that was a very attractive part of Caltech. I was very happy when I applied and got admitted. Then I had to decide between a couple places. But, for me, the way Caltech was structured, I thought it was a good match for my personality.

ZIERLER: What were your motivations at that point? What kind of science did you want to do, or were you really open-minded when you started the graduate program?

STONE: I was somewhat open-minded, but because of my undergraduate training, which was pretty strong in what's called fluid mechanics and heat transfer and a little applied math, I was oriented toward the fluid mechanics side. Caltech had a very strong program. There was a man named Gary Leal; a man named Eric Herbolzheimer at the time, who then moved to Exxon. The applied math department at the time had a very strong group of faculty: Philip Saffman, Gerry Whitham. Dan Meiron was a young professor at the time. Don Cohen was a very, very strong applied mathematician. There was a very strong community of people, and then people in mechanical engineering, e.g., Ted Wu. I was already in that space of fluid mechanics, and I was pretty sure that was the kind of thing I enjoyed. Although at the time when I started, I found that I wasn't that good at the math side, and I would struggle a lot with physical intuition. That's why, having a laboratory project that Gary asked me to work on, I think, helped my physical intuition, and helped me mature as a researcher.

ZIERLER: I wonder if you can explain the importance, in retrospect, of gaining that ability in physical intuition.

STONE: I think in many real problems, there are lots of things that impact what you see. It could be a fluid mechanics problem. It could be any problem. There are lots of things that affect it, and so the question is, how do you explain it qualitatively? How do you explain it quantitatively? Normally, if I have some problem, and there are eight different things I can think of that might matter, often, it's only two or three of them that control really what you see, and so you have to make assumptions. Learning to think physically about that, I think, is really important. I mentioned before we started that there's a story. As a professor, I've done research on a number of areas. One area I've worked on involves so-called thin film flows. In this area, there's an approximation and a very beautiful theoretical result that goes back to the names of Landau, the great Russian physicist; Levich, who was his student in Russia but came to the US, and there's an institute named after him at the City University of New York; and Derjaguin, a great physical chemist; and a man named Francis Bretherton. I had worked on this thin film problem as a young professor, and though I'm surely not an expert, I know a lot about this problem. But as a student, I was first exposed to it because I sat in on a course that Professor Saffman taught in applied mathematics at Caltech. In applied math, you learn not only exact analytical things but how you make approximations. How you make approximations is formalized in a methodology called asymptotics.

Professor Saffman—actually, I remember this. I was sitting in the back of the room. I think I was auditing a class. Professor Saffman set up this problem. He described his assumptions. He went through all the math. At the end, he ends up with a formula. Near the end of the lecture, when he got into the end, one of the students, one of the very good applied math students, raised his hand, and said, "Professor Saffman, when you started the problem, you made a set of assumptions, and that let you arrive at the result you've now shown us how to derive. But how do you know the final result is right?" Saffman, who was really a magnificent applied mathematician and, I think, someone who had great physical insight, paused. He looked at the class, and then he said—and this is close to word-for-word, I think—he said, "Normally, what happens is after you've developed your theory, someone does an experiment, and often they get a different answer. In getting a different answer, it helps you realize which of your assumptions you've erred on, so you correct your assumptions, and then you could redevelop the theory." To me, it really showed a powerful message, and that is, for many problems, you need hints. Maybe they're going to come from numerical simulations. Maybe they're going to come from experiments. But real understanding is some integration of them. I really like this area of asymptotics. I'm not as good at it as real strong applied mathematicians. But I value it a lot because it emphasizes trying to get insights, and trying to understand how approximations lead to simplified results that aren't just a graph in someone's paper. It helps you understand it qualitatively and quantitatively. But I think it's this interplay that I really think is valuable.

ZIERLER: Howard, as a grad student, beyond chemistry and chemical engineering, was there research elsewhere on campus that was important for you at GALCIT, for example?

STONE: GALCIT, and the applied math program, actually, there was a lot of interchange between those two. The mechanical engineering program, and what was done in the Thomas building—maybe it still is—the chemical engineering department, the environmental engineering department had several very strong people in fluid mechanics-related fields. There were even one or two people, although I didn't interact with them at the time, in physics, Mike Cross and a couple others who were doing fluid mechanics-like problems. It was really many places on campus. I didn't have much interaction as a graduate student in chemistry, although chemical engineering is part of chemistry at Caltech. I had a little, maybe. But I had a lot more, seminar-wise, in mechanical engineering, in particular, in applied math, and a little in GALCIT. Anatol Roshko was there. Hans Liepmann, was there. You might know some of these names.

ZIERLER: Of course.

STONE: I sat in on classes they taught. Hans Hornung, Paul Dimotakis, Tony Leonard, now Mory Gharib, John Dabiri, Beverley J. McKeon, it's a who's who of fluid mechanics, because fluid mechanics in that sense is this extremely broad subject. It pretty much hits almost every area of engineering but also crosses over into physics and, as I said, some into biology.

ZIERLER: Were computers embraced among the fluid dynamicists at that point, or is it still a bit early on?

STONE: I was there starting in 1982, and my memory is—it'd be interesting if you knew more from the history—that when I started, there was an NSF-sponsored computer center, maybe over in one of the biology buildings. You had to generally go over there, and there'd be a few monitors, and that's where you did a little computing, if I remember. Then shortly after that, or maybe around the same time in the early '80s, '83, '84, labs started to have their own little monitors appearing. The first experiment I worked with in chemical engineering, which, as I said, was this magnificent experiment, effectively a robot, built by Barry Bentley with a small camera to do imaging, and then a PDP-11/23 with maybe—I don't know—a one-megabyte hard disk or whatever to do data storage. It was right around the early '80s, as I remember things, that at least monitors started to appear in some personal computers in different labs in engineering.

ZIERLER: What was the process for figuring out who your thesis advisor would be?

STONE: I pretty much got there thinking I was going to do fluid mechanics, and there were probably at the time maybe three names that were related. Gary Leal, I think, was the person whose work I thought was the best fit for what I was interested in. Eric Herbolzheimer and John Seinfeld did all kinds of interesting things in environmental engineering, so I actually took the environmental engineering sequence that John, Rick Flagan, and the late Glen Cass taught. But I think, all along, I was leaning towards Gary, and that was a really good fit for me.

ZIERLER: To go back to that question about applications and basic research, was the research environment at Caltech in chemical engineering, was it basically entirely fundamental? Were people thinking about applications at all?

STONE: I think so. Certainly, the focus was fundamental science and engineering, fundamental ideas, courses that taught you fundamental principles. That was certainly the spirit in the air. But I think several of them worked on problems inspired by some problems in industry. I don't know how much interaction they had with people in industry, but I can imagine there would've been some. I'm not sure I was exposed to much at the time, but I'm sure there was some.

ZIERLER: How did you get your thesis project going?

STONE: Gary had this project involving how small drops of liquid deform when a fluid is sheared. He had a graduate student, as I said, named Barry Bentley building an experiment to study this in a very fundamental way that built on an original paper from the 1930s by G.I. Taylor—I don't know if you know that name—the great G.I. Taylor. Gary was the person who realized you could use modern tools in computers and mechanical engineering to build an automated version of this experiment G.I. Taylor had first demonstrated. That was a project then that I took over. Gary also had a side that did very sophisticated numerical simulations, so I also learned that.

ZIERLER: In terms of putting the thesis together, what aspects required laboratory work and instrumentation?

STONE: All the experimental work, measuring how different shear forces affect small suspended droplets, that was all experimental. There was some image processing to be done because, as I said, there was an early digital camera connected to the experiment. I forget if it was 60 pixels by 60 pixels, or whatever. I did a little image processing based on that. There was a still camera that took lots of photographs, so there was lots of, if I remember right, developing film, and then projecting the film up so I could make measurements based on a calibrated scale. All the experimental side on how small droplets deform in given shear fields was experimental, and then the numerical simulations—and I think I tried a little theory—built on work in the field for how do you simulate problems of that type. It was a great training ground because there were lots of things to learn that allowed me, when I took my next steps, to learn new ideas, and apply ideas like this in other areas.

ZIERLER: Howard, what was the research culture like in the laboratories at Caltech? Were graduate students working with each other? Were there professors involved? Were you basically by yourself at times? How did all that work?

STONE: The fluid mechanics group at the time, Gary's group and Eric Herbolzheimer's group, and there might have been another group, we were in the basement of maybe it was called Fairchild. Is that the chemical engineering building?


STONE: We were in the basement. There was a very good community. There were a lot of people around, so it was easy to talk to people. Lots of people were working on related problems, so there were lots of topics to talk about. The groups had weekly group meetings, as I recall. Maybe it was every two weeks. I think it was every week. There were lots of interesting subjects to listen to. There was a department seminar every week. As I said, there were seminars in applied math all the time. There were lots of subjects to hear discussed, and there was a very nice community of people. From my perspective, it was really a great community that way, because you learn often more from your colleagues than necessarily direct one-on-one from your advisor just because you're around your colleagues so much. There were a lot of my colleagues, a few older, a few younger, that I learned a lot from.

ZIERLER: You said previously that most people now would consider you a theorist. It sounds like, in graduate school, you were much more on the experimental side.

STONE: I did experiments because the experimental apparatus had been built, and I was successful in not breaking it.

ZIERLER: [laugh]

STONE: I didn't have really the talent to build it from scratch, so I don't want to give anyone the wrong impression. I did a lot of experiments at the time with this apparatus, but I don't want you to get the impression that I actually built it. I actually benefited from the fact that the person who built it—Barry Bentley, who then went off and started a very successful company—really built a very robust experiment that lasted many, many years, and used very early technologies at the time to allow a very innovative experiment to be done at that time.

ZIERLER: Were there any theories in fluid dynamics or related fields that served as an intellectual guidepost for this experimental work?

STONE: As I said, the experimental work that made up my thesis started already with G.I. Taylor, who was a very influential physicist in the early parts of the 20th century. He had already thought about and worked on some of these problems. Because there are lots of problems in industry that have emulsions, whether it's oil droplets and water, because you're thinking about salad dressing or separating oil from water because of production of hydrocarbons, or olive oil processing where you have to take the water out to get a purer olive oil, there are lots of problems that have droplets of one phase in another phase. There was a research arm in the literature where people had been studying this experimentally. Then as computer tools got better with simulations, and sort of in between, there were some approximate theories that one could use. Several of them were actually started by G.I. Taylor. My advisor Gary Leal came from the side of fluid mechanics where a man named Andy Acrivos (Professor at Stanford and later City University of New York) had pioneered numerical methods for some of these problems. We were exploring and using them. I'm not sure we were inventing new methods, but we were trying to apply them to new problems.

ZIERLER: What would you say your principal findings or contributions were in the thesis?

STONE: In the thesis? That goes back a long time. In the area of fluid mechanics, it was recognized that if you have a globule of fluid, and you shear it, then you could stretch it out. There was a theory due to Lord Rayleigh, the great Lord Rayleigh, that if you have a cylindrical thread of fluid, it's unstable, and it forms little droplets. That was very well understood for how you go from a cylinder to spheres. In our experiments, because we started with a finite droplet and we deformed it, it was always of finite length. What we saw was a combination of the middle trying to become spheres, and the ends trying to return the thread to its initial shape, sometimes breaking up, and sometimes just recovering itself. We realized that because shapes were finite, and not an ideal infinite cylinder, that what you got at the end was some combination of the ends having some dynamics, and the middle undergoing the so-called the capillary instability. That was one thing we found out, and people, I think, over the years will refer to that idea. I'm not sure we picked the best words for it when we described it. We called it end pinching because the ends sometimes broke and sometimes didn't, and so people sometimes refer to that. I'm not sure that's profound, but it was the realistic feature that you had this finite object; not an infinite object. Then I did some other work in the thesis with so-called surface active agents. I put them in a numerical simulation. That was some early work, trying to understand how to struggle with this topic of interfacial chemistry that was new to me. I'm not sure there's anything that profound in it. I'm not a profound kind of person, but I learned a lot. I think the most important thing was I learned about many new topics and how the subjects fit together, and I learned a set of tools that I think could then be widely applied in other problems, and I think that was really the big advantage…well I'm not sure advantage but a nice tool I learned. Then I started as an Assistant Professor at Harvard, and someone talked to me about swimming microorganisms. I already knew the math that I needed, and I could do something when it came up in other areas. It was just a great set of tools that I was then able to utilize in other areas.

ZIERLER: I'll test your memory. Who was on your thesis committee?

STONE: Good question. Gary Leal; I think John Brady, because John had moved to Caltech at that point; Philip Saffman; and I'm not sure who the fourth was.

ZIERLER: Now this story about Saffman and Delbrück, is there an origin? Were you aware of any of these collaborations while you were in graduate school?

STONE: No, I wasn't aware at all. I was certainly aware of Saffman, and I might have heard the name Delbrück. But when I got to Harvard, I forget exactly when, but my first couple years at Harvard, I was doing research on various topics. I think it was spring of my second, third, or fourth year, I got a letter from a man named Harden McConnell, who was a Professor of Chemistry at Stanford. Harden McConnell, if you don't know the name, a Caltech graduate who worked with Linus Pauling, passed away maybe six years or seven years ago, was one of the great physical chemists of the 20th century. Anyway, so, he writes me a letter. He was a professor at Stanford. He wrote me a letter because he was studying membranes, cell membranes. Often in cell membranes, you see patches called domains—phase domains is what they refer to—that when you look at them from above, it looks like a droplet in liquid, but it's a separate lipid phase in a membrane. McConnell was interested in how they changed shape. Now, in his case, he thought they changed shape because of electrical effects. But one of his postdocs, a woman named Ka Yee Lee, who I think is now provost at the University of Chicago, she had been a grad student at Harvard when I was a young professor, and so she knew of some of my work. She had made this remark to McConnell, "Oh, your pictures look like Howard's research." He had written me this letter because, like I said, I had done work on drop deformation. McConnell writes me this letter, with some of his papers, and I couldn't understand a word of them.

It was the summer. My research group was like one or two students. I decided I was going to learn about it, and so I started to read his papers. That led me to the fluid mechanics literature because there's a paper by Saffman alone, and then Saffman and Delbrück on fluid mechanics of lipids or domains in membranes, and how domains in membranes translate. In the biophysics world, it's called the Saffman-Delbrück problem. Over several years, I became knowledgable about the math and the physics around the Saffman-Delbrück problem. Now, I should say that I spent a whole summer working on it after receiving McConnell's letter. I typed up my notes in three or four pages. I sent them to McConnell, and I had made an assumption because I had read it in the Saffman-Delbrück problem. I had made an assumption that some parameter was small. It was in the Saffman-Delbrück problem but not in McConnell's problem. McConnell wrote me back a two-sentence letter. I got it like in September at the end of the summer. I was a young untenured assistant professor. The letter read something like, "Dear Professor Stone, thank you for your interest in my problem. You assumed epsilon was .01. In my problem, it's 100. Thank you for your interest in my problem."

ZIERLER: [laugh]

STONE: This is a true story. The letter is on my desk when a professor from the physics department came over to chat with me about something. She looked at it. She reads the letter, and she said, "Do you know who he is?" I said, "I have no idea who he is, but I just spent three months working on his problem, and this is what I get." She looked at me, and says, "He's famous. You're not. Your career just ended." I'm not sure what I should do with that, except that McConnell was really a wonderful scientist. He was just dedicated to the problems he studied. He wrote me again, and said, "If you're ever in the area, please stop by." My parents, as I said, were in San Jose. The next time I went to visit my parents, I wrote some people in chemical engineering at Stanford, and said I'd like to come by for a few hours. I wrote McConnell that I'd like to see him. The chemical engineers were shocked when I told them, "Oh, I have to go leave. I have a meeting with McConnell," because McConnell scared everyone apparently on campus. He was this intellectual giant. But, with me, he was just always incredibly gentle, and just wanted to talk about these research questions.

I eventually figured out the difference between the Saffman and Delbrück work, and McConnell's problem, and we wrote two papers together. The consequence of it was, on this and some other problem—there were a couple other things I worked on that went back to Saffman—that when Saffman retired, they had a retirement party. His former students organized the retirement party. His former students all knew I had worked on a couple problems he had worked on, so they invited me to the retirement party. It was at Saffman's home in Pasadena. Sorry if I'm telling too many stories. At this retirement party, at one point during the retirement party, I noticed Professor Saffman standing alone in his front yard, so I walked over, and I said hello. He remembered me. We talked. I said, "I've worked on a couple things related to what you've done. You probably might know this but, in particular, I've become an expert on what's called the Saffman-Delbrück problem." I said, "I'm just curious. You're an applied math Professor. Delbrück was a professor of molecular biology. Did you work together because Caltech is such a small and collegial place, and everyone likes each other?" He looked at me, and he said, "I can't comment on other people liking each other. I don't know. But what I can say," and then he points to the house next door. He said, "Max Delbruck, he lived next door. We used to rake the leaves together, walk the dog together. We talked all the time. That's why we ended up working on this problem together, because he lived next door." When Saffman passed away, they had a memorial service for him. Although I was not one of his students, the people who organized it asked me to give the leadoff talk, because I had this history in my research of working on problems that tied back to things Saffman did. As part of giving this talk, I told the story. How could I not? I told the story.

At the end of the talk, two women raised their hand. The first said, "I just want you to know that I'm Philip Saffman's daughter." I think she said, "I was a Caltech undergraduate in biology. When I applied to graduate school, several people who interviewed me in graduate school asked me, "Are you the daughter of Saffman who worked on this biological problem of membranes?" and I responded "Oh, no, my father's a mathematician. He would never work on problems like that." It was only later, she said, over dinner one day that she asked her father, "Did you actually work on a problem involving biology?" Then he said, yes, and they talked about it. Now, for me, that was always important because it reminds me of how little I knew about what my father did until I got much older. Anyway, she told her story, and after she finished, the other woman raised her hand. She said, "I want you to know that the only reason I'm here is because I married an applied mathematician, but I'm Max Delbrück's daughter. I can assure you," she said, "he never raked the leaves."

ZIERLER: [laugh]

STONE: For me, this problem, although it just happened by accident, it just told me all these wonderful stories about the intellectual life and some of the personal life. Over the years, I've read more about these people, and I even wrote a paper about some of Saffman's scientific work. I was asked to contribute an article to a journal in recognition of Saffman. What most people do is they write the paper on the subject they are currently working on, and then they write an acknowledgement. I said, "I'll only do this if I can write about the science of Philip Saffman." It took a long time, but I wrote a research paper that was on the fluid mechanics of Professor Saffman, and not the all of it because it turned out he switched fields halfway through his career. But, anyways, for me, it was this great intellectual exercise. It was really fun. Then, over the years, I've learned a lot more about Max Delbrück, who really has an amazing story.

ZIERLER: Yeah, he sure does. From biology to physics, and all that he discovered, it is remarkable.

STONE: Yeah. This again is probably unrelated. Do you know who he worked with before leaving Germany?


STONE: I can send you the photograph if you want. Lise Meitner.

ZIERLER: Oh, of course.

STONE: He worked before they both had to flee Nazi Germany. There's a plaque that commemorates this that I discovered this summer, because of a friend, on a building outside Berlin. Anyway, I learned that this summer, and I sent it to a few people I know who also have Caltech connections, of course, to Delbrück. For me, there are lots of things that connect the intellectual life too. You're a historian. You understand this much better than me.

ZIERLER: Howard, to get back to the narrative, after you defended—

STONE: Sorry about that. I get off track easily.

ZIERLER: No, it's wonderful. I live for these stories. When you finished the dissertation, were you pursuing postdocs and faculty appointments in tandem, or what did that look like?

STONE: No, at the time, it was not so common, I think, to do postdocs in fluid mechanics, but it was becoming more common. I received an NSF postdoctoral fellowship to go to Cambridge, England, in the department of applied math and theoretical physics. That's one department at Cambridge. It's probably best known because at the time Stephen Hawking worked on the theoretical physics side. But the applied math program is superb. It's probably more than half the department, and almost all of it is fluid mechanics. I went there to work with a man named John Hinch who had worked with Gary Leal. There was a man named George Batchelor who led the program for many years. It's a very distinguished fluid mechanics group, and so I went there for a year. At the time, I was just starting, I think, to interview for faculty positions. I'm trying to remember. I think maybe right before leaving Caltech, I had done a couple interviews for faculty positions, and I got an offer in the middle of my postdoc year.

ZIERLER: Was Cambridge a good experience?

STONE: It was incredible. It was amazing. I would tell people, for me, the most amazing thing was, first of all, John Hinch was a remarkable advisor. But Cambridge—I don't know if they still do it so regularly—but it was this old building. You always ran into people because it was this sort of, in a way, oddly organized old building. But because of it, you were always running into people. They had teatime twice a day. Most people went for 20 or 30 minutes, and there were these little tables that had white tops that you could write on. Teatime, for me, was amazing as I had never seen anything like this. It was sort of like Jeopardy. The answer was on the table, and so you had to figure out what the questions were. I discovered pretty quickly that, as I told you I like people, and so I met many people from the department. But most people in the department sat with their own research group. I had the advantage that since I was coming from the outside, I wasn't so tied to a research group.

To a first approximation, Monday, I would sit with one research group, and Tuesday, I would sit with another research group, and Wednesday, I would sit with another one. I just got exposed to lots of terminology. I got exposed to fluid mechanics problems of the interior of the earth, fluid mechanics problems of oceans and atmospheres, fluid mechanics problems of the different groups. I was working within this area of low Reynolds number flows and physicochemical hydrodynamics, and it was just this eye-opening experience about all these different problems in science and engineering, all being studied through the lens of fluid mechanics. For me, it was great. I remember when I was a young professor at Harvard, I was at some social function, and someone came up to me during a conversation. I said, "I was a postdoc in Cambridge." The person said, "They waste so much time at tea." I said, "Are you kidding? That was the most educational time of the day." I would joke, "I probably learned more at teatime in one year than all my years of education because there were so many interesting exchanges, and people would write what they were thinking, and so you just really got exposed to a lot." I tried over the time to recreate that, but that requires a small community, and you really have to want to make the time to do that.

ZIERLER: Beyond Harvard, where else were you looking for faculty positions?

STONE: It's probably hard. I interviewed at Penn. I didn't get an offer. I interviewed, I think, at UCLA and maybe at Santa Barbara. I can't remember. But the only offer I had was from Harvard, and so I took it, and I was just lucky, as lucky could be. I worked in a group of faculty members in what was called a mechanics group. The people included: John Hutchinson, Jim Rice, the late Bernard Budiansky, the late Lyell Sanders, the late Fred Abernathy, Dick Kronauer, the late Tom McMahon, who was also an author. It was just an incredibly supportive group. In Harvard at the time, the engineering and applied science department, as I mentioned, had a very strong material science group led by the late David Turnbull and Frans Spaepen and Mike Aziz, and a super applied physics group.

I started attending the applied physics seminars. I forget what it was called. I think it was called the Lunchtime Condensed Matter Seminar Series led by Bert Halperin, David Nelson, at the time David Vanderbilt, and then later Daniel Fisher. I just learned, again, a ton because there were these very interactive seminars, and it was often the conversations around the seminars that were just very helpful for me to learn some things or be exposed to some ideas. It's true that many of the research groups I visit in Europe often come from the physics side of the world; that is a little less common in the States. But a lot of that was part of my exposure as a young professor at Harvard. I had this great mechanics group around me, and they were really intellectually inspiring, and very strong groups in material science and applied physics. It wasn't a large community, so it was a little easier at the time to kind of wander. I didn't have a big research group. There wasn't a lot of pressure put on me to raise a lot of money. I really benefited from all of that, if you like, intellectual freedom.

ZIERLER: What department did you join when you got to Harvard?

STONE: That was the thing. At the time, the School of Engineering and Applied Sciences didn't have departments. At the time, it was just called the division of applied sciences. I think that was for two years, my first two years, and then it changed to the division of engineering and applied sciences. Internally, it had a computer science group, a mechanics group, an electrical engineering group, an applied physics group, a material science group, and an environmental science and engineering group. But it didn't have departments. Everyone reported to the dean. It had a loose internal structure where courses and things were worked out, but it didn't have official departments. That was great.

ZIERLER: Your appreciation during your time in Cambridge, for all of the applications in fluid dynamics, did that affect your research agenda as a young assistant professor, would you say?

STONE: Probably in the sense it made me more aware that it was okay to think outside the box, and that these principles are applied in many areas. Recall I mentioned that Howard Berg, who is a Caltech name, was in the biology department at Harvard and was studying swimming microorganisms, but the person he did it with originally was Ed Purcell, the Nobel Laureate in physics for magnetic resonance imaging, NMR, MRI. In the field of swimming microorganisms, if you want, one of the first papers you read is by a Nobel Laureate in physics, Ed Purcell. I think I was probably aware of some of this, and so I think the Cambridge experience of seeing fluid mechanics in many ways might have made me more appreciative that you could apply these principles. I think because I didn't have a big research group at the time, it was just a little easier for me maybe to feel that it was okay to explore a little more broadly. I had the time and freedom to do that.

ZIERLER: Howard, what were your experiences like as an untenured assistant professor at Harvard? You hear all these stories.

STONE: Say again.

ZIERLER: Your experiences as an untenured assistant professor at Harvard. You hear all of these stories about the very low tenure rate, that Harvard is really for more senior faculty. What were your perspectives on that, given the length of your time ultimately at Harvard?

STONE: You certainly heard those stories. I won't repeat one or two of them because they're pretty sad, that were told to me both by people at Harvard and MIT. I think what made it a little easier for me is, one, I was single for most of the time.

ZIERLER: You could work 24/7 if you wanted to?

STONE: I could work. I also think I felt less family-related pressure, that if something didn't work out—at least, this is what I told myself. I'm not sure that was mentally the most healthy thing to do. As I said, the mechanics group at the time—and I think I mentioned all the names—it was this incredibly supportive group that made you feel intellectually welcome. Also, I should say, the other people that I mentioned in material science and applied physics, they all made me feel intellectually welcome. Although you heard these stories, I never felt that from my immediate colleagues—I didn't think I was going to get tenure. I'm not sure that was necessarily in the cards. But I always felt supported, and I always felt welcomed and part of the community. I could tell other stories. I'm not sure they're appropriate. Then I was fortunate enough to get tenure. That did eventually happen.

ZIERLER: Did you want—?

STONE: I think I was sort of res…I'm not sure "rescued" is the right word. But, for the one, in the group I was around, it was a very supportive community.

ZIERLER: You said at the beginning you didn't have a very large research group. Did you want to take on more and more postdocs and graduate students over the years?

STONE: Yes, some part of that. I've never been the most successful fundraiser. The American system requires that you raise money to support your group. But what happened at some point, I had a couple students. I'd get a grant or, every once in a while, I got an industrial grant, which was really informative for me. If I remember, a company came to visit Harvard. It was called PPG, the former Pittsburgh Plate and Glass. They had material science questions, but they all involved materials coating surfaces, and so they ended up getting directed to me and a colleague named Gareth McKinley. Gareth eventually moved to MIT; he is a very distinguished, wonderful colleague. This company visited, and they liked their visit, and so they invited Gareth and I to spend a day at their company, which I now realize we were unpaid consultants like six times over two years. But it tied into something that I knew about, which was coating flows. I think they found the feedback useful. After one of these visits, I mentioned to Gareth, "This is ridiculous. We keep making these one-day visits. The only way we can learn more is to get a research grant." Gareth had a very gentle way to suggest that they do this. They eventually came to Harvard and offered a research grant to explore this problem of coating flows, which is a wonderful area. It's got fundamental things. Like I said, it's tied to fluid mechanics. The names associated with it start with Landau—Landau and extend through Derjaguin and Bretherton. If I remember how this then went, I was so excited to get this grant. I was so thrilled by it that, in order to set up the experiment that we had to do, I think I spent $20,000 or $30,000 buying the necessary equipment. Then I learned at some point that the Harvard lawyers hadn't agreed with the PPG lawyers and, four months later, I was told there was no contract. It had never gotten signed, and so I was in the hole for $20,000 or $30,000 of equipment that we were building in my lab. Then the miracle happened was that the head of the company's research arm came to Harvard the following year, and asked to visit the lab. I said, "OK." He saw what we'd set up, and he said, "This looks great." I said, "I don't know if it's great or not. You guys never provided any money for it. I've had to pay for it myself." He went back, and got the lawyers to stop quibbling, and to agree, then funded it, and said then the first $20,000 or $30,000 that they could spend is to pay for the equipment they already bought. That led to several research papers. I became very expert, in my own way, at this kind of problem that sits between industry and academia. For me, it was a wonderful experience. But my research group was small, and so I got off track a little. Then somewhat later in my career, Dave Weitz moved to Harvard. Dave is a very distinguished, insightful, and energetic colleague. George Whitesides, I eventually met. George is one of the great chemists; actually, might have a Caltech connection.

ZIERLER: He does. He's an alum.

STONE: Yeah. George became a very good friend, and his son, it turns out, is a Princeton graduate, who's on my visiting committee in mechanical and aerospace engineering. There are all these wonderful connections again. But George and Dave succeeded at some point in raising a lot of money for a group of people to study an important problem related to material science and biofilms – it was a great research program - and that allowed me to hire a couple people. My group eventually grew over the years through some combination of my efforts and also very generous colleagues. Now it's probably too big. But I probably benefited a lot from the fact that it started much smaller.

ZIERLER: Howard, you've been recognized at Harvard in a number of ways for your commitment to teaching. Did you develop that skill in real time? Were you always interested in pedagogy and the undergraduate experience?

STONE: I'm not sure I was always interested in the undergraduate experience, but I've always enjoyed talking to people about problem solving. Even as an undergraduate, I would volunteer tutor for different classes. In fact, I have some very embarrassing memories of blunders I made in front of 40 kids. I realized it was a blunder. I just couldn't figure out how to get out of it at the time. But I've always enjoyed that, and so I always found teaching interesting. In my view, at least in the work I do, teaching has helped my research enormously because it filled in lots of holes in understanding in fluid mechanics, physical chemistry, and other physics that I was then able to use to my benefit in thinking about research problems. I always enjoyed it. I probably spent more time worrying about it, thinking about problems. But I also got lucky. At Harvard, I taught a thermodynamics class my first three years. I've always enjoyed thermodynamics. It filled in a lot of things for me. I then taught an applied math class, and I got notes from John Hutchinson. His notes were just magnificent, in the way of showing how you do things without necessarily worrying about too many details but showing you how things fit together, and so I learned a lot from that. I enjoyed it. For me, I've just always found teaching and research, they go together. Where I struggle, and I think many have struggled, is time. Everything takes time, and there's just a finite amount of time. Maybe I benefited early on from, again, maybe being single, maybe having a smaller research group, so I had fewer time commitments there. I enjoyed the community. I've always found teaching has always helped my research, every time I do it. The hardest thing now is finding time, and trying to remind myself to take the time to think about my lecture, and not say, "Oh, I know this. I'm just going to walk in, and talk about it." Although teaching is very nonlinear: I have many experiences where I spent hours preparing the lecture, and it came out a disaster, whereas other times, I had so little time, and I put it together at the last minute, and it worked perfectly. Teaching is, I think, this very nonlinear thing. It is improved when you develop a rapport with the class because the more back and forth there is, the easier it is for you to engage, and for you to judge how you're getting across the ideas. COVID had a huge impact, I think, on kids' preparation and otherwise. But I've always enjoyed teaching, still to this day. Although, as I said, now, just finding time is harder. I know, at least in my case, it helped my research enormously, not in the sense of solving a research problem. It's not trivial like that. It's more it helped me fill in intellectual connections between subjects, and made me more able to understand different things that I was seeing in research and research papers.

ZIERLER: Your work on lipids, would you say that was your entree point into biological-oriented research?

STONE: Yeah. If you don't count swimming microorganisms, then a couple of these papers I wrote with McConnell, at least, made me aware of some of these questions and just this whole research field. I should say, when I first started to study this, I was reading papers like on early experiments done on the diffusion of lipids in membranes. Very early on when I was reading this, I went to a Harvard football game, the only Harvard football game I ever went to. I went alone, and I was sitting there, and a miracle happened. I introduced myself to the guy sitting next to me, and he (John Dowling) was a professor in molecular biology at Harvard. Anyways, at some point, he asked me, "What are you working on?" I said, "I'm working on this and that, and I'm now trying to learn about membranes. I received this letter from Harden McConnell." It turned out, he (Dowling) knew the whole history of how molecular biologists were first studying the diffusion of lipids in membranes. One of the first papers was done at Johns Hopkins, and another one was done at Harvard. He told me this all during the football game. I don't remember anything about the football game. But when I left, I knew a lot about this early history. There was a paper by Poo and Cone and papers by some other people. Really, it was this small miracle because it informed me about something about this early information that gave scientists the idea that there were fluid mechanical concepts—he didn't use those terms; I use those terms—in trying to understand the diffusion of lipids in membranes. That was related to the question McConnell was asking.

ZIERLER: Howard, your work in surfactants and things like hydrocarbons, was that related to the rise of sustainability research, environmental science?

STONE: No. We've done some work recently where we've connected surfactants to an environmental problem. But, originally, it came up again because of this intersection of physical chemistry and fluid mechanics. If you just take a drop of water, and you release it in some typical cooking oil, it will tend to sediment. You can measure the speed at which it sediments. There's a theory for the sedimentation of a droplet in another liquid, and that theory is called the Hadamard-Rybczynski theory. Hadamard was this great French mathematician at the turn of the century, 19th to 20th century. Rybczynski was a Polish mathematician who worked on the problem independently. Anyway, so, there's a theory, but the theory never agrees with the experiments. This, in some areas of math and physics, is a called a paradox. You do an experiment, and you get a number that's 50% off from where it should be. Why is that? Eventually, people realized it was because of surfactants, that there's always some contaminants in fluids. They accumulate on the interface between the water and air, or between the oil and water, and they change the properties of the interface. That effect when it happens is called a change in surface tension, and it's given a name called the Marangoni effect. Marangoni was an obscure 19th century Italian physicist. But it's related to the wine tears that we talked about at the very beginning. They're very common fluid mechanics problems involving ordinary systems where surfactants change what you see, and the only way you can understand an experiment is if you account for them. Sometimes we study that just because that's a common kind of problem. You try to do experiments, and what takes place is influenced by the interface.

The kinds of things you learn if you ever study this problem goes back to the original Millikan oil drop experiment. Millikan, another great, Caltech name: in order to get an idea for the magnitude of the electric charge, he was doing experiments where oil droplets were sedimenting between two electrodes. He needed to know the speed of the droplet, and how it was affected, and then what the electrical forces were. This kind of model comes up in different places. In fact, the sedimentation problem of a particle, which is a standard fluid mechanics problem, it's part of three Nobel prizes: the Millikan oil drop; Brownian motion, where the Nobel Prize went to Jean Baptiste Perrin, although the theory was Einstein's; and then the original theory that gave you molecular weight of proteins. Scientists did rapid centrifugation, which had the proteins translating through a fluid, and they needed a sedimentation result in order to estimate molecular weights. Some of these fluid mechanics problems have this really wonderful connection to other areas of physics. Again, there's a story, and you would love it. Most people don't learn these connections. But when I learn it now, I tell these stories when I teach because they connect fluid mechanics to all these other fields.

ZIERLER: Sure. It's all nature. Nature doesn't care about these academic distinctions.

STONE: Yeah. Surfactants are this feature. They have this big impact on many fluid mechanics problems. There's a beautiful terminology now that a few people use. Todd Squires—I don't know if you know the name—Todd is a professor at Santa Barbara, but he was a postdoc at Caltech with John Brady. Todd has written a beautiful review article where he ties together how surfactants affect many problems, but because you don't see them, they're this hidden variable that you have to account for in order to understand what's going on. He's written, I think, a paper with the term—it's either in the title or in the paper—this hidden variable. Surfactants are often that way. They're at the interface. They're molecular sized. You don't see them, but they can have a big effect on what you observe.

ZIERLER: Howard, tell me about your decision to move to Princeton in 2009.

STONE: I'm not sure what I want to say in public. The easiest one, my family, we were always very happy in Boston. But my wife has family in New York, and we decided to make the move. I'll always miss Harvard, but moving to Princeton was probably one of the best things I ever did. The first one was marrying my wife, Valerie. But I've been very happy at Princeton, and it is where our two wonderful daughters, Taylor and Blaise, grew up. I've had many, many wonderful collaborations with people all over campus. Right now, I have interactions, collaborations with five different groups of molecular biology. They're probably appalled at how little biology I know. But I get to work with Bonnie Bassler and Ned Wingreen, and Zemer Gitai, Josh Shaevitz, and Sabine Petry. They're all great scientists. They're serious. They're great collaborators. They're beautiful writers. But I also get to collaborate with people in other engineering and physics departments. It's just been intellectually a wonderful time. It's a smaller community, and so getting to campus is easy. Going home is easy. Getting to the airport is easy. I like the train (Amtrak) that runs along the east coast. I've found many parts of combining family and work very appealing to live in a smaller community.

ZIERLER: Would you say that you got even more interdisciplinary by the time you got to Princeton or as a result of coming to Princeton?

STONE: Maybe. I found it easier, maybe, in ways. But I think I was always open to many of these ideas. In fact, the first collaboration I had at Princeton, I think, was with Bonnie Bassler. I had met Bonnie while I was still at Harvard because, right before moving, she visited. Rich Losick, who's a biology professor at Harvard—a wonderful, wonderful man, and we had co-authored a paper together—when Bonnie visited, he made sure I met Bonnie. That was really a good way to start. But the ironic thing was when I was at Princeton, very early on—if I remember right—Howard Berg came to give a seminar. I decided I would go to the seminar because I knew Howard, and I met Zemer Gitai, as a young professor, because he was there at the seminar. Then the weird thing was the very next day, I think, I met Zemer at the supermarket because it's Princeton. Princeton's a small place. There's a finite probability, anytime you go to the supermarket, you run into a professor. I ran into Zemer. I just met him the day before. Then two days later, I met him again at the gym. By this time, we had a collaboration, three conversations in the course of a week. He is someone I still collaborate with to this day. I think just something about the small community, and not everyone wants to collaborate, but there's certainly a lot of people who enjoy collaboration. The university has been very good about setting up programs that give people a chance to get together.

ZIERLER: Is there a line of research that you've pursued in the past 10 or 15 years that you think was only possible as a result of coming to Princeton?

STONE: I know there's a line of research that probably happened because I was here. It could have maybe happened somewhere else. A number of years ago, I advised a graduate student in chemical engineering. Her name's Akanksha Thawani. But before coming to Princeton, I think I had to Skype with her four or five times when she was trying to make decisions about where to go to graduate school. She was from India, and she was trained in theory, and she wanted to study swimming microorganisms and things like this is, which was what she was trained in. In our last Zoom conversation, she said—this is my recollection—"If I come to Princeton, will you be my advisor?" I said, "I've never made this promise to anyone, but I will say that if you decide to come to Princeton, yes, I will be your advisor." She came to Princeton, and when she came to Princeton, she said to me, "I'd like to start working with you." I said, "I think that's great." I should say, Akanksha, she is brilliant. I said, "But if I were you, before I made any decision about advisors, what I would do is I'd go meet some more professors. I think that's good for you. If I was your age, I'd go meet some of the professors in molecular biology because it's an area that is—I love fluid mechanics—but this is an area that's evolving rapidly."

She took that to heart. She met some professors, and she ended up meeting a young professor who had just joined Princeton, named Sabine Petry, who studies spindle formation. The construction of the spindle inside a cell allows a cell to divide. Sabine and her postdoc had discovered something about how the spindle is constructed that no one had ever observed before. Sabine agreed to co-advise Akanksha, so Akanksha had us as co-advisors, and we would meet every two weeks. I like walking, so I would always generally walk over to Sabine's office. Although there was a lot of molecular biology in what Akanksha did, she became an experimentalist, but she has this very, very strong quantitative training. We ended up learning something about the mechanics of how the spindle is created. Because I would occasionally go to Sabine's group talks, a couple years ago, I attended the PhD thesis defense one of her students. At the end of the thesis defense, the person put up a picture of the microtubules that are part of the spindle, and what it looks like is that there are little droplets of liquid decorated along the microtubule. I went back, and talked to my other graduate students, Bernardo Gouveia, who also works with me and with Sabine. I said, "That picture looks just like what's called the Rayleigh capillary instability in fluid mechanics. It looks just like what you get when you take a cylinder of liquid, and put it on a cylinder. It breaks up into droplets." Bernardo and I started to work on it. We went to talk to Sabine about it. Josh Shaevitz, who's a colleague, collaborated, and pushed back a little in good ways. We ended up discovering that in this topic, a new area of molecular biology, that is called the formation of biological condensates—I don't know if you've heard this term, but it's now an area that is growing up in molecular biology, which argues that inside the cell, you don't just have membrane-encapsulated organelles, but you have phase-separated regions, which act as nucleating centers for other activities. What we believe we discovered was phase separation on the microtubule because it's a liquid phase that then breaks up into droplets. Those droplets co-localize nucleating factors that allow spindle branches to form. It was maybe one of the first examples that showed a functional application of how phase separation inside a living system can impact function.

Because of that, I got interested in learning more about this area of liquid-liquid phase separation as it's described in biophysics. I have one or two side projects now on it. I've co-authored a paper with my colleague Cliff Brangwynne. The reason it's really useful to be at Princeton when learning about biological condensates is Cliff, who's in chemical engineering, is one of the modern people who first recognized this idea about 12 years ago. There's a seminar series every two weeks on it. There are several groups in molecular biology that have found phase separation in other biological systems, and several very strong theory groups. By being here, and working with Sabine, we stumbled on this problem. We then recognized it had other value, and it led to other collaborations. That could happen other in places because the topic is discussed at many places. It's very active. But in particular I was fortunate to be at Princeton because there was a lot of interest in this kind of problem.

ZIERLER: In making the move from Harvard to Princeton, did you bring graduate students with you? Did you see an opportunity to build up a new group?

STONE: Yeah, a couple people moved with me. They were instrumental into designing my lab, setting up my lab. I got really lucky. I had a wonderful group at Harvard. One or two of them were about to start their own labs, and they said, "No, we're happy to help you design your lab because I'd rather make the mistakes in your lab so that I don't make them when I design my lab." So Bill Ristenpart, Laurent Courbin, Sigolene Lecuyer, Jiandi Wan, Rachel Pepper, and Alison Forsyth, among others, all helped make my new lab at Princeton possible.

ZIERLER: [laugh]

STONE: Several of my group were magnificent. One of them took trips with me to Princeton to talk to the architects. Several of them helped think about how to lay out the lab. It was just fantastic. Then one or two moved with me to Princeton. In particular, Jiandi Wan moved with me. He's now a professor at UC Davis, where I was an undergrad. Jiandi helped me set up the lab when we first arrived. A couple of my graduate students moved with me. Then I had this—I don't know. You can decide how to use this. Then I heard these classic academic remarks that I found ridiculous. One of my students who was early in their career, they were going to move to Princeton. But in order to get a degree at Princeton, they needed to complete the course requirements. One of my colleagues in engineering said, "No, we can't accept a Harvard course. We're at Princeton, and no courses are as good as Princeton courses." They actually said that. I said, "Are you crazy? These are courses they took in the mechanics group at Harvard from John Hutchinson and Jim Rice. These aren't just great researchers. These are great educators." I had to argue for this. I still can't believe in my mind that I have colleagues that think that, somehow, the courses here are any different than the courses almost anywhere else, such as peer institutions. It's just wrong. But I had to spend my time with these arguments. I couldn't believe it to this day. But this is true in many places that people have these unrealistic expectations of how good the education is at one institution relative to a peer institution.

What I'm sure is individual courses might be a little different the student's course record was fine, and the courses they had were wonderful courses. They looked a little different, but that's because you and I, even in the same field, you'll teach a slightly different course than your colleague. But, anyway, I had a few people move with me, and they were instrumental into getting me restarted. I remember my first year or so thinking, "Am I ever going to have a research group again? What's going to happen?" But I got really lucky. One really good person (Doug Holmes) came. He was from a smaller community. As I recall he said, "I'm not attracted by big cities. I want to be in a smaller community." He was great. About the same time I was joined my Margarita Staykova, a wonderful biophysicist from Europe. I was really lucky that way. I've had really great students and great postdocs, and I've been really fortunate, and then also had many good collaborations.

ZIERLER: Howard, I asked earlier about technology and instrumentation. In comparing your experiences setting up at Harvard as a junior faculty, and then at Princeton, coming as a tenured faculty from Harvard, how might that distinction come into sharp focus some of those advances in technology and computation when you were setting up at Princeton?

STONE: What I had realized at Harvard, I was very lucky. I had a colleague at Harvard, Dave Weitz, who had a lot of valuable experimental systems, not just good optical microscopes but confocal microscopes, several rheometers. I realized in order to do the work I needed to do, I should have my own. I never had that at Harvard. I had an optical microscope, but I never had a confocal. As part of my start-up, I said I'm going to get a confocal, and Dave or one of his group, I think, helped me think through what that would mean, and a couple of my postdocs helped. We got a rheometer, and I was fortunate enough when I came here, there was a colleague in chemical engineering, Bob Prud'homme, who's an expert on rheometry. Rheometry is involved with measurements of the mechanical properties of materials, whether they're solids or liquids. Bob said, "I have a rheometer. It's from this company. These break occasionally, so what would be ideal is if you get a rheometer also from the same company so that when mine breaks, we can use yours, and when yours breaks, you can use mine."

That's what we did, and it worked out perfectly. We got a different rheometer but from the same sort of company. It uses the same software. It uses the same kind of tools. When another professor joined chemical engineering, they then also did that, so that now we have a community so that when one person's equipment is down for a little while, the community can use someone else's. I did that. We got a confocal so that we could do the work. We got a couple instruments that, I think, I didn't know enough about as a younger professor. But after seeing the successful colleagues, and the instruments they were utilizing to do some of the work, we were able to get it in my own lab rather than just having to necessarily just use it in another lab. Now several of us at Princeton, though our students mostly work in their own group, but they know what's available in other groups, and we just let our students come and use the equipment so that we can do the best research possible.

ZIERLER: Coming back to the impact of COVID that we talked about earlier, what happened to your lab? Were you able to keep things running? Did you have to shut down completely? Did your work become more theoretical by nature during this time?

STONE: I guess two things happened. The thing that happened to everyone at Princeton and pretty much elsewhere in the US was, from March to at least June or July, the lab was closed, so everyone was at home. But what did happen to me that's very COVID-related was that in March 2020, one of my former postdocs, a man named Manouk Abkarian, who's from Montpellier, France—Manouk is a super experimentalist, and he's an expert on blood flow—he was planning to do a sabbatical at Princeton. On March 1st, he arrived in Princeton with his family. He's an experimentalist. He had been a postdoc with me 20 years before. He's a very dear friend. Ten days later, the university closed. Here he was, this world-class experimentalist, and he's living in an apartment on campus with his family, trying to homeschool his kids who are now at home, who are French children educated previously in French going to American school.

But I would talk to Manouk every day by Zoom and, obviously, the theme we're talking about was COVID. Very early on, we ran across news reports of people getting sick due to close interactions, and we started to talk about how speech might affect COVID transmission. In fact, the first article we ran across on it was from about—I could tell you a story about it—but it was from like February 1st. During March, we then were talking by Zoom almost every day about what it would mean for you and I to have a conversation, and how that might affect transmission of a virus. Right around the time, although the CDC didn't talk about it, and the World Health Organization didn't talk about it, there were news reports about a possible aerosol transmission. I'll get around of answering your question now. In April, I decided to write a short grant to the NSF. They had a window for what are called rapid grants to respond to COVID. I wrote one of these, and submitted it, and I think I submitted it on a Wednesday. The program manager wrote back. He said he's got too many of these. He probably won't even look at it. My proposal was something like fluid mechanics of speech as a driver for COVID transmission [the actual title was "RAPID: Flow Asymmetry in Human Breathing and the Asymptomatic Spreader." I could send you the title if you want.


STONE: That afternoon, The New York Times ran a piece with an artist's rendition of how aerosols might be driving COVID. Then a day or so before or a day after, the National Academy of Sciences had a brief committee report on the possibilities of aerosol transmission, which, at the time, the public health community didn't pay attention to. I copied this to the program manager, Ron Joslin, who was a very thoughtful person. I said, "I understand you have a lot. But the topic I'm talking about is the one in the news. It's not about coughs. It's not about sneezes. It's not about your hands. It's about the air." He wrote back something like "If you can turn it into a six-page version rather than the eight-page version, I can consider it under this other funding mechanism, and that's being done in the next four days or so." I turned it around overnight, I think, maybe two days. I submitted it on a Friday, and I think on Monday or Tuesday, I got an email that said, "You've got a grant."

Then I had to write Princeton. Now I have to be careful what I say because Princeton did say, "If you got a grant that related to COVID, you could open your lab for one person." I then wrote the University administration, which of course was under a lot of pressure since given the rapid realization of the extent of the health threat. I'd like to think I have a gentle personality. I said, "I've received a grant from the NSF to study airflows that come from speech, and how that might be creating an environment that allows droplets to transmit virus." Then I signed it, "I hope to make a difference." Someone wrote back, and said, "We're not opening labs for hope." I was furious. That was just my personality getting in the way. I forwarded The New York Times article. They let me open the lab for one researcher at a time and my colleague Janine Nunes was instrumental in helping throughout the COVID-19 pandemic (and before and after) in helping to keep my lab running. Then this is where luck would have it. Manouk Abkarian, this wonderful friend and experimentalist, he is just a super experimentalist. I think this is fair to say. The first careful images you can see about how when you speak, you have no idea, but you create an environment around you that envelopes the head of someone two meters away. Certain sounds you make, including sounds that are called plosives, like if you say "papa" or "Peter" or "Peter picked a peck" or "pizza," create a vortex out of your mouth. Mory Gharib and John Dabiri, two leading Caltech professors, by the way, would be very happy with this because they have written about the fluid mechanics of vortices. Plosives typically create a vortex that propagates already a meter or two in a second, two seconds. We wrote, I think, what's fair to say, one of the first papers documenting the fluid mechanics of how your environment is changed by speech. Everyone's ignored it, I think, for the most part. But it certainly plays a role, I think, in how you make yourself safer. But, in part of this then, we needed numerical simulations.

Manouk had a colleague named Simon Mendez, and so we started to have Zoom meetings every week. One graduate student (Nan Xue) from my group could go in when Manouk wasn't there to help set up the labs so we could do flow visualization experiments. Manouk then did the experiments on his own. He had a high-speed camera, and he had a laser sheet, and he would speak next to the laser sheet, and then you could image the flow by seeing what was happening in the laser sheet. A group at NIST had published a paper showing how you could use a laser sheet in a clever way to see droplets, and so we utilized that for speech. Then, one of my graduate students, Fan Yang, tried to get some simulations going, and did some modeling for me. Then Simon did the simulations using his sophisticated software. We were meeting every week if not more frequently to discuss our results and ideas. We wrote an early paper (June 2020), trying to help people illustrate how when you talk, things that come out of your mouth can change the environment around you. That is one reason not only to keep away from someone—that's the social distancing—but to reduce the time you're near them. We even wrote a paper saying there's social distance, but there's another axis, which is time. You could have your interaction, but you want to minimize both of these, if you can. COVID-19 turned out to be very impactful, not only in the bad way it impacted society, but in this other way, we tried to learn more about fluid mechanics of speech. It turns out, there's a lot about acoustics but very little about what the airflow was that gets created. I still have a side project around that that one of my very talented postdocs, Junshi Wang, is trying to look at links to language and speech, and how that affects the airflow. That's a long story short.

There was a bad side. My lab was closed, just like everyone else's, basically until the fall 2020. But we did get a window starting in May where we were allowed one person at a time in the lab. For me, that turned out to be one of my graduate students who helped set up an experiment. Then my magnificent collaborator Manouk Abkarian, who then also realized at one point, by the way, that if you go into the literature about all these droplets that spread diseases, and this is 100 years of research, what the research would say, and I'm sure it's probably true, is that droplets come from deep in the lungs, or droplets come from your airways. That's probably true. But there's a third thing people don't talk about, and I think—I might be wrong—the first group to point this out was Manouk and I. Manouk, in doing all this imaging, started to take a camera, and point at his mouth all the time. Whenever his lips were wet, what he would always see was that when your wet lips open, there are little droplets being created right at the lips. You know this because you have some friends that are always the spitters.

ZIERLER: [laugh]

STONE: We imaged all that. We imaged how that when your lips are typically wet, it's not uncommon that you create little filaments that then break up—again, the so-called capillary instability—and that you're also forming droplets right at your mouth. Now, they might be bigger droplets, but they are another source of droplets that get created when you're speaking, and your airflow carries them away. We documented that also with imaging. Anyway, COVID-19 is something now that I think about this other problem that I've never really worked on before but how airflows might be affecting disease transmission. Now everyone's more aware of it, but most people think about it in the context of have a good circulation in the room. Have more airflow. To be honest, a lot of the transmission probably happens because you're too close to someone for a long enough time that you breathe in something they're breathing out. People don't probably talk about that as much as they could. We've tried to document that, and that was an outcome of COVID-19 that impacted my research.

ZIERLER: Howard, now that we've worked right up to current research, for the last part of our talk, a few retrospective questions, and then we'll end looking to the future. To go back to the topic that obviously brings us together, what did you learn at Caltech, either in the way that research collaborations happen or motivations in looking at the fundamentals, what has stayed with you that's really informed your research career ever since your time at Caltech?

STONE: I think I was certainly influenced by my advisor Gary Leal, in the way he looked at problems. I think, as I said, when I was at Caltech, there were a lot of problems that were informed by fluid mechanics. The thing that I found appealing to me, I think, over the years was this interplay of experiment and theory and/or simulation that I got a flavor for at Caltech, and that I saw in different places at Caltech. On the theory side, as I mentioned once, there's this approach, called asymptotics, which is some combination, if you don't mind, of math and art, because approximation requires some intuition and a feel for things. Then you need to be good enough at math that you can go through the details. I'm not sure I'm good at either of them, but I certainly appreciate it. At some intersection of all these things, I first got this probably at Caltech. As an undergraduate, I got the start of an education, but you really have to work on problems on your own to fill in the education. I think I saw a lot of that at Caltech, and I think it gave me—and I mentioned this earlier—a set of tools that maybe I didn't understand everything but I could build on.

I was fortunate enough to be able to have time and colleagues to help me do that. Then as a young professor, I just had lots of impactful experiences, where I got to use these tools. The thing I probably didn't see as much at Caltech is how to choose problems. I think, as a young professor, I got to see a little bit more about choosing a different set of problems. When you're a graduate student, you can only do a finite number of things. I remember this in particular because when I was a young professor at Harvard, there was a postdoc in physics at Harvard who came and sat in on a course I taught for a year. His name was Armand Ajdari. He ran a lab for a long time at École Supérieure de Physique et de Chimie Industrielles in Paris, but then he was head of research at Saint-Gobain, which is a large French company, and is now head of research at Arkema, another international company. But I remember he would come and talk to me, and occasionally he would ask me questions, and I'd give him papers to read. What was always interesting is he'd often come back a few days later, and he'd say, "The papers you gave me were really interesting, but that wasn't the question I was asking." I started to appreciate a lot more just the range in depth of the kinds of questions that you can ask to get insights into different kinds of problems. That's sometimes where our different fields come into play. The physicist asks a different kind of question than maybe the mechanical engineer or the chemical engineer or the chemist. They're all working around maybe the same problem, but they approach it from different perspectives. I started to get a better feel for that. I think that enriched me intellectually to appreciate a wider avenue of questions because it's really the questions that drive what we learn. I got some of that at Caltech. I think Caltech maybe did more exposing me to a set of tools, and then over the years, I was able to get a exposure to a wider set of questions and also how to ask questions.

ZIERLER: I won't burden you with all of the ways that you've been recognized in your career, but I wonder if you can reflect on the distinctions of really discipline-specific awards, like the Batchelor Prize, versus being elected into the National Academy.

STONE: Obviously, if you're lucky enough to get awards, that's a wonderful experience. It means a lot. They're not unrelated. How do I want to say it? These academies are wonderful organizations, and I do some volunteer work for them. But, historically, the people in these academies have nominated the people they know, which are the people in their fields, and many of our fields are pretty narrow. It's not so surprising that now the academies are realizing that there are areas that aren't represented at all. There are wonderful, talented people outside the traditional areas that aren't recognized. I happen to work in an area which I think is viewed as important in fluid mechanics, so that led to discipline-specific prizes. But I also happen to work in an area where there was some representation in some of the academies, which made it, I think, possible historically to be nominated, because there are many, many talented people who aren't nominated, and sometimes that's simply because they're outside of the traditional means to be nominated. I think the academies now are trying hard to broaden that pool of talented people, so I don't want to demean anything. I just want to recognize that if you get awards, I'd like to think that there's value in the work I've done that led to it, and that that was rewarded, and recognized, because of the people I've worked with who contributed to the work we've published. But I also recognize that I'm in an area where very talented people are part of the academies, and so I was fortunate enough to be part of that. If you can understand what I'm trying to say, I certainly feel honored to have been awarded, but I certainly recognize that there are many, many talented people who haven't had the fortune I've had. But it doesn't mean they're not as talented if not more talented than I am.

ZIERLER: Howard, a theme of our talk, of course, has been interdisciplinarity. Do you think that there's something unique to fluid dynamics as a leading sub-discipline in this greater trend throughout the sciences in interdisciplinary studies?

STONE: I think what's true is that there are lots of problems where fluid mechanical ideas have an impact. Because I don't know it so well, I do not work myself on climate-related questions, but I do understand the themes of climate research, and these climate models are full of fluid dynamics. That's one of the great challenges of modern day. I can understand the concepts, and there are also chemistry concepts in there, obviously. But it certainly has a lot of fluid mechanical concepts in it. I don't think the people who would study it think of themselves as fluid mechanics people for the most part—some do. There are problems involving water, flooding, wildfires. I'm co-editor with Parviz Moin at Stanford of the Annual Review of Fluid Mechanics, and the range of topics that have fluid themes is enormous. I think it's a great training ground. But to really impact a problem, you have to dig deep in a discipline-specific way. I think you need both. I certainly enjoy learning both, both the discipline-specific areas, and just learning more about fluid mechanics as a field. But I should say before—I can't believe I haven't mentioned it till now—I wouldn't be the kind of person I am or the view I have towards education if it wasn't for my parents, so I should definitely say that. They were immigrants who met in this country, both from Jewish families in Germany who happened to meet when they came to America, and they certainly always valued education. I think somehow that got imprinted a bit somewhere along the way for me, and I certainly value that, and probably nudging me towards engineering—although I didn't appreciate it at the time—were my parents said, "You need a profession so that when you finish your education, you will have an income or whatever."

ZIERLER: In all the research that you've been involved in, where do you think you've had the greatest impact in however you might define that?

STONE: Can I pass on that?

ZIERLER: What about that's given you the most satisfaction?

STONE: Maybe it's transmitting these fluid dynamics ideas to other people this inter…and I'm not alone in this, don't get me wrong, but emphasizing the interplay of theory and experiments, and emphasizing the value of clear communication in trying to do that. I think in some areas, I might be appreciated because I've taken the time to try to be a good teacher. Whether or not I am a good teacher, I don't know, but I've taken the time to try to help other people learn some of these ideas, and I think there's some people that have valued that.

ZIERLER: That's an answer about the transmission of scientific knowledge more than any specific research project, is what you want to emphasize?

STONE: Maybe, because I think a common theme in some of the work we do is this interplay of experiment and theory or modeling. If there is a discipline, it would be in this area of physicochemical hydrodynamics or soft materials where showing how the principles of fluid mechanics can help address other scientific problems in different disciplines.

ZIERLER: Finally, Howard, last question. Looking to the future, in however long a timescale you define a research agenda, or in the kinds of things that your graduate students and postdocs are doing that really set the tone for the group, what are the big projects or research questions you have yet to take on? What do you want to do next?

STONE: I'm interested in learning more about this interplay of different areas of classical physics. The one that comes up that I've mentioned is this interplay of fluid mechanics with elastic materials. There's a big subject of that. Most of us learn about one or the other, and there are a subset of people that sort of work in between. I've enjoyed learning about that, in part, because living systems, soft, living systems have vascular systems. I'm interested in trying to still explore some of these questions. I have a collaboration with two scientists in The Netherlands, Toby Kiers and Tom Shimizu, who study the fungus network that connects plants, and it has active fluid flows in this vascular network, and we're trying to understand that. I still enjoy trying to explore the way fluid mechanics helps understand other problems that come up in nature. From an intellectual point of view, I like understanding the way classical physics ideas intersect, because that comes up in so many natural problems. I'm, again, more interested in problem solving than any specific problem, I think. That's at least what I tell myself.

ZIERLER: There's still more than enough to keep you busy though, is what you're saying?

STONE: You're very kind.

ZIERLER: [laugh] Howard, this has been a great conversation. I want to thank you so much for doing this.

STONE: Thank you.