Studying the Squishy Stuff: A Conversation with Sujit Datta

Sujit Datta's eyes light up and he smiles when he talks about "squishy" things like mucus and other polymeric fluids. The professor of chemical engineering, bioengineering, and biophysics at Caltech studies how bacteria, fluids, and other soft and pliable materials move and behave in the complex environments where they are typically found—from soils and sediments to biological tissues and gels. Datta's group, lovingly nicknamed "The Squishy Lab," uses a combination of experiments, theoretical modeling, and computer simulations to better understand the fundamental principles governing these systems and their applications.

Datta completed his undergraduate work in mathematics and physics at the University of Pennsylvania, where he also completed a master's degree in physics. He then obtained his PhD in physics at Harvard University with a focus on fluid dynamics and instabilities in soft and disordered media such as porous underground rocks and chemical microcapsules.

When Datta joined the Caltech faculty in 2024, it was a homecoming of sorts. He had most recently been at Princeton University, starting in 2017 as an assistant professor; in 2023, he was promoted to associate professor and director of graduate studies of chemical and biological engineering. But prior to that, he was a postdoctoral scholar in chemical engineering at Caltech, studying biophysical processes in the gut in the lab of Rustem Ismagilov, the Ethel Wilson Bowles and Robert Bowles Professor of Chemistry and Chemical Engineering; Merkin Institute Professor; and director of the Jacobs Institute for Molecular Engineering for Medicine.

Datta's training in a variety of disciplines is apparent in his current work, which relies on concepts and methodologies from fields ranging from microscopy and materials science to chemical dynamics and polymer physics. His group's findings are relevant in diverse areas ranging from medicine and public health to biotechnology, energy, and sustainability.

Datta was recently named chief editor of the journal Reviews of Modern Physics. He has also received numerous awards and honors including the American Institute of Chemical Engineers' Allan P. Colburn and 35 Under 35 Awards, several honors from the American Physical Society; the American Chemical Society's Unilever Award; and a National Science Foundation CAREER Award.

We sat down with Datta outside the Red Door Café on the Caltech campus to ask him about his work and what is so special about all this squishy stuff.

First, can you tell us about the overarching themes of your work and why you are so fascinated by the physics of squishy things?

Soft materials, or "squishy" things, are fascinating because they're ubiquitous, yet they can behave in counterintuitive ways. Take something as familiar as toothpaste. We typically learn about materials being solids, liquids, or gases. So, is toothpaste a solid or a liquid? The answer is both. It flows like a liquid when you squeeze it out of the tube, but it holds its shape like a solid when you don't. This behavior reflects toothpaste's microscopic composition: It is a mixture of tiny mineral particles and spaghetti-like molecules called polymers, components that are easily deformed and not strongly bonded together. Because of all the many weak interactions between these microscopic components, toothpaste exists in a middle ground between typical solids and liquids. It can transition between acting like one or the other with the slightest squeeze.

What really drives my research is the recognition that for decades, most studies of squishy things have focused on them in controlled, idealized laboratory settings. We study bacteria in test tubes, we test polymers in simple geometries, we examine gels in isolation. But in the real world—whether it's bacterial biofilms growing in the thick mucus in our lungs or polymer solutions being pumped through complex pore networks in underground rock formations to remove trapped contaminants—these materials typically exist in complex environments. My group focuses on trying to understand how environmental factors change the behavior of squishy materials and how these materials in turn change their environments.

Can you provide an example of the kind of problem that fascinates you?

One of our recent studies examined how bacteria grow in polymeric solutions like mucus. I got interested in this because of its connection to cystic fibrosis, where the mucus in the lungs is thicker, more concentrated. Cystic fibrosis patients often die because of infections that arise in that more concentrated mucus. But most lab studies of bacteria focus on their behavior in simpler polymer-free liquid cultures. So we said, "OK, let's start studying how bacteria grow in those kinds of mucus to see if we can get some clues as to why this is the case."

What we found is that in mucus, bacteria unexpectedly grow into unusual cable-like structures, eventually knitting themselves together to form something like a living gel. This is not at all what we see when they grow in other types of liquid. In regular liquid, cells grow, eventually they divide into two other cells, and then they separate from each other and just diffuse away. And that process repeats, leading to a dispersion of disconnected cells. In mucus, the cells still grow and divide, but instead of separating and diffusing away, we found they remain stuck to each other, end to end. And as that process continues, they grow into these beautiful long cables that help them stick together.

Your work involves not only experimentation but also simulations and theory. Can you talk about how those different parts of your work interact?

Typically, guided by some kind of hunch or curiosity, we design very careful experiments to visualize things that we couldn't see before. And that often leads to some kind of unexpected discovery because nature is infinitely surprising and way more interesting than anything we could ever imagine.

So when we see something cool in our experiments, we want to explain why it happened. That's where theory and simulations come in. We draw on ideas from all sorts of different fields—from biological physics, polymer physics, statistical physics, fluid dynamics, physical chemistry, whatever's necessary. Guided by clues from the experiments, we use these ideas to try to develop theoretical models that have the minimal essential ingredients to recapitulate the phenomena that we see in the experiments. And if we are successful, then we can make additional predictions that we can also test. It's a little feedback loop.

A lot of the time, our first idea is wrong. And so then we go back to the drawing board and ask, "What did we miss?" We put that in, test predictions, and continue. Once we get to a point where we feel like we've actually captured the essential ingredients, then we have a mathematical model that can hopefully help generalize what we found more broadly. That's the approach we take in our research.

Can you give me some other "greatest hits" from your lab?

Oh yeah, I can give you a bunch. I love everything that we do—it's hard to pick. But I'll give you a couple. A few years ago, we asked a very simple question: How do bacteria grow? Not on a 2D surface as we typically study in the lab, but in a 3D environment, like many natural bacterial habitats.

Over the years, we developed an experimental system to be able to visualize and study bacterial behavior in 3D. We can even 3D print microbial communities and do all sorts of crazy stuff with them.

We used the system to simply watch bacteria growing in 3D. And what we found is, unlike growth on a 2D surface, bacterial colonies grow into rough shapes that kind of look like broccoli. That's something that's been seen in various natural manifestations of bacteria, but people haven't been able to explain why. Guided by the experiments, we were able to come up with a simple model to explain why this happens. It reflects a fundamental feature of cell growth in 3D, which is when a colony of cells gets to be big enough, the cells on the outside consume the nutrients that are getting in from the surrounding environment, causing the cells on the inside to become starved. Because growth is just localized to a thin, surface layer, random fluctuations in growth dominate and give rise to this weird broccoli shape. And that was something that we were able to capture using a simple model. And with this new understanding, we have a way to quantitatively frame additional scientific questions. For example, we're continuing to follow up on how these growth patterns change in communities of different cell types and what the physiological implications are.

Another example comes from an entirely different focus in my group, which is fluid dynamics and transport processes in nonliving systems. People use polymer solutions in many industrial applications, and they pump them through porous media—porous rocks, or soil, or filters. Back in the 1960s, people noticed that when you pump one of these solutions through a porous medium, its viscosity goes up—it thickens. It's like the faster you pump them, the harder it gets to pump them, which is unexpected. So, that had been a puzzle for a long time.

So, we designed careful experiments to directly visualize the flow inside porous media, and we were surprised to see that these fluids actually created something that looks like turbulence. They created chaotic flows—even though these are conditions where you would never expect turbulence to arise. We developed a very simple model to explain that observation, tested it, and iterated. It turns out that all the sloshing around in the turbulent-like flow causes a lot more energy loss and an increase in fluid viscosity. And using our final model, we were able to come up with predictions for a given polymer solution and a given porous medium. We could predict what the viscosity increase would look like quantitatively. So, in addition to revealing something fundamental about fluid dynamics in complex spaces, our study provided guidelines for practitioners who use these kinds of fluids in real-life applications.

We like to ask if you have any hobbies outside of your work.

I do! Back in the day, I was a competitive kickboxer. Those days are over, but I still take an active interest in running and fitness. Exploring Los Angeles with my 6-year-old is one of my biggest hobbies. And I really enjoy cooking. Everything I do during the day is very precision oriented, so cooking is where I let the chaos reign.