From tiny particles to dancing droplets: Yale engineers explore the hidden physics of fluids
This story originally appeared in Yale Engineering magazine.
From the time you wake up in the morning, your day is filled with fluid mechanics. There’s the way that toothpaste (a non-Newtonian liquid) flows out onto your toothbrush. That chaotic combining of fluids when you pour cream in your coffee? That’s called turbulent mixing. How the air circulates around you, the clouds form above you, and raindrops splay out on your car’s windshield are all matters of fluid mechanics. It’s a field of research that affects everything from the pharmaceutical industry to predicting the weather to designing motor vehicles and airplanes.
Amir Pahlavan and Bauyrzhan Primkulov, both Mechanical Engineering faculty members, have made fluid mechanics a key focus of their research. The two have collaborated on a number of projects, including those that could advance in-situ mining and geothermal energy.
Postdoctoral associate Mobin Alipour in the Pahlavan lab pours liquid in the darkened lab to reveal the hidden physics of fluid motion and interfaces.
Photos: Tanner Pendleton
Whether it’s within the micron-scale labyrinth of soil or the kilometer-scale swirls of the ocean, fluids carry everything from nutrients and microbes to pollutants, droplets, and microplastics. But in nature, what gets transported often doesn’t simply go with the flow. In Pahlavan’s lab, the unifying question is deceptively simple: Why don’t things go with the flow, and what are the invisible forces steering them? By combining theory, simulations, and visual lab experiments, his team looks for general rules that connect microscopic mechanisms to large-scale outcomes.
A central theme of the lab is that chemical gradients — variations in salt, nutrients, or other dissolved substances — create “landscapes” that can guide motion of tiny particles known as colloids, and bacteria through complex porous environments such as soils, aquifers, and biological tissue. These gradients may be caused by outside sources such as seawater intrusion, irrigation, or contamination. But they can also be generated internally as the environment evolves: when things dissolve or crystallize in water, by microbial metabolism and secretion that reshape local chemistry, and by freeze–thaw cycles that concentrate solutes and redirect flow pathways.
How chemical gradients influence the transport of colloids and bacteria in porous environments, however, has received little attention.
“Colloids are everywhere, from clay particles and microplastics in subsurface environments to synthetic nanoparticles used in drug delivery and environmental remediation,” Pahlavan said. “Bacteria are equally ubiquitous, living in the pores of soils and sediments and even within our bodies; their ability to sense and move along chemical cues can strongly shape how they spread, attach, and form biofilms in these complex environments.”
Not always going with the flow
To isolate these effects in a controlled, measurable way, Pahlavan’s lab builds miniature porous worlds using microfluidics. They develop small chips patterned with networks of obstacles that mimic the winding geometry of real porous media while allowing Pahlavan and his research team to see what’s happening in real time. Within these networks, the lab studies how chemical gradients steer suspended colloids — a process known as diffusiophoresis — and how microorganisms navigate nutrient landscapes via chemotaxis (the way bacteria sense good and bad chemicals in their surroundings and steer themselves accordingly). The goal is to develop ways to predict when chemistry acts as an active driver that determines the direction of particles and cells and where they accumulate.
The same “not going with the flow” question also plays out in far larger scales in the ocean. With tabletop models that feature a very thin layer of saltwater, magnets and a small electric current, the researchers create two-dimensional flows that resemble surface ocean dynamics as seen from above. The motion can be chaotic. It stretches and folds material into long filaments, dramatically amplifying spreading even in the absence of turbulence. Pahlavan’s group uses laboratory experiments and modeling to explore how particles, algae, droplets, and floating objects move in these flows, and how properties such as inertia and shape determine whether material follows currents, concentrates into patches, or separates into distinct pathways.
A particular focus is on objects that can change as they move. Droplets can break up and coalesce, shifting their size distribution in real time and feeding back on how they spread and where they end up. Floating material can also become collective: in the ocean’s marginal ice zone, for example, sea ice can behave like a driven, interacting granular layer — more like a crowd of colliding, jostling pieces than a smooth surface. By treating these systems as coupled problems, the lab aims to identify the mechanisms that set spreading rates, patchiness, and accumulation zones, with relevance to phenomena ranging from microplastic dispersal and algal blooms to ice floe dynamics.
Across porous mazes and ocean-like swirls, the lab’s goal is to uncover the hidden rules that govern the movement of particles, large and small. By linking controlled experiments to theory and simulations, Pahlavan aims to make these movements more predictable, and eventually more controllable across the environments that matter most, from the pores of soil and tissue to the surface of oceans.
As a student in Iran, Pahlavan had been encouraged to go into electrical engineering. But something about mechanical engineering called to him.
“It can expose you to many kinds of problems,” he said. “For me, it was more natural. And it helps that you can visualize things!”
Science that looks like art
Throughout Bauyrzhan Primkulov’s lab in Mason Laboratory are various contraptions that manipulate tiny drops of liquid into countless different states. Drops of water get stretched to their limit, until they inevitably break apart and give way to even tinier droplets shooting in all directions. Elsewhere, drops dance across a vibrating film of liquid, generating dazzling patterns. Slow-motion camerawork captures it all in minute detail.
“You see something, you like it, you’re not sure why. There was something aesthetic about these problems that initially drew me in.
Bauyrzhan Primkulov
It’s all part of Primkulov’s mission to shed light on the many mysteries presented by fluid mechanics. This work has the potential to provide technical solutions to current energy and environmental challenges. In addition to that, the experiments are pretty mind-bending. Primkulov, who grew up in Kazakhstan, said he discovered the beauty of fluid mechanics while a graduate student.
“There was the usual fluid mechanics, but then I took another course on fluid mechanics that had to do with interfacial phenomena, where surface tension is important,” he said. “You would get very, very visually appealing experiments, something similar to art. You see something, you like it, you’re not sure why. There was something aesthetic about these problems that initially drew me in.”
Some of his work has clear practical applications, like his research on in-situ mining, in which a solution is pumped underground to dissolve minerals and bring them to the surface. Others tackle fundamental science, asking fascinating questions along the way.
“I try to keep a balance. I think most problems can be interesting if you spend enough time on them. I try to do projects where I clearly see the application, but I think it’s important to do problems where maybe I don’t see the applications yet, but the problem has an interesting feature you don’t usually see at this scale.”
Sometimes a click pen is all you need
Primkulov said he tends to bounce between experiments and theory, each refining the other until he knows it’s right.
“It always starts in the lab — you see something that you can’t quite explain,” he said. “So then you play around with an experiment, and you start working on the models. You make a guess in your model, then you go to experiment to test it out. Then eventually, you refine the experiment, you refine the model, and then once the two agree, that’s a very good feeling. I’ve been chasing that ever since it happened for the first time.”
Recently, his lab demonstrated how liquid drops can overlap with the uncanny world of quantum mechanics. Specifically, they showed how vibrating droplets can serve as what they call a “hydrodynamic analog” of the Kapitza-Dirac effect, a theory from quantum science in which a beam of particles is diffracted by a “grading” made of a standing wave of light. By shaking a thin film of oil with specific levels of vibrations, oil droplets placed on the pool don’t merge at first. Instead, they bounce and create waves, which then cause them to “walk” across the liquid. Their own waves are guiding these droplets and can occasionally show statistics very similar to those of quantum-scale particles, Primkulov said.
Using a simple click pen, the lab can create precise, tiny droplets that hover, bounce, and "walk" across a vibrating liquid bath.
And what happens if you infuse these droplets with nanoscale particles of iron, and then place them in a magnetic field? His lab is currently working on that also.
In addition to making discoveries like these, Primkulov is making it easier for other labs to make their own. Conventional methods for droplet generation often rely on specialized or costly equipment, which significantly limits accessibility. But Primkulov found an even better way to generate single droplets from a vibrating liquid bath: the humble click pen. With it, they can reliably produce droplets on a stationary bath and then apply them to vibrating baths. By varying the retraction length and pen tip radius, they can control the dimensions of the liquid ligament and access a wide range of droplet sizes. It’s an approach that easily allows them to make droplets with tunable sizes and investigate bouncing droplets for a wide range of experiments.
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Published Date
Jun 4, 2026
