By Kimberly Mann Bruch and Scott Paton, SDSC Communications
A bubble floating on the ocean looks calm and delicate, like a tiny glass dome catching the sunlight. But inside its thin skin, fast, powerful flows of liquid are moving because of tiny chemical differences you can’t see. A recent Princeton University study, using the ACES system at Texas A&M University through the U.S. National Science Foundation’s ACCESS program, uncovered simple rules that explain when these fragile films stretch, thin or suddenly pop.
What Is a Surfactant?
Surfactants are molecules that like to sit at surfaces, such as the boundary between air and water. In the ocean, they come from natural sources like phytoplankton and microbes, as well as from human pollution and runoff. These molecules gather at the water’s surface and form a kind of chemical “skin” on bubbles and waves.
When surfactants are spread unevenly, they pull more strongly in some places than others. This difference in pull, called surface tension, makes water rush from areas with more surfactant to areas with less. Scientists call this motion Marangoni flow, but you can think of it as the surface trying to smooth itself out.
“At first glance, a soap film or sea foam seems gentle,” explained Luc Deike, an associate professor in Princeton’s Mechanical and Aerospace Engineering Department and the High Meadows Environmental Institute. “But when surface tension gradients are strong in low-viscosity liquids like water, the motion turns violent.”
From Slow Syrup to Wild Water
In a thick liquid like syrup, these flows move slowly and are quickly damped out. In water or seawater, however, the liquid is much less viscous, so inertia takes over and the flows can speed up dramatically.
The Princeton team created a mathematical model of a very thin liquid film, like the skin of a bubble, that suddenly gets a patch of extra surfactant. The film then begins to thin and spread outward, and the changes in thickness, speed and surfactant concentration all sharpen together over time.
Shocks in a “Calm” Liquid
Under conditions where inertia dominates, the math that describes this flow starts to look a lot like the equations used for compressible gases. In this analogy, surfactant behaves like pressure and the film thickness behaves like density. That means sharp, wave-like fronts (shocks) can form, where differences in the film suddenly pile up, similar to how a traffic jam forms on a busy highway.
Near the point where the film is about to rupture, its behavior becomes universal, meaning it follows the same patterns no matter what the original surfactant distribution looked like. “You don’t usually associate shocks with liquids at rest,” Deike said. “But the math shows they’re unavoidable without added physics.”
Surfactant properties then step in to shape what actually happens. The exact relationship between surfactant amount and surface tension can either soften these shocks or make them stronger.
How Nature Keeps Things from Blowing Up
In the real world, films don’t develop infinite spikes or infinitely thin regions. Effects such as the film’s bending stiffness and the ability of the liquid to stretch smooth out the extreme behavior predicted by the simplest equations. Instead of razor-sharp shocks, the system develops moving boundaries where conditions change quickly but not infinitely.
Behind these moving boundaries, the surfactant spreads out and becomes more uniform, while ahead of them, the film remains thicker and intact. The way the film thins near these boundaries follows specific, predictable laws. “There’s order emerging from complexity,” Deike said. “Regularization yields universal rules.”
Why This Matters: Ocean and Industry
Ocean waves constantly create bubbles that rise and burst at the surface. When a bubble cap breaks, it throws tiny droplets (sea spray aerosols) into the air. These droplets can help form clouds, move chemicals and organisms from the ocean into the atmosphere and influence both climate and air quality. The surfactants produced by plankton play a major role in how these bubbles form and burst.
Lead author and Princeton graduate student Jun Eshima said U.S. National Science Foundation ACCESS allocations on the ACES supercomputer system were crucial. “It enabled parameter sweeps to verify predictions via simulations,” he noted, referring to the ability to test many different conditions quickly.
The same physics show up in many industries, including:
- Chemical manufacturing, such as emulsions and paints
- Pharmaceuticals, such as aerosols and tiny coated particles
- Food and beverages, like foams and whipped products
- Energy technologies, including sprays and desalination processes
- Semiconductor manufacturing, where ultra-thin liquid films coat surfaces
Understanding how thin films behave helps engineers design more reliable processes, reduce waste when films fail and better control how products form and break up.
Back to That Shimmering Bubble
That glistening bubble you see at the beach is more than just pretty. Inside its film, only microns thick, complicated flows and shock-like fronts are playing out in fractions of a second. The Princeton study, Similarity Solutions and Regularisation of Inertial Surfactant Dynamics, published in the Journal of Fluid Mechanics, shows that even this complex behavior follows clear physical rules.
To learn more about getting your own allocation on ACCESS, visit our Get Started page.
Resource Provider Institution(s): Texas A&M University
Resources Used: ACES
Affiliations: Princeton University
Funding Agency: NSF
Grant or Allocation Number(s): OCE140023
The science story featured here was enabled by the U.S. National Science Foundation’s ACCESS program, which is supported by National Science Foundation grants #2138259, #2138286, #2138307, #2137603, and #2138296.