&Bullet; physics 14, s65

A new technique for measuring the displacement of tiny particles in a liquid shows a previously unknown hydrodynamic flow pattern that could provide information about the mechanical properties of the interface.

The thermal movements of tiny particles sitting on the surface of a liquid can induce currents. These currents are key to understanding a wide variety of hydrodynamic phenomena, from the self-organization of particles on droplets to the properties of lipid membranes in living cells. Now researchers have found a way to visualize these flow fields for colloids that thermally wobble on the surface of a water film in order to better understand the mechanical properties of this liquid interface [1] .

A small particle on a liquid surface dances when liquid molecules hit it. This dance then drives currents in the liquid far away from the particle. In principle, researchers could see these currents by scattering other particles on the liquid and watching them move. But there is a problem: these additional particles are also thermally triggered by molecules. With many independent thermal dances, it can be difficult to reconstruct the fluid flows.

To get around this problem, Mehdi Molaei, Kathleen Stebe, and their colleagues at the University of Pennsylvania developed a technique that tracks the displacements of each particle in the liquid. They repeatedly analyzed video recordings of micrometer-sized colloids at an air-water interface, each time using a different colloid as a reference point. By computing displacement correlations between approximately


The team never before reconstructed visualized hydrodynamic patterns that swirl at the fluid interface with colloid pairs.

These patterns show that the interface acts like an incompressible 2D layer, behavior the team attributed to traces of contamination from surfactants – contaminants that lower the surface tension of a liquid. While it was already known that surfactants can make the surface of a liquid incompressible, the new experiments indicate that there is no way to eliminate this effect: The team found that even with just under 1000 surfactant molecules per square micrometer – a concentration in which the surfactant was not detectable – the behavior persisted.

–Christopher Crockett

Christopher Crockett is a freelance writer based in Arlington, Virginia.


  1. M. Molaei et al., “Interfacial flow around Brownian colloids”, Phys. Rev. Lett.126, 228003 (2021).

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