&Bullet; physics 14, p78

Some micrometer-sized spheres experience unexpected vibrations when placed in an electric field – a movement that engineers could use to build tiny robots.

Engineers have started to develop micrometer-sized particles that move and change shape without any external drive. One technique for producing this so-called “active matter” is to subject dielectric particles in a liquid to a strong electric field, which, as researchers have shown, causes the particles to rotate spontaneously. However, little is known about the dynamics of this system. Now, Kyle Bishop of Columbia University, New York, and colleagues are experimentally observing that dielectric particles suspended in a liquid above a flat electrode oscillate back and forth in ways that popular models do not explain [1] .

In their experiment, the team distributed plastic balls with diameters from 1 to 50

$\mu$

m in a weakly conductive hydrocarbon solution containing a surfactant. They clamped some of the solution between two pieces of conductive glass and observed what was happening with a microscope.

The team observed that some balls began to roll like a golf ball when the electric field exceeded a certain value, which was dependent on the viscosity of the liquid and the polarizability of the ball. This movement came from an unsymmetrical charge distribution over the sphere. As the field increased further, the team observed that other spheres began to swing back and forth like pendulums.

The researchers model the system and predict that the oscillations begin when the so-called boundary layer around a sphere – a layer of liquid in which the equilibrium of the ions and their movement differs from the mass – is as thick as the radius of the sphere. In the case of larger balls, this layer becomes too thin for the balls to allow vibrations.

–Sophia Chen

Sophia Chen is a freelance science writer based in Columbus, Ohio.

## References

1. Z. Zhang et al., “Quincke oscillations of colloids on planar electrodes”, Phys. Rev. Lett.126, 258001 (2021).

## areas of expertise

Soft Matter Fluid Dynamics

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