Swarming of Self-propelled Particles on the Surface of a Thin Liquid Film

Pototsky A, Thiele U, Stark H

Abstract

We consider a colony of self-propelled particles (swimmers) in a thin liquid film resting on a solid plate with deformable liquid-gas interface. Individual particles swim along the surface of the film predominantly in circles and interact via a short range alignment and longer-range anti-alignment. The local surface tension of the liquid-gas interface is altered by the local density of swimmers due to the soluto-Marangoni effect. Without the addition of swimmers, the flat film surface is linearly stable. We show that a finite wave length instability of the homogeneous and isotropic state can be induced by the carrier film for certain values of the rotational diffusivity and a nonzero rotation frequency of the circular motion of swimmers. In the nonlinear regime we find square arrays of vortices, stripe-like density states and holes developing in the film.