Birotor hydrodynamic microswimmers: From single to collective behaviour(a)

Abstract

Abstract A microswimmer composed of two oppositely rotating strongly coupled colloids in solution is here termed as birotor and investigated by means of hydrodynamic simulations. The related flow fields, swimmer velocities, and rotational diffusion are controlled by the properties of the fluid, the swimmer geometry, rotation frequency, and also by the substrate friction. Resulting from mutual hydrodynamic and steric interactions, birotor pairs might follow one another, or more frequently rotate around each other. For larger number of interacting swimmers the continuous formation and dissolution of small and rotating aggregates dominates the collective dynamics. The birotors motion is hydrodynamically enhanced at short distances, such that the average velocity of the swimmers shows to increase with density for the investigated range of densities. This is compensated by a decrease of rotational diffusive time, making that the overall effective diffusion decreases with density. These results constitute the first systematic analysis of the birotor microswimmer, which could be also further modified as an easy to manipulate active particle for various potential applications.