Self-organization of clusters of spheroidal squirmers*

The “squirmer model” is a classical hydrodynamic model for the motion of interfacially-driven microswimmers, such as self-phoretic active colloids or green algae. Recently (Poehnl and Uspal, Phys. Rev. Fluids, 2023), we found that stable bound pairs can occur for identical squirmers with oblate shape and non-axisymmetric interfacial actuation, as well for shape-heterogeneous squirmers (e.g., a prolate squirmer and an oblate one) with axisymmetric actuation. Here, using analytical theory and numerical calculations, we consider self-organization of small clusters of particles. For instance, we show that oblate squirmers can form an immotile polygonal cluster. In this type of cluster, the centers of the particles are located on the vertices of a polygon, and the particle axes are oriented towards the center of the polygon. Using coarse-grained simulations, we consider how this tendency to cluster affects the collective behavior of many swimmers moving in a two-dimensional layer.