Gravity-induced dynamics of a squirmer microswimmer in wall proximity

Abstract

We perform hydrodynamic simulations using the method of multi-particle collision dynamics and a theoretical analysis to study a single squirmer microswimmer at high Péclet number, which moves in a low Reynolds number fluid and under gravity. The relevant parameters are the ratio $α$ of swimming to bulk sedimentation velocity and the squirmer type $β$. The combination of self-propulsion, gravitational force, hydrodynamic interactions with the wall, and thermal noise leads to a surprisingly diverse behavior. At $α> 1$ we observe cruising states, while for $α<1$ the squirmer resides close to the bottom wall with the motional state determined by stable fixed points in height and orientation. They strongly depend on the squirmer type $β$. While neutral squirmers permanently float above the wall with upright orientation, pullers float for $α$ larger than a threshold value $α_{\mathrm{th}}$ and are pinned to the wall below $α_{\mathrm{th}}$. In contrast, pushers slide along the wall at lower heights, from which thermal orientational fluctuations drive them into a recurrent floating state with upright orientation, where they remain on the timescale of orientational persistence.