Gravity-induced dynamics of a squirmer microswimmer in wall proximity

We perform hydrodynamic simulations using the method of multi-particle collision dynamics and a theoretical analysis to study a single squirmer microswimmer at high Peclet number, which moves in a low Reynolds number fluid and under gravity. The relevant parameters are the ratio alpha of swimming to bulk sedimentation velocity and the squirmer type beta. The combination of self-propulsion, gravitational force, hydrodynamic interactions with the wall, and thermal noise leads to a surprisingly diverse behavior. At alpha > 1 we observe cruising states, while for alpha < 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 beta. While neutral squirmers permanently float above the wall with upright orientation, pullers float for alpha larger than a threshold value alpha(th) and are pinned to the wall below alpha(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.