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 $\alpha \gt 1$ we observe cruising states, while for $\alpha \lt 1$ 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 ${\alpha }_{\mathrm{th}}$ and are pinned to the wall below ${\alpha }_{\mathrm{th}}$ . 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.

Keywords

• microswimmer dynamics
• low-Reynolds-number flows
• swimming under gravity
• hydrodynamic wall interactions
• 47.63.Gd
• 47.63.mf