Gravity-induced dynamics of a squirmer microswimmer in wall proximity

TL;DR

This study combines hydrodynamic simulations and theoretical analysis to explore how gravity influences the behavior of a microswimmer near a wall, revealing diverse motional states depending on propulsion and squirmer type.

Contribution

It provides new insights into the complex interplay of self-propulsion, gravity, and hydrodynamics affecting microswimmer dynamics near surfaces.

Findings

Neutral squirmers float with upright orientation.

Pullers float above the wall when  > _{th}.

Pushers slide and intermittently float near the wall.

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 P\'eclet 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 $\alpha_{\mathrm{th}}$ 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.