Hydrodynamic oscillations and variable swimming speed in squirmers close to repulsive walls

TL;DR

This study uses lattice Boltzmann simulations to explore how hydrodynamics and short-range repulsive forces influence the motion, trapping, and speed of squirmers near walls, revealing complex behaviors like hydrodynamic trapping and oscillations.

Contribution

It demonstrates the critical role of near-field hydrodynamics and repulsive interactions in controlling squirmer dynamics near surfaces, a novel insight for active matter research.

Findings

Hydrodynamic trapping of squirmers at walls due to hydrodynamics and repulsive forces.

Hydrodynamic oscillations observed in pushers near surfaces.

Pusher speed increases near surfaces; pullers exhibit speed transitions based on interaction range.

Abstract

We present a lattice Boltzmann study of the hydrodynamics of a fully resolved squirmer, radius R, confined in a slab of fluid between two no-slip walls. We show that the coupling between hydrodynamics and short-range repulsive interactions between the swimmer and the surface can lead to hydrodynamic trapping of both pushers and pullers at the wall, and to hydrodynamic oscillations in the case of a pusher. We further show that a pusher moves significantly faster when close to a surface than in the bulk, whereas a puller undergoes a transition between fast motion and a dynamical standstill according to the range of the repulsive interaction. Our results critically require near-field hydrodynamics; they further suggest that it should be possible to control density and speed of squirmers at a surface by tuning the range of steric and electrostatic swimmer-wall interactions.