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 a > 1 we observe cruising states, while for a < 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 a th and are pinned to the wall below a 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.

Gravity induced formation of spinners and polar order of spherical microswimmers on a surface

“…Generally, these results show good agreement with the simulations using multi particle collision dynamics (MPCD) method [53]. The tendency of the pushers to float further away from the wall comparing to pullers can be captured by far-field hydrodynamic effects [53] and qualitatively understood by considering the swimming mechanism. Pushers create a stronger flows behind the particle while with pullers the flow is more pronounced on the top hemisphere.…”

Collective dynamics in a monolayer of squirmers confined to a boundary by gravity

“…In this article we address a monolayer of squirmers that forms under strong gravity at the bottom surface of the simulation box by performing parallelized simulations with up to 10 8 fluid particles and up to several thousand squirmers. In contrast to our previous work [42,46] where the squirmers can leave the bottom surface to swim upwards, in this article gravity is so strong that they are constrained to the bottom surface. Here, the squirmers either point upwards or tilt against the surface normal so that they move along the surface.…”

“…Since they are tilted against the wall normal, they move in the horizontal plane and their collective motion initiates swarming. The tilted orientation is expected for pushers at heights between the validity of lubrication theory and far-field approximation [41,42]. As we will discuss…”

“…In recent articles we focussed on single microswimmers * jan-timm.kuhr@tu-berlin.de † holger.stark@tu-berlin.de in moderate gravitational fields [42] and on their collective sedimentation [46]. For our studies we used squirmers as model microswimmers [47][48][49][50], the swimmer type of which can be continuously tuned from pushers over neutral swimmers to pullers.…”

Frequency-dependent higher-order Stokes singularities near a planar elastic boundary: Implications for the hydrodynamics of an active microswimmer near an elastic interface

“…Transport properties of active agents near a no-slip rigid planar wall reveal various interesting features [66][67][68][69][70][71][72][73][74][75][76][77][78][79][80][81][82][83] , including their escape from the wall, a stationary hovering state, or gliding along the boundary maintaining a constant orientation during their navigation. Interestingly, flagellated bacteria display circular swimming trajectories close to surfaces as a consequence of hydrodynamic couplings 84 .…”