Hydrodynamic interaction of microswimmers near a wall

Abstract

The hydrodynamics of an archetypal low-Reynolds number swimmer, called “squirmer”, near a wall has been numerically studied. For a single squirmer, depending on the swimming mechanism, three different modes are distinguished: (a) the squirmer escaping from the wall, (b) the squirmer swimming along the wall at a constant distance and orientation angle, and (c) the squirmer swimming near the wall in a periodic trajectory. The role of inertial effects on the near-wall motion of the squirmer is quantified. The dynamics of multiple squirmers swimming between two walls is found to be very different from a single squirmer. Near-wall accumulation of squirmers are observed. At a relatively small concentration c = 0.1, around 60 – 80% of the squirmers are accumulated near the walls and attraction of pushers and pullers towards the wall is stronger than neutral squirmers. Near-wall squirmers orient normal to the wall, while in the bulk region, the squirmers are mostly oriented parallel to the wall. At a high concentration, c = 0.4, the percentage of the near-wall squirmers is around 40%. The orientation angle of squirmers in the bulk region is more uniformly distributed at high concentrations. In the near-wall region, pullers repel each other, while pushers are attracted to each other and form clusters.

Microswimmers near surfaces

“…The squirmer model has also been employed to study the effects of the hydrodynamic near field on the swimming motion near surfaces [44,45].…”

Dimensionality matters in the collective behaviour of active emulsions

“…Accordingly, numerical and analytical simulations report mostly on two-dimensional (2D) systems: either rigorous 2D geometries [12][13][14][15], or the change in rotational diffusion properties in pure Brownian particles with varying degrees of freedom [16], or zconfined quasi-2D disks or rods with coupled hydrodynamic interactions [17][18][19][20][21]14], or swimmers confined to two dimensions interacting by three-dimensional hydrodynamic fields [22]. In the case of full three-dimensional (3D) systems, numerical simulations either exclude hydrodynamic interactions [23,24] or are restricted to small system sizes [25]. In experimental studies, various artificial systems have been reported on, each showing interesting individual features.…”

Microalgae Scatter off Solid Surfaces by Hydrodynamic and Contact Forces

“…One of the most basic types of interaction is the scattering off a solid plane. Bacteria and other microswimmers with rearmounted flagella ("pusher" type) are well known to accumulate spontaneously on planar surfaces [12], a phenomenon that has been equally well explained by theories based on either purely steric [7] or hydrodynamic [6,13] interactions. New experiments are finally prising these two effects apart, with results in clear support of the latter [14,15].…”

“…Qualitatively, m ≠ 1 signals an interaction. Theoretical studies based on far-field hydrodynamics for puller microswimmers skimming off planar and spherical surfaces [13,[21][22][23] predict consistently a repulsive reorientation of the microorganism's trajectory. Indeed, this can be directly observed from the angular deflection βðθ in Þ measured in our experiments (Fig.…”