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 we observe cruising states, while for 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 and are pinned to the wall below . 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.
Highlights
The fact that active particles are inherently in non-equilibrium has stimulated experimental [1, 2, 3, 4], theoretical [5, 6] and numerical [7, 8, 9, 10] research in the last decade
We perform hydrodynamic simulations using the method of multi-particle collision dynamics and a theoretical analysis to study a single squirmer microswimmer at high Peclet number, which moves in a low Reynolds number fluid and under gravity
The decisive factors for the observed motional states are hydrodynamic interactions with the no-slip surface, gravity, and thermal noise, which are usually present in experimental systems
Summary
The fact that active particles are inherently in non-equilibrium has stimulated experimental [1, 2, 3, 4], theoretical [5, 6] and numerical [7, 8, 9, 10] research in the last decade This is true for fluid systems at low Reynolds number, where swimmers on the micron scale are considered, i.e. biological organisms [11, 12] and synthetic particles [13, 14] as well as continuum models thereof [15]. In this article we report on full hydrodynamic simulations of a single squirmer under gravity close to bounding walls and supplement it by a theoretical analysis. We first introduce the main phenomenology observed in our simulations
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