Understanding the Onset of Oscillatory Swimming in Microchannels

TL;DR

This paper reveals that higher-order hydrodynamic moments cause oscillatory trajectories of micro-swimmers in channels, emphasizing the importance of quadrupole moments and enabling new experimental characterization methods.

Contribution

It demonstrates the role of higher-order hydrodynamic moments in swimmer trajectories, providing a simple model and insights into swimmer behavior in confined geometries.

Findings

Oscillatory trajectories caused by hydrodynamic quadrupole moments.

Behavior occurs far from walls, not requiring lubrication theory.

Model enables trajectory-based characterization of swimmer properties.

Abstract

Self-propelled colloids (swimmers) in confining geometries follow trajectories determined by hydrodynamic interactions with the bounding surfaces. However, typically these interactions are ignored or truncated to lowest order. We demonstrate that higher-order hydrodynamic moments cause rod-like swimmers to follow oscillatory trajectories in quiescent fluid between two parallel plates, using a combination of lattice-Boltzmann simulations and far-field calculations. This behavior occurs even far from the confining walls and does not require lubrication results. We show that a swimmer's hydrodynamic quadrupole moment is crucial to the onset of the oscillatory trajectories. This insight allows us to develop a simple model for the dynamics near the channel center based on these higher hydrodynamic moments, and suggests opportunities for trajectory-based experimental characterization of swimmers' hydrodynamic properties.