Multi-Ciliated Microswimmers -- Metachronal Coordination and Helical Swimming

TL;DR

This study uses mesoscale hydrodynamics simulations to analyze how metachronal coordination in multi-ciliated microswimmers influences their swimming behavior, revealing dependence on wave parameters and resulting in helical trajectories.

Contribution

It introduces a detailed simulation approach to study the effects of metachronal wave patterns on microswimmer motion and trajectory shape, highlighting the role of cilia coordination.

Findings

Swimmers exhibit smooth motion and spin around their axis.

Chirality induces helical swimming trajectories.

Swimming velocity increases with cilia number, following a sublinear power law.

Abstract

The dynamics and motion of multi-ciliated microswimmers with a spherical body and a small number N (with 5 < N < 60) of cilia with length comparable to the body radius, is investigated by mesoscale hydrodynamics simulations. A metachronal wave is imposed for the cilia beat, for which the wave vector has both a longitudinal and a latitudinal component. The dynamics and motion is characterized by the swimming velocity, its variation over the beat cycle, the spinning velocity around the main body axis, as well as the parameters of the helical trajectory. Our simulation results show that the microswimmer motion strongly depends on the latitudinal wave number and the longitudinal phase lag. The microswimmers are found to swim smoothly and usually spin around their own axis. Chirality of the metachronal beat pattern generically generates helical trajectories. In most cases, the helices are thin and stretched, i.e. the helix radius is about an order of magnitude smaller than the pitch. The rotational diffusion of the microswimmer is significantly smaller than the passive rotational diffusion of the body alone, which indicates that the extended cilia contribute strongly to the hydrodynamic radius. The swimming velocity vswim is found to increase with the cilia number N with a slightly sublinear power law, consistent with the behavior expected from the dependence of the transport velocity of planar cilia arrays on the cilia separation.