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 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.Graphic abstract
Highlights
Cilia and flagella are the ubiquitous machinery in eukaryotic cells and organisms to generate fluid flow and to propel cells and microorganisms in a fluid environment [1,2]
Examples for the beating dynamics with phase lag χ = −77◦ with latitudinal wave numbers kφ = 0 and kφ = 1 are shown in Figs. 3 and 4, respectively
A metachronal wave is imposed for the cilia beat, for which the wave vector has both a longitudinal, kθ, and a latitudinal, kφ component
Summary
Cilia and flagella are the ubiquitous machinery in eukaryotic cells and organisms to generate fluid flow and to propel cells and microorganisms in a fluid environment [1,2]. While many eukaryotic flagella have the beat pattern of a sinusoidal traveling wave and are usually employed to propel single cells like sperm [3], cilia typically have two distinct phases in their beat cycle—the power and the recovery stroke—and often work together in pairs like in Chlamydomonas reinhardtii [4], or in large cilia carpets. It has been shown that the transport efficiency can be much higher than for perfect beat synchronization, related to the fact that always a fraction of the cilia is in the power stroke, avoiding a forward-backward motion of the fluid. The synchronization of the beat of the two flagella of Chlamydomonas reinhardtii arise from an elastic mechanical coupling at their basal foot [24,25]
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