  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.
