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Abstract

This thesis deals with the collective motility of microswimmers in complex environments.
We study the motility of a single alga in complex environments, the hydrodynamic
interactions between microswimmers, the collective effects of the run-reverseflick
swimming strategy, and the statistical effects of an active Brownian particle
exhibiting two motility stages.
We investigate the swimming behavior of the green alga Chlamydomonas reinhardtii
in confinement and find an increased probability of the cell swimming close
to the confining wall. We discovered that the near-wall swimming probability scales
with the local wall curvature. The model that we propose, consisting of an asymmetric
dumbbell, describes the near-wall swimming accurately and does not require any
fitting parameter. In fact, we found that the important ingredient to the curvatureguided
navigation is the torque stemming from the asymmetry of the organism.
Hydrodynamic interactions between microswimmers can also play an important
role in their collective behavior. To investigate the effects of hydrodynamic interactions
we propose a new model based on an asymmetric dumbbell that takes into
account the hydrodynamic flow fields of puller- or pusher-type microswimmers. We
explore the corresponding nonequilibrium phase diagram and find density heterogeneities
in the configuration of swimmers. In fact, we find a maximum heterogeneity
at intermediate filling fractions and high Péclet number. Using simulations with only
hydrodynamic and only steric interactions between the swimmers we show that the
maximum in heterogeneities of swimmers stems from a competition of hydrodynamic
and steric interactions. This result is supported by an analytical theory that we
propose. Importantly, this maximum represents an optimum for microswimmers’ colonization
of their environment.
Bacteria have different swimming strategies for finding nutrition. Escherichia coli
bacteria follow a run and tumble strategy, whereas Vibrio alginolyticus bacteria have
a run-reverse-flick pattern. We study the collective effects of the run-reverse-flick
strategy from a theoretical point of view using molecular dynamics simulations and
analytical theory. We present the collective diffusion coefficient of the system and find
using both approaches that there is maximum in collective diffusion at a forward-tobackward
runtime ratio of 1.2. Intriguingly this is the same runtime ratio that was
found experimentally for Vibrio alginolyticus.
We study the statistical effects of a microswimmer that can switch from a highly
motile state to a low motility state. By solving the underlying Fokker-Planck equation
we find the mean square displacement as well as the intermediate scattering
function analytically, which we verify using Brownian dynamics simulations. We find
an interesting subdiffusive behavior of the mean square displacement and point out
implications for experimental systems. The intermediate scattering function that we
find shows non-ergodic effects that resemble the properties of a supercooled liquid.