Collective Microswimmer Motility in Complex Environments

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

Anomalous cooling and overcooling of active systems

The phenomenon that a system at a hot temperature cools faster than at a warm temperature, referred to as the Mpemba effect, has been recently realized for trapped colloids. Here, we investigate the cooling and heating process of a self-propelling active colloid using numerical simulations and theoretical calculations with a model that can directly be tested in experiments. Upon cooling the particles' active motion induces a Mpemba effect. Transiently the system can even exhibit smaller temperatures than its final temperature, a surprising phenomenon which we refer to as activity-induced overcooling

Thermisch-oxidatives Alterungsverhalten von hydriertem Nitril-Butadien-Kautschuk

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Maximum in density heterogeneities of active swimmers

Suspensions of unicellular microswimmers such as flagellated bacteria or motile algae can exhibit spontaneous density heterogeneities at large enough concentrations. We introduce a novel model for biological microswimmers that creates the flow field of the corresponding microswimmers, and takes into account the shape anisotropy of the swimmer's body and stroke-averaged flagella. By employing multiparticle collision dynamics, we directly couple the swimmer's dynamics to the fluid's. We characterize the nonequilibrium phase diagram, as the filling fraction and Péclet number are varied, and find density heterogeneities in the distribution of both pullers and pushers, due to hydrodynamic instabilities. We find a maximum degree of clustering at intermediate filling fractions and at large Péclet numbers resulting from a competition of hydrodynamic and steric interactions between the swimmers. We develop an analytical theory that supports these results. This maximum might represent an optimum for the microorganisms' colonization of their environment

Barrier-mediated predator-prey dynamics

The survival chance of a prey chased by a predator depends not only on their relative speeds but importantly also on the local environment they have to face. For example, a wolf chasing a deer might take a long time to cross a river which can quickly be crossed by the deer. Here, we propose a simple predator-prey model for a situation in which both the escaping prey and the chasing predator have to surmount an energetic barrier. Different barrier-assisted states of catching or final escaping are classified and suitable scaling laws separating these two states are derived. We discuss the effect of fluctuations on the catching times and determine states in which catching or escaping is more likely. We further identify trapping or escaping states which are determined by hydrodynamics and chemotactic interactions. Our results are of importance for both microbes and self-propelled unanimate microparticles following each other by non-reciprocal interactions in inhomogeneous landscapes

A new solution for the determination of the generalized couplingcoefficient for piezoelectric systems

Recently, novel damping devices based on shunted piezoceramics have been investigated. Piezoceramics are therefore embedded into the mechanical structure and convert some part of the kinetic vibration energy into electrical energy. Subsequently, this energy is dissipated in the electrical network that is connected at the electrodes of the piezoceramics. The network is designed with the aim to maximize the dissipation, which results in a damping effect on the mechanical system. Alternatively, the converted energy can be stored and utilized to power electronic devices like sensors for health monitoring, called Energy Harvesting. In both cases, the converted energy and the damping performance depend on the so called generalized electromechanical coupling coefficient K. It is therefore crucial to maximize this factor. Besider the piezoelectric material properties, the coupling coefficient also depends on the vibration mode of the piezoceramics. Only for a constant mechanical strain distribution in the whole volume the generalized coupling coefficient K is equal to the material coupling k. In all other cases, K is smaller than k. This publication presents a general derivation of the generalized coupling coefficient K for an arbitrary, uniaxial deformation of the piezoceramics, which is based on the potential energy stored in the piezoceramics. The general result is applied to a piezoelectric bending bimorph and verified by a finite element model