Taxis of Artificial Swimmers in a Spatio-Temporally Modulated Activation Medium

Contrary to microbial taxis, where a tactic response to external stimuli is controlled by complex chemical pathways acting like sensor-actuator loops, taxis of artificial microswimmers is a purely stochastic effect associated with a non-uniform activation of the particles' self-propulsion. We study the tactic response of such swimmers in a spatio-temporally modulated activating medium by means of both numerical and analytical techniques. In the opposite limits of very fast and very slow rotational particle dynamics, we obtain analytic approximations that closely reproduce the numerical description. A swimmer drifts on average either parallel or anti-parallel to the propagation direction of the activating pulses, depending on their speed and width. The drift in line with the pulses is solely determined by the finite persistence length of the active Brownian motion performed by the swimmer, whereas the drift in the opposite direction results from the combination of ballistic and diffusive properties of the swimmer's dynamics.Comment: 19 pages, 6 figures; Entropy (in press

Taxis of Artificial Swimmers in a Spatio-Temporally Modulated Activation Medium

Contrary to microbial taxis, where a tactic response to external stimuli is controlled by complex chemical pathways acting like sensor-actuator loops, taxis of artificial microswimmers is a purely stochastic effect associated with a non-uniform activation of the particles' self-propulsion. We study the tactic response of such swimmers in a spatio-temporally modulated activating medium by means of both numerical and analytical techniques. In the opposite limits of very fast and very slow rotational particle dynamics, we obtain analytic approximations that closely reproduce the numerical description. A swimmer drifts on average either parallel or anti-parallel to the propagation direction of the activating pulses, depending on their speed and width. The drift in line with the pulses is solely determined by the finite persistence length of the active Brownian motion performed by the swimmer, whereas the drift in the opposite direction results from the combination of the ballistic and diffusive properties of the swimmer's dynamics

On the Properties of Self-Thermophoretic Janus Particles: From Hot Brownian Motion to Motility Landscapes

This thesis investigates several phenomena that are associated with (self-)thermophoretic Janus particles with hemispheres made from different materials serving as a paradigm for active propul- sion on the microscale. (i) The dynamics of a single Janus sphere in the external temperature field created by an immobilized heat source is studied. I show that the particle’s angular velocity is solely determined by the temperature profile on the equator between the Janus particle’s hemispheres and their phoretic mobility contrast. (ii) The distinct polarization-density patterns observed for active-particle suspensions in activity landscapes are addressed. The results of my approximate theoretical model agree well with exact numerical and measurement data for a thermophoretic microswimmer, and can serve as a template for more complex applications. The essential physics behind the formal results is robustly captured and elucidated by a schematic two-species “run- and-tumble” model. (iii) I investigate coarse-grained models of suspended self-thermo- phoretic microswimmers. Starting from atomistic molecular dynamics simulations, the coarse-grained de- scription of the fluid in terms of a local molecular temperature field is verified, and effective nonequilibrium temperatures characterizing the particle’s so called hot Brownian motion are mea- sured from simulations. They are theoretically shown to remain relevant for any further spatial coarse-graining towards a hydrodynamic description of the entire suspension as a homogeneous complex fluid.In dieser Arbeit untersuche ich mehrere Phänomene, die im Zusammenhang mit (selbst-)thermo- phoretischen Janusteilchen auftreten. Diese Teilchen bestehen aus zwei Halbkugeln mit unter- schiedlichen Materialeigenschaften und dienen in dieser Arbeit als Musterbeispiel für aktive Fort- bewegung auf der Mikroskala. (i) Die Dynamik eines einzelnen Janusteilchens im externen Temper- aturfeld einer ortsfesten Heizquelle wird untersucht. Es wird gezeigt, dass die Winkelgeschwindigkeit des Teilchens ausschließlich durch das Temperaturprofil am Äquator zwischen den Hemisphären des Janusteilchens und dem Unterschied ihrer phoretischen Mobilitäten bestimmt wird. (ii) Ich befasse mich mit den charakteristischen Polarisations- und Dichteprofilen, die für aktive Teilchen in Aktivitätslandschaften beobachtet werden. Die Ergebnisse meines approximativen theoretis- chen Modells stimmen gut mit exakten numerischen Lösungen und Messdaten für einen ther- mophoretischen Mikroschwimmer überein und können als Vorlage für komplexere Anwendungen dienen. Die wesentliche Physik hinter den formalen Ergebnissen wird durch ein schematisches Zwei-Spezies-“Run-and-Tumble”-Modell erfasst und erklärt. (iii) Ich untersuche Coarse-Graining- Modelle von suspendierten selbst-thermophoretischen Mikroschwimmern. Ausgehend von atom- istischen molekulardynamischen Simulationen wird die grobkörnige (coarse-grained) Beschreibung des Fluids in Form eines lokalen molekularen Temperaturfeldes verifiziert. Anschließend berechne ich effektive Nichtgleichgewichtstemperaturen, die die sogenannte heiße Brownsche Bewegung der Teilchen charakterisieren, und vergleiche diese mit Simulationsdaten. Es wird gezeigt, dass diese effektiven Temperaturen für jede weitere räumliche Vergröberung hin zu einer hydrodynamischen Beschreibung der gesamten Suspension als homogenes komplexes Fluid relevant bleiben