Active mixtures interacting with walls and asymmetric obstacles

Abstract

Active matter is composed of a large number of self-propelled particles, with examples encompassing bacterial swarms, animal flocks, and autophoretic colloids. In this thesis, we study a mixture of fast and slow self-propelled particles in two scenarios: (i) in the presence of two parallel flat walls and (ii) in the presence of a regular array of large asymmetric (half-disk shaped) obstacles. For this purpose, 2D simulations of active Brownian particles were carried out. The system has two types of particles, each one characterized by its own self-propulsion speed. To isolate the effects of speed diversity, the system-average self-propulsion speed is kept unvaried as the degree of speed diversity is varied. Due to their persistent motion, particles accumulate around the objects in a phenomenon known as wetting. Stationary segre- gation arises since faster particles are more likely to occupy new available spaces. For degrees of speed diversity ≥ 30%, we observe a transition where the self-propulsion of the slower particles becomes too weak and thus these particles start to accumulate more easily over a layer of faster particles rather than near the wall. For the walls, we find that the segregation process evolves in two stages: a fast dynamics, where the wetting layer grows via aggregation of fast and slow particles at different rates, and a slow dynamics, characterized by the relaxation of the thickness and the composi- tion of the wetting layer towards the stationary state. Also, we extended a kinetic theory previously used for motility-induced phase separation in one-component sys- tems in order to include wetting by active mixtures. With excellent quantitative agreement, our simulations and theory show that, by increasing speed diversity, the wetting layer thickness decreases strongly, whereas its composition is weakly non- monotonic. For asymmetric obstacles, particles traveling from the curved to the flat side of the half-disk obstacle spend less time trapped than in the opposite direction. As a result, directed motion emerges spontaneously. We find that the corresponding rectification current is amplified when the degree of speed diversity is increased. In the active-passive limit, the passive particles still undergo directed motion dragged by the active ones. Due to rectification, segregation profiles are different between the curved and flat sides. Near the obstacle corners, pairs of vortices that further contribute to rectification are observed. Their vorticities also increase with speed diversity. Our results provide useful insights into the behavior of active matter in complex environments.