Chemotaxis of self-phoretic active particles and bacteria [article]

Oliver Pohl, Technische Universität Berlin, Technische Universität Berlin, Holger Stark
2016
Chemotaxis in systems of active particles is a most interesting topic to study. Typically, these phenomena are encountered in cellular systems. However, recently, motion along chemical gradients in form of diffusiophoresis in systems of activated colloids has been observed. Such out-of-equilibirium systems give rise to novel types of collective behavior, for example, dynamic clustering emerges. Theoretical studies have been mainly concentrated on collective behavior without diffusiophoresis,
more » ... hough such systems are much harder to realize experimentally. We here introduce a model of an assembly of active colloids which incorporates diffusiophoresis. The latter gives rise to interactions between the colloids, which are transmitted by a dynamically evolving chemical field. We show that for Janus particles diffusiophoresis leads not only to translational attraction (or repulsion), but also to an effective torque, which orients the particles towards (or away) from each other. In addition, we take into account that interactions may be screened in crowded regions because the chemical field cannot diffuse freely. In numerical simulations we show that this model reproduces dynamic clustering, whenever particles direct their motion away from each other. We classify this state in dynamic clustering 1 and 2 by means of their cluster size distribution functions. In addition, we observe rapid transitions to so-called collapsed states. By mapping our model to the Keller-Segel equations, we rationalize this phenomenon. A deeper investigation of the collapsed state reveals a pulsating collapsed cluster for certain parameter configurations. Finally, we calculate association and dissociation rates of clusters and show that they serve as indicator for dynamic clustering and its transitions. Many Bacteria perform chemotaxis by actively biasing their particular run-and-tumble random walk, where they increase the run length when swimming up a nutrient gradient. This "classical" chemotaxis strategy [...]
doi:10.14279/depositonce-5409 fatcat:ucftvcsqmjdw7d7okz2b5y2mby