Collective phenomena in active Brownian particles with feedback controlled interaction rules

Abstract

The formation of groups by animals or other living organisms is a widespread phenomenon in nature. Individuals coordinate their actions to gain an advantage compared to being isolated, for example regarding the protection from predators or the optimisation of foraging. The organisation principle is often poorly understood because it is not feasible to control the interactions between individuals. Therefore, investigating their collective behaviour demands versatile model systems that allow to control the interactions precisely. Due to their outstanding experimental accessibility, active Brownian particles are suitable candidates for this purpose.<br />In this thesis, I present the experimental realisation of programmable interaction rules in a suspension of active Brownian particles with light-activated self-propulsion. Using feedback controlled laser illumination, the propulsion velocity of each particle can be adjusted individually. In addition, asymmetric illumination allows to influence the propulsion direction by active reorientation. With different interaction rules, the performance and versatility of the experimental system is demonstrated.<br />First, the applicability of feedback control is confirmed by navigating single particles towards a target using different control strategies. The efficiency of these strategies is compared regarding possible limitations of resources. Subsequently, collective phenomena in a group of active Brownian particles are investigated with interaction rules inspired by biological systems. Imitating the quorum sensing of bacteria, particles that change their motility as a response to the local density, can form regions with enhanced particle density. Size, density and shape of these regions strongly depend on the specific response of the particles. Furthermore, active Brownian particles organise in cohesive collective states when following an interaction rule which is motivated by the behaviour of social animals. Detailed investigations of the observed rotating swirls, show that they are remarkably robust against internal and external perturbations. In addition, the transition between swirls and unordered swarms is analysed, revealing analogies to a second-order phase transition in thermal equilibrium.<br />These findings highlight the versatility of individually controlled active Brownian particles as a model system. The ability to test interactions under experimental conditions can be expected to be highly relevant for future research of collective behaviour.