Prof. Jaakko Timonen, Aalto University, Physics Department

Active particles are self-propulsive objects that can interact with each other and with their environment – leading to formation of various fascinating out-of-equilibrium collective states. Examples of active particles include both biological ones such as flocking birds, schooling fish and swarming microbes and also synthetic ones based on various physical and chemical propulsion mechanisms. In this talk, I will discuss our recent experiments and results on a model 2D system consisting of confined self-propulsive colloidal particles (Quincke rollers) under a uniform in-plane biasing force. The system can be driven from a solid-like (crystalline) phase to a gas-like phase by varying the ratio between the propulsive force and the biasing force. Between these two limits, a dynamic equilibrium is formed. The equilibrium is partially analogous to the sedimentation-diffusion equilibrium of passive particles, but also shows some features that are not present in the classic sedimentation-diffusion equilibrium.