MPIDS Colloquium: Active Brownian Particles: A minimal model for active matter

A striking non-equilibrium phenomenon is the clustering and phase separation of dense, phoretically propelled colloidal particles. The simplest model that reproduces this phase separation in computer simulations is active Brownian particles (ABPs): spherical (discoid) particles that are propelled by a constant force along an orientation that undergoes free rotational diffusion. This model incorporates the two basic physical ingredients of micro-swimmers: volume exclusion and persistence of motion. In the first part I will sketch the theoretical approach to the observed phase separation, whereby the propulsion speed takes the role of an inverse temperature. Indeed, the large-scale behavior of ABPs can be mapped systematically onto an effective free energy. Recently, thermodynamic aspects in the theoretical modeling of active matter have moved into focus with the hope that general principles and universal bounds for autonomously driven matter might be formulated, much in analogy to conventional thermodynamics. In the second part I will briefly describe the perspective stochastic thermodynamics has to offer using active Brownian particles as an illustration.