Coarse-graining suspensions of active Brownian particles

On a mesoscale, suspensions of locally heated colloidal particles can to some extent be described like ordinary isothermal systems with effective parameters that can be predicted for sufficiently symmetric setups. This still holds, to a lesser degree, for asymmetrically heated ("Janus”) particles that turn into active, self-propelling microswimmers, due to unbalanced thermal gradients. I will discuss informative signatures of the local nonequilibrium, accessible to mesoscale observers. Some increasingly coarse-grained paradigmatic situations that we have recently considered comprise active Brownian heat engines, polarization-density patterns emerging in activity gradients, ensuing active ratcheting, and time-reversal symmetry breaking fluctuations in non-reciprocal field theories.