Flocking and re-orientation transition in the q-state active Potts model.

We study the active q-state Potts model (APM) with q = 4 in two-dimensions. The active particles can change their internal states and undergo a nearest neighbor biased diffusion leading to a flocking model. The flocking dynamics is exploited to probe the liquid-gas phase transition in the APM with an intermediate liquid-gas co-existence phase. We compute the phase diagram for the APM and analyze the effect of the self-propulsion velocity on the particle band formation and its orientation in the co-existence phase, where a novel re-orientation transition from transversal to longitudinal band motion is revealed. We further construct a coarse-grained hydrodynamic descriptions of the model which validates the findings of microscopic simulations.