Activity waves in condensed excitable phases of Quincke rollers.

Traveling waves are universal in excitable systems; yet, the microscopic dynamics of wave propagation is inaccessible in conventional excitable systems. Here, we show that active colloids of Quincke rollers driven by a periodic electric field can form condensed excitable phases. Distinct from existing excitable media, condensed excitable colloids can be tuned reversibly between active liquids and active crystals in which two distinct waves can be excited, respectively. In active liquids, waves propagate by splitting and cross over each other, like sound waves, in collision. In active crystals, waves annihilate or converge, like shock waves, in collision. We show that the microscopic dynamics of sound waves is dominated by electrostatic repulsions while the dynamics of shock waves is encoded with a local density-dependent memory of propulsion. The condensed excitable colloids with tunable and controllable dynamics offer unexplored opportunities for the study of nonlinear phenomena.