Spontaneous shock waves in pulse-stimulated Quincke rollers*

Microscopic Quincke rollers – colloids suspended in a weak electrolyte and energized by an electric field – are a popular realization of synthetic active matter: interacting self-propelled particles. The pulsating electric field generates a multitude of novel patterns not observed in the system stimulated by a constant field. Shock waves emerge spontaneously in local high-density regions and move faster than the average particle speed. The shock waves occur when the roller's translational and rotational decoherence times become comparable. In turn, this time ratio is controlled by the electric pulse duration. The experiment is supported by the computational modeling highlighting the role of the particle collisions and hydrodynamic flows. Our results provide insight into the design of reconfigurable active matter systems.