Spontaneous shock waves in pulse-stimulated flocks of acitive particles

Active matter demonstrates complex spatiotemporal self-organization not accessible at equilibrium and the emergence of collective behavior. Fluids com- prised of microscopic Quincke rollers represent a popular realization of synthetic active matter. Temporal activity modulations, realized by modulated external electric fields, have been recently suggested as an effective tool to expand the variety and complexity of accessible dynamic states in active ensembles. Here, we report on the emergence of shock wave patterns composed of coherently moving particles energized by a pulsed electric field. The shock waves emerge spontaneously and move faster than the average particle speed. Combining experiments, theory, and simulations, we demonstrate that the shock waves originate from intermittent spontaneous vortex cores due to a vortex meandering instability. They occur when the rollers' translational and rota- tional decoherence times, regulated by the electric pulse durations, become comparable. The phenomenon does not rely on the presence of confinement, and multiple shock waves continuously arise and vanish in the system. Our findings highlight the importance of the interaction timescales in the emergence of dynamic patterns under temporally modulated energy injection. The results may stimulate design strategies for reconfigurable self-assembled active architectures.