  Wave-exciting is a universal phenomenon in physical and biological excitable
systems. Here we show that colloidal systems of Quincke rollers which are
driven periodically can condense into active liquids and active crystals, in
which waves can be excited. In active liquids, the waves propagate antiparallel
to local density gradients via the splitting of dense bands, and cross over
each other in collision as sound waves do. The waves in active crystals have a
sharp front like that of shock waves, and propagate parallel to local density
gradients. The shock waves annihilate or converge as they collide. Detailed
investigations on microscopic dynamics reveal that in sound waves, the dynamics
of rollers is dominated by electrostatic repulsions; in shock waves, the
dynamics is encoded with a density-dependent collective memory. These findings
demonstrate a realization of excitable colloidal systems with tunable dynamics.
This is of great interests in exploring the principles of self-organization and
the fabrication of active functional materials.
