Activity Waves in Condensed Phases of Quincke Rollers

TL;DR

This paper demonstrates that Quincke roller colloids can form active liquids and crystals exhibiting wave phenomena similar to sound and shock waves, revealing new insights into self-organization in active matter.

Contribution

It introduces a new active colloidal system where waves are excited in condensed phases, showing distinct wave behaviors in liquids and crystals, and links microscopic dynamics to wave propagation.

Findings

Waves propagate antiparallel to density gradients in active liquids.

Shock waves in active crystals have sharp fronts and collide like classical shock waves.

Microscopic dynamics are dominated by electrostatic repulsions and density-dependent memory.

Abstract

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.