Nonequillibrium shape fluctuations and motility of a droplet enclosing active particles*

Ensembles of motile colloids, driven by Quincke electrorotation to roll on a solid surface, have become a popular model of active matter since the direction of the individual colloid motion is not predetermined. We study the collective dynamics of the Quincke rollers in soft confinement by enclosing the rollers inside a liquid droplet sandwiched between two surfaces. We find that the rollers self-organize into a single vortex that uniformly fills the drop. The droplet interface exhibits strong fluctuations with power spectrum consistent with active fluctuation driven by particle-interface collisions. Broken detailed balance confirms the nonequilibrium nature of the shape dynamics. The rollers activity also gives rise to droplet net motion, which is superdiffusive.