Interrupted Motility Induced Phase Separation for aligning active colloids.

Switching on large activity in a rather dense system of active Janus colloids, we observe fast clustering, followed by clusters aggregation towards full phase separation. The phase separation process is however interrupted when large enough clusters start breaking apart. Following the cluster size distribution as a function of time, we identify four successive dynamical regimes. Tracking both the particle positions and orientations, we characterize the structural and alignment ordering present in the growing clusters, and thereby unveil the mechanisms at play in these regimes. In particular, we identify how alignment between the neighboring particles is responsible for the interruption of the full phase separation. This experimental study, which provides the first large-scale observation of phase separation in active colloids, combined with particle scale analysis of the local mechanisms, points at the new physics observed when both alignment and short-range repulsion are present.