Dynamic Clustering in Active Colloidal Suspensions with Chemical Signaling

In this Letter, we explore experimentally the phase behavior of a dense active suspension of self-propelled colloids. In addition to a solidlike and gaslike phase observed for high and low densities, a novel cluster phase is reported at intermediate densities. This takes the form of a stationary assembly of dense aggregates-resulting from a permanent dynamical merging and separation of active colloids-whose average size grows with activity as a linear function of the self-propelling velocity. While different possible scenarios can be considered to account for these observations-such as a generic velocity weakening instability recently put forward-we show that the experimental results are reproduced mathematically by a chemotactic aggregation mechanism, originally introduced to account for bacterial aggregation and accounting here for diffusiophoretic chemical interaction between colloidal swimmers.