We’re excited to share our latest publication in Nature Communications, where we investigate the dynamics of mixed active-passive systems. In these systems, the passive particles are buffeted around by the active components like swimming microorganisms or synthetic active particles. These suspensions are fascinating both at a fundamental level (how can we describe/prescribe the average behaviour of the passive particles?) and -possibly- for future technological applications (directed transport at the microscopic scale). Here we show that confining an active-passive system leads to a non-uniform distribution of the passive species in a predictable way. We then use confinement to induce the mixed system to spontaneously un-mix and separate out the passive components! This is Steve’s first paper, in collaboration with Raphaël Jeanneret (LPENS Paris, France) and Idan Tuval (IMEDEA UIB-CSIC, Spain).