Spontaneous demixing of mixed active-passive suspensions.

Understanding the properties of active matter is currently driving a rapid growth in soft- and bio-physics. Some of the most important examples of active matter are at the microscale, and include active colloids and suspensions of microorganisms, both as a simple active fluid and as mixed suspensions of active and passive elements. In these systems, recent work has started to provide a window into new phenomena including activity-induced depletion interactions, phase separation, and the possibility to extract net work from active suspensions. Building on current research in our group exploring the physics of colloid-microswimmer interactions we are interested in understanding how external control of the dynamics of the active component can be used to alter the transport of passive cargo. Here we report on new experiments studying the behaviour of active-passive systems under spatial confinement. We show that the spatial inhomogeneity in swimmers’ distribution and orientation resulting from confinement has a dramatic effect on the spatial distribution of passive particles, with the colloids accumulating either towards the boundaries or towards the bulk of the sample depending on the size of the container. We show that this can be used to induce the system to de-mix spontaneously.