Impact of long range repulsive interactions on the stochastic dynamics in ensembles of active and passive particles

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In this experimental project we are going to study the collective behavior in a quasi-two dimensional colloidal ensemble consisting of active and passive particles under the impact of fluctuations both with respect to particle distribution and thermal noise. The active particles are photo-responsive and can generate long-range repulsive interactions, the strength of which can be tuned on demand and over a broad range, from weakly repulsive competing with other attractive particle interactions, to strongly repelling fully governing pattern or phase formation. The origin of the long-range repulsive interactions is diffusioosmotic in nature (light driven diffusioosmotic flow, LDDO) and related to generation of local flow around each active colloid. Active particles can either be fully "dressed" with a radially directed flow field, or rendered Janus-type, self-propelling colloids by partially inactivating their surface. The unique feature of our approach is that the long range mutual repulsion and also self-propulsion can be tuned by convenient external optical stimuli (intensity, irradiation wavelength) such that a broad variety of experimental situations can be realized. Currently, there are some theoretical studies and predictions related to phase separation behaviour of ensembles of active and active/passive particle mixtures that could not yet be captured by experiment, for instance motility induced phase separation. Since we can make the strength of mutual interaction and self-propulsion tuneable and time-dependent, one can significantly extend the phenomenology of non-equilibrium behaviour of colloidal ensembles in experimental study and in particular address the aspect of fluctuations, related to the central question in this project: how does the non-equilibrium, dissipative nature of the flow patterns dictate the particle distribution in mutually interacting colloidal ensembles?