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Emergent structures and dynamics in suspensions of active colloids
Active fluids, such as suspensions of self-propelled particles or of cytoskeletal filaments and molecular motors, are a fascinating example of soft matter displaying non-trivial collective behaviours which represent issues in non-equilibrium statistical physics. Moreover, the recent development of techniques to assemble miniaturized devices has led to an ever-growing interest for micro- and nano-scale engines that can perform autonomous motion (”microrobots”), as, for instance, self-phoretic colloids, for which the propulsion is induced by the generation of a chemical species in a reaction (locally) catalyzed at the particle surface. I will show results from numerical simulations of suspensions of active colloids, described as resolved finite size particles, interacting with a scalar field according to a coarse-grained model. The numerical implementation has been tested against exact results on the chemotactic behaviour of an isolated particle as well as on the self-propulsion mechanism. The study on suspensions of several particles suggests the existence of a kind of non-equilibrium phase transition from a jammed state to the formation of aggregates, at changing a parameter (dubbed phoretic mobility) accounting for the particle-”fuel” chemical affinity). I will stress how to single out the effect of dynamical correlations among particles arising from hydrodynamics, with respect the quorum-sensing-like coupling due to the concentration field, on the geometry of colloidal cluesters as well as on the dymamics of the jammed state.