Active microrheology in confined systems_

Active microrheology in confined systems: from superdiffusive to giant diffusive fluctuations

Talk by Prof. Gleb Oshanin, LPTMC, University Pierre & Marie Curie, Paris, France

In active microrheology one monitors the response of a host medium to the presence of a tracer particle (TP) pulled by an external force f.

In non-equilibrium statistical mechanics, the same situation is modelled in a thought experiment in quest for the generalised fluctuation-dissipation relations. Recent MD studies of the anisotropic TP dynamics in liquids confined in 3D capillary-like geometries revealed an astonishingly violent response of the medium resulting in a super-diffusive broadening (SDB) of fluctuations of the TP position along the bias, which behaviour did not receive a convincing explanation so far. Via numerical simulations we demonstrate that the same SDB emerges in confined 2D stripe-like geometries, used in lab-on-chip devices, both for dense liquids and strikingly, also for granular fluids with non-elastic collisions. We present a coherent theoretical description based on a “diffusive defect” picture which a) reproduces and explains the observed superdiffusive growth, b) shows that it is specific for confined geometries only, and is absent in unbounded 3D systems, and c) shows that it is a (quite extended in time for dense systems) transient regime, which paves the way to an ultimate diffusive behaviour with a giant diffusion coefficient, orders of magnitude larger than for f =0 (passive microrheology).

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