Out of Equilibrium Dynamics of Driven Colloids in Viscoelastic Media

Abstract

Viscoelastic media belong to our everyday live and are highly relevant in different fields like medicine, industries or biology. Compared to Newtonian fluids, viscoelastic solutions reveal long stress relaxation times similar to the timescale of the colloidal motion. Already a humble shearing excites a non-equilibrium state, since the bath cannot relax rapidly like in Newtonian fluids.In this thesis, we present straightforward procedures for measuring the response of a viscoelastic medium via active microrheology on colloids using optical tweezers. We find that the viscoelastic bath leaves its nonlinear fingerprints already in equilibrium measurements, e.g. for a freely diffusing colloidal probe or a particle in a static confinement. In particular, we measure a strong dependence of the viscosity experienced by the colloidal probe on the trapping strength of the optical potential. Surprisingly, a mere gentle shear velocity that drives the surrounding fluid just slightly out of equilibrium, opens a variety of novel and astonishing phenomena, like a new oscillatory state appearing on a timescale, which is a multiple of the fluid's structural relaxation time. Further, we discover a new shear rate regime, where the viscosity acting on the colloidal probe is remarkably increased compared to that of a freely diffusing particle. Our findings are supported by a minimal model, which considers overdamped Langevin dynamics in combination with a stochastic Prandtl-Tomlinson model.