Danilo Forastiere — Università di Padova #
Probing the hidden structure of polymer fluids via active microrheology #
Recent experiments in active microrheology allow for the experimental investigation of the nonlinear viscosity of complex fluids, such as micellar fluids or polymer solutions.  Depending on the probe's drag speed, multiple regimes may emerge, which still lack a microscopic understanding.  Via Brownian molecular dynamics simulations, we are able to relate the different frictional regimes of the probe particle to changes in the microscopic structure of the polymer fluid, currently inaccessible to microrheological experiments. In particular, we identify hydrodynamic and non-hydrodynamic degrees of freedom of the polymers successively brought out of local equilibrium by the probe, and we suggest a more general phenomenology involving the onset of shear-thickening for long enough polymer chains.  Our findings derive from studying a general mesoscopic model of complex fluids in which a spherical probe dragged by a moving harmonic trap interacts with many elastic chains of soft monomers. Our numerical study of the resulting Brownian dynamics reveals that such a model reproduces the phenomenology observed in the experiments and allows us to identify three different dynamical regimes in terms of the average motion and fluctuations of the probe particle. These three regimes and their typical time scales are then interpreted using simple theoretical considerations based on the dynamics of a few microscopic degrees of freedom of the polymers.