Colloids in active fluids: simulations of microrheology experiments

Active fluids are suspensions of particles that absorb energy from their surroundings (or from an internal reservoir) in order to do work. They represent a novel type of out-of-equilibrium system and they are found predominantly, even though not solely, in biological contexts. I will present results concerning the simulation of an experiment in which a colloidal probe is pulled through an active nematic fluid and show that the drag on the particle is non-Stokesian (not proportional to its radius). I will explain this in terms of the disruption of orientational order around the probe particle and show how this leads to the prediction of a regime in which a large enough particle in contractile fluid (such as an actomyosin gel) can show negative viscous drag in steady state, which is confirmed in simulations.