  We simulate an experiment in which a colloidal probe is pulled through an
active nematic fluid. We find that the drag on the particle is non-Stokesian
(not proportional to its radius). Strikingly, a large enough particle in
contractile fluid (such as an actomyosin gel) can show negative viscous drag in
steady state: the particle moves in the opposite direction to the externally
applied force. We explain this, and the qualitative trends seen in our
simulations, in terms of the disruption of orientational order around the probe
particle and the resulting modifications to the active stress.
