  We investigate the response of a dense monodisperse quasi-two-dimensional
(q2D) colloid suspension when a particle is dragged by a constant velocity
optical trap. Consistent with microrheological studies of other geometries, the
perturbation induces a leading density wave and trailing wake, and we use
Stokesian Dynamics (SD) simulations to parse direct colloid-colloid and
hydrodynamic interactions. We go on to analyze the underlying individual
particle-particle collisions in the experimental images. The displacements of
particles form chains reminiscent of stress propagation in sheared granular
materials. From these data, we can reconstruct steady-state dipolar flow
patterns that were predicted for dilute suspensions and previously observed in
granular analogs to our system. The decay of this field differs, however, from
point Stokeslet calculations, indicating that the finite size of the colloids
is important. Moreover, there is a pronounced angular dependence that
corresponds to the surrounding colloid structure, which evolves in response to
the perturbation. Put together, our results show that the response of the
complex fluid is highly anisotropic owing to the fact that the effects of the
perturbation propagate through the structured medium via chains of
colloid-colloid collisions.
