Small amplitude active oscillatory microrheology of a colloidal suspension

We measure the microviscosity of a colloidal suspension using active, oscillatory laser tweezer microrheology. Our results are compared to a new theoretical model that explicitly accounts for different bath and probe particle sizes and the three independent contributions to the stress: direct interactions (collisions) between the probe and suspension particles; indirect interactions between suspension particles; and the Einstein contribution of the suspension particles. Notably, direct and indirect interactions give rise to two characteristic and highly separated dimensionless frequencies or Peclet numbers as the probe diameter becomes large relative to the average suspension particle diameter, but are identical in the limit that the diameters are equal. The experimental microviscosities are consistent with indirect interactions dominating the direct bath-probe collisions in the large probe limit. This enables a reinterpretation of previous drag microrheology experiments in the large probe limit, further confirming the promise that microrheology can, by actively driving the probe particle, be adapted to measure the nonlinear rheology of complex fluids.