Recently, it has been shown that the force exerted on a probe particle translating through a bath suspension arises from two primary contributions: direct collisions of bath particles with the probe, and indirect interactions, which result from the deformation of the suspension in the fluid surrounding the probe [1]. Notably, only the indirect interactions have analogs to the suspension dynamics found in macrorheology. Although the indirect contributions should dominate as the probe particle size becomes much larger than the bath particle size, direct interactions are still of interest for the limitations they impose on active microrheology in the non-linear regime. In this talk, I will discuss simultaneous measurements of the drag force on a colloidal probe and the corresponding non-equilibrium suspension microstructure in the limit where the bath and probe sizes are similar. Earlier, we showed that force thinning occurs simultaneously with the development of an anisotropic suspension microstructure, and that the force thinning extends to surprisingly large Peclet numbers compared with theory [2]. Nonetheless, I will show that the results are in good agreement with recent simulations [3] and even elucidate the role of near-field hydrodynamic interactions on direct collisions.

[1] T. M. Squires, Langmuir 24, 1147 (2008).

[2] T. M. Squires and J. F. Brady, Phys. Fluids 17, 073101 (2005).

[3] I. C. Carpen and J. F. Brady, J. Rheol. 49, 1483 (2005).