Active microrheology of a colloidal suspension in the direct collision limit

Abstract

The single-point active nonlinear microrheology of a colloidal suspension is measured using laser tweezers in the limit that the diameter of the probe particle approaches the diameter of the bath suspension particles. The microviscosity thins as the probe velocity (and corresponding microrheological Péclet number) increases. This thinning behavior correlates with the development of a nonequilibrium suspension microstructure surrounding the probe particle, in which a boundary layer forms on the upstream face of the probe and a wake depleted of bath particles trails the probe. The magnitude of the microviscosities and the thinning behavior are in good agreement with Brownian dynamics simulations reported by Carpen and Brady [J. Rheol. 49, 1483 (2005)]. The microviscosity increment collapses onto a single curve for all volume fractions when scaled by the contact distribution of bath particles around the probe. Scaling the microviscosity increment yields values lower than the dilute theory; furthermore, it plateaus at significantly higher Péclet numbers. The latter effect is corrected by rescaling the Péclet number with the suspension collective diffusion coefficient in place of the bath particle self-diffusivity. The magnitude of the microviscosity increment suggests the theory overestimates the frequency of bath-probe collisions. The presence and role of hydrodynamic interactions and the effect of the soft repulsive potential are discussed.