(102e) Microviscosity Thinning in Colloidal Suspensions

In this talk, I will discuss the single-point active non-linear microrheology of colloidal suspensions, with a focus on identifying the correct Peclet number for these measurements. The microviscosity of a suspension of buoyancy matched colloidal PMMA particles is characterized by measuring the drag force exerted on colloidal micro-probes as they are held by laser tweezers in a uniform flow. The microviscosity thins as the probe velocity (and corresponding microrheological Peclet number) increases. Consistent with theory [1], this thinning behavior correlates with the development of a non-equilibrium 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 microviscosity increment collapses onto a single curve for all volume fractions when scaled by the contact distribution of bath particles. However, the scaled microviscosity increment plateaus at significantly higher Peclet numbers than expected. This can be understood by rescaling the Peclet number to account for the bath suspension effective volume fraction by the suspension collective diffusion coefficient in place of the bath particle self-diffusivity.

[1] T. M. Squires and J. F. Brady. Phys. Fluids, 17:073101, 2005.