(163b) Direct Bath-Probe Interactions In Active Non-Linear Microrheology

Optically trapped micrometer-sized melamine probe particles are held stationary in a uniformly flowing suspension of fluorescent PMMA particles, enabling the simultaneous measurement of the drag force by video microscopy and the suspension microstructure by confocal microscopy. The "microviscosity" agrees with bulk rheology at low velocities and shows a pronounced thinning behavior as the velocity increases. The thinning behavior is directly correlated with the development of anisotropic bath suspension structure around the probe. Trailing the probe, there is a void or wake with a low density of bath particles which eventually closes due to Brownian motion. At the leading edge of the probe, a high-density boundary layer forms which wraps partially around the sides. While these trends are in agreement with recent theory, at higher Peclet numbers, this boundary layer is compressed and persists further around the probe than expected. Lastly, the relative sizes of the bath particle excluded volume and hydrodynamic radii are manipulated by reducing the ionic strength of the solvent. Altogether, this combination of rich rheological behavior and easily tunable properties is an ideal starting point for understanding the influence of direct interactions between bath and probe particles in active microrheology. The consequences of these interactions for microrheological measurements of non-linear rheology will be discussed.