(123b) Active and Nonlinear Microrheology in the Large-Probe Limit: Direct Vs. Bulk Effects

In passive microrheology, the linear viscoelastic properties of complex fluids is inferred from the Brownian motion of colloidal tracer particles. Active (but gentle) forcing can be also be used to obtain such linear-response information, and less gentle forcing has recently been employed to probe nonlinear material properties. Here we address a variety of issues that arise in the context of this technique -- most generally, what exactly can be measured, and how can such measurements be interpreted? Using a model system (a large colloidal probe pulled through a dilute colloidal suspension), we examine the different sources of stress upon the probe particle, including both direct probe-bath collisions and rheological stresses from the bulk. We discuss the relationship between such measurements and the analogous macro-rheological measurements, drawing analogies with well-known macro-rheological phenomena whenever appropriate. Finally, we discuss simple modifications for active microrheology that would simplify the interpretation, or would allow further rheological information to be obtained.