Active microrheology of colloidal suspensions: simulation and microstructural theory

Accelerated Stokesian Dynamics (ASD) simulation and a microstructural theory are applied to study structure and the viscosity of hard-sphere Brownian suspensions in active microrheology (MR).We consider moderate to dense suspensions, from near to far from equilibrium conditions.The theory explicitly considers many-body hydrodynamic interactions (HIs) in active MR, and is compared with ASD.Two conditions of moving the probe with constant force (CF) and constant velocity (CV) are considered.The structure is quantified using the probability distribution of colloidal particles around the probe, g(r), which is computed as a solution to the pair Smoluchowski equation (SE) for 0.2<\phi<0.50, and a range of Peclet numbers (Pe), describing the ratio of external force on the probe to thermal forces.Results of ASD and theory demonstrate that a wake zone depleted of bath particles behind the moving probe forms at large Pe, while a boundary-layer accumulation develops upstream.The wake length saturates at Pe>>1 for CF while it continuously grows in CV.This contrast in behavior is related to the dispersion in the motion of the probe under CF conditions, while CV motion has no dispersion.This effect is incorporated in the theory as a force-induced hydrodynamic diffusion flux in the pair SE. We also demonstrate that, despite this difference of structure in CF and CV, g(r) near the probe is set by Pe, for both CF and CV resulting in similar values for their viscosity.Using the theory, the structural anisotropy and Brownian viscosity near equilibrium are shown to be quantitatively similar in both CF and CV motions, which is in contrast with the dilute theory which predict distortions and Brownian viscosities twice as large in CV, relative to CF.This difference arises due to the many-body interactions associated with the equilibrium structure in the moderate to dense regime.