  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.
