Heterogeneous solvent dissipation coupled with particle rearrangement in shear-thinning non-Brownian suspensions.

Dense non-Brownian suspensions exhibit significant shear thinning, although a comprehensive understanding of the full scope of this phenomenon remains elusive. This study numerically reveals intimate heterogenous coupled dynamics between many-body particle motions and solvent hydrodynamics in shear-thinning non-Brownian suspensions. In our simulation systems, we do not account for frictional contact forces, reflecting experimental conditions under low shear rates where shear thinning occurs, while hydrodynamic interactions are directly incorporated using the Smoothed Profile Method. We demonstrate the spatially correlated viscous dissipation and particle motions; they share the same characteristic length, which decreases with increasing shear rate. We further show that, at lower shear rates, significant particle density changes are induced against the incompressibility of the solvent, suggesting the cooperative creation and annihilation of gaps and flow channels. We discuss that hydrodynamic interactions may substantially restrict particle rearrangements even in highly dense suspensions, influencing the quantitative aspects of macroscopic rheology.

Microrheology of active suspensions.

We study the microrheology of active suspensions through direct hydrodynamic simulations using model pusher-like microswimmers. We demonstrate that the friction coefficient of a probe particle is notably reduced by hydrodynamic interactions (HIs) among a moving probe and the swimmers. When a swimmer approaches a probe from the rear (front) side, the repulsive HIs between them are weakened (intensified), which results in a slight front-rear asymmetry in swimmer orientation distribution around the probe, creating a significant additional net driving force acting on the probe from the rear side. The present drag-reduction mechanism qualitatively differs from that of the viscosity-reduction observed in sheared bulk systems and depends on probing details. This study provides insights into our fundamental knowledge of hydrodynamic effects in active suspensions and serves as a practical example illuminating distinctions between micro- and macrorheology measurements.