Orientation statistics and effective viscosity of suspensions of elongated particles in simple shear flow

The significance of the particle aspect ratio on the statistics of the particle orientation and effective viscosity of a non-dilute suspension in simple shear flow is discussed and investigated by dynamical simulation. It is argued that, in the absence of an imposed length scale or screening dimension due to bounding surfaces, assumed periodicity, or long-range particle interactions, the concept of rotational diffusivity is not appropriate in the limit of infinite dilution for non-colloidal systems. Numerical simulations are carried out based on an improved implementation of the boundary-element method for particulate Stokes flow, featuring the iterative solution of the master linear system for the particle contour traction and particle linear and angular velocities, based on particle-cloud clustering. The numerical results confirm that the transition from a nearly-ordered to the random state due to particle interactions occurs on a time scale that is consistent with the theoretical estimate, and demonstrate that the particle eccentricity reduces the effective viscosity of the suspension and skews the orientation probability density distribution in the direction of the unperturbed shear flow.