Hydrodynamic stress on small colloidal aggregates in shear flow using Stokesian dynamics.

The hydrodynamic properties of rigid fractal aggregates have been investigated by considering their motion in shear flow in the Stokesian dynamics approach. Due to the high fluid viscosity and small particle inertia of colloidal systems, the total force and torque applied to the aggregate reach equilibrium values in a short time. Obtaining equilibrating motions for a number of independent samples, one can extract the average hydrodynamic characteristics of the given fractal aggregates. Despite the geometry of these objects being essentially disordered, the average drag-force distributions for aggregates show symmetric patterns. Moreover, these distributions collapse on a single master curve, characteristic of the nature of the aggregates, provided the positions of the particles are rescaled with the geometric radius of gyration. This result can be used to explain the reason why the stress acting on an aggregate and moments of the forces acting on contact points between particles follow power-law behaviors with the aggregate size. Moreover, the values of the exponents can be explained. As a consequence, considering cohesive force typical for colloidal particles, we find that even aggregates smaller than a few dozen particles must experience restructuring when typical shear flow is applied.