Microrheology of Suspensions: Oscillations in Viscosity of Sheared Suspensions of Uniform Rigid Rods

Abstract

Previously developed theory predicts that when a dilute suspension of monodisperse axisymmetric rigid particles is subjected to a simple shear flow, assuming no particle interaction and negligible Brownian motion, the orientation distribution function undergoes undamped oscillation of frequency twice that a particle rotation about the vorticity axis. Thus, the instantaneous rheological properties of the suspension, such as the intrinsic viscosity which is obtained as averages with respect of orientation, should also oscillate with time. Experiments on model suspensions confirm these oscillations which, however, turn out to be damped, presumably as a result of: 1) a small spread in particle shape; 2) particle interactions; 3) rotary Brownian motion; and 4) nonuniformity of shear flow.

In all cases the rotating particles came to an equilibrium distribution of orientations and of the associated rheological properties.

Experiments on the viscosity oscillation due to the distribution of orientations in suspensions of nearly monodisperse rigid rods subjected to steady shear flow are discribed. The significance of the particle concentration, particle axis ratio, initial particle orientation and shear rate are shown and discussed.