(467g) Oscillatory Torsional Flow of Concentrated Suspensions

Torsional flows of monomodal concentrated suspensions between parallel disks were previously considered to maintain uniform particle distributions. However, recent studies extending the period of observation suggest that shear-induced particle migration or other microstructural ordering can occur over long time scales in steady and oscillatory torsional flows. In this study, we observe particle motion and rheology in oscillatory torsional flow of a 0.4 particle volume fraction suspension over a duration of 4800 oscillations. The oscillation amplitude covers the transition range below the minimum strain for the formation of steady-state microstructure and the oscillation frequency is also varied. We detect slow outward drift of tracer particles, typically accompanied by gradual increase in the measured torque. We compare measured particle drift velocities to predicted values estimated from the suspension balance model. The results of this study can also assist in parameter selection for rheological measurements of suspensions in the parallel disk geometry.