(232g) Particle Migration and Torque Evolution in Oscillatory Torsional Flows of Concentrated Suspensions

In this study, we observe particle motion and rheology in oscillatory torsional flow of a monomodal 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, radially outward drift of tracer particles, which is mainly a function of the oscillatory strain amplitude with slight dependence on the frequency of oscillation. We compare measured particle drift velocities to predicted values estimated from the suspension balance model. Simultaneously, we capture the torque evolution. At the largest strain amplitudes and lowest frequencies, the torque increases as the flow evolves, as was previously observed in steady flow, and a local minimum is detected at intermediate strain values, in agreement with other studies of oscillatory flow. In summary, the results suggest the interplay at intermediate oscillation amplitudes between radial particle migration driven by the overall particle stress balance and microstructural rearrangement driven by local self-assembly of the particles into layers.