Date: Wednesday 16th of October 2019, 13:00.
Location: INB3305 (Isaac Newton Building, #14).
‘Unsteady flow and liquid migration in shear thickening suspensions’
by John R. Royer, School of Physics and Astronomy, University of Edinburgh, Edinburgh, Scotland.
Our understanding of suspension rheology has been transformed over the past few years, with an emerging consensus that friction-like sliding constraints between contacting particles dominate the rheology at high concentrations. These friction-like contacts form as repulsive forces between particles are overcome, leading to an increase in viscosity with stress, known as shear thickening. While this new understanding of shear thickening can capture idealised simple shear flows, in real industrial settings more complex flow geometries abound.
Here we investigate suspension flow through constrictions, extruding shear thickening cornstarch suspensions through a narrow die. While suspensions at low to moderate concentrations flow homogeneously, at high solids concentrations liquid migration can occur, where the suspension becomes more dilute as it passes through the constriction. During liquid migration, the extruded material maintains a steady concentration, independent of time or the initial concentration. At low flow rates, this critical concentration is controlled by a characteristic shear rate in the die, giving a universal phase diagram for the onset of liquid migration. We can predict this phase boundary by combining the Wyart-Cates model for the shear thickening suspension rheology with the ‘suspension balance’ describing relative solid-liquid flow in response to local stress gradients, thus demonstrating a framework to apply our new understanding of suspension rheology to more complex flow geometries.