Michiel Hermes, School of Physics & Astronomy, The University of Edinburgh, UK. Professor Itai Cohen, host

Seminar Title: Shear thickening of dense suspensions and the transition from collodial to granular rheology

Abstract: Shear thickening happens in many suspension systems and is more often a problem e.g. during extrusion or pipe flow than that that it is useful. Although it is well characterized it is not well understood, often hydrodynamics are mentioned but no quantitative theory has yet emerged that can predict the flow properties of hard spheres. I will discuss measurements on the shear thickening of corn starch as well as model colloidal systems and compare them with a recent model by Wyart and Cates. This model describes shear thickening by the formation of frictional contacts between particles and the transition from a frictionless to a frictional flow curve. We have extended this model to fit our data with a minimal set of parameters. I will explain how the need to counter Van der Waals forces results in shear thickening for all hard particles and why shear thickening is often followed by shear thinning. Another aspect of the Wyart and Cates model is that it predicts sigmoidal flow curves and discontinuous shear thickening between flowing states. The inherent instability of these sigmoidal flow curves prohibits their direct measurement. However, we have measured signatures of sigmoidal flow curves as well as discontinuous sheer thickening between flowing states, exactly as predicted.