Condensed Matter Seminar: Drop impact: complex fluids under extreme stress

Complex fluids exhibit a variety of exotic flow behaviours under high stresses, such as shear thickening and shear jamming. Rheology is a powerful tool to characterise these flow behaviours over the bulk of the fluid. However, this technique is limited in its ability to probe fluid behaviour in a spatially resolved way. Here, I will show how we can utilize ultrahigh-speed imaging and the free-surface geometry in drop impact as a new tool for studying the flow of dense colloidal suspensions.  In addition to observing Newtonian-like spreading and bulk shear jamming, we observe the transition between these regimes in the form of localized patches of jammed suspension in the spreading drop. This system offers a unique lens with which to study shear-thickening fluids, allowing us to obtain flow information in a spatially-localized manner, so that we can observe coexisting solid and liquid phases. Furthermore, we capture shear jamming as it occurs via a solidification front traveling from the impact point, and show that the speed of this front is set by how far the impact conditions are beyond the shear thickening transition.