Drop impact of yield-stress fluids

The normal impact of a drop of yield-stress fluid oil a flat rigid surface is investigated experimentally. Using different model fluids (polymer microgels, clay Suspensions) and impacted surfaces (partially wettable, super-hydrophobic), we find a rich variety of impact regimes from irreversible viscoplastic coating to giant elastic spreading and recoil. A minimal model of inertial spreading, taking into account all elasto-viscoplastic rheology, allows explaining in a single framework the different regimes and scaling laws. In addition, semi-quantitative predictions for the spread factor are obtained when the measured rheological parameters of the fluid (elasticity, yield stress, viscosity) are injected into the model. Our Study offers a means to probe the short-time rheology of yield-stress fluids and highlights the role of elasticity oil the unsteady hydrodynamics of these complex fluids. Movies are available with the online version of the paper (go to journals.cambridge.org/flm).