Impact of shear-thinning and yield-stress drops on solid substrates.

The behaviour of shear-thinning and viscoplastic fluid drops impacting on solid substrates as compared with that of Newtonian drops is studied experimentally by means of high-speed imaging. In particular, the investigation focuses on the morphological aspects of drops after inertial spreading. While the impact morphology of drops of shear-thinning fluids turns out to be qualitatively similar to that of Newtonian fluids, viscoplastic drops can exhibit central drop peaks at the end of inertial spreading. The influence of yield-stress magnitude on drop impact behaviour is qualitatively established by measuring the size of these central drop peaks. The peaks indicate that drop deformation during impact is localized: within a threshold radius, shear-stress effects will not be large enough in magnitude to overcome yield-stress effects, and therefore viscoplastic fluids within this region will not deform from the drop shape prior to impact.