Experimental study of the impact dynamics of polymeric hollow droplets
The impact dynamics of hollow droplets, while influential in applications such as coating and spraying, remain less explored than their dense counterparts. In particular, the impact dynamics of viscoelastic hollow droplets have yet to be fully explored. This study presents an experimental investigation into the impact of hollow Newtonian (water) and viscoelastic (polymeric solution) droplets on a solid surface at different impact velocities and polymer concentrations. We demonstrate two hallmark features of hollow droplet flattening: the formation of a central counter-jet and the final deposition, both associated with the entrapped air bubble. For Newtonian droplets, the counter-jet exhibits rapid growth and breakup due to capillary instabilities. Introducing polymer additives fundamentally alters this behavior: viscoelasticity affects the counter-jet's height and velocity, delays bubble rupture, and inhibits droplet detachment. Crucially, we observe the emergence of beads-on-a-string structures during filament thinning, a signature of the competition between elastic and capillary forces. By systematically varying the polymer concentration and impact velocity, we identify the conditions under which three distinct outcomes occur: deposition, partial deposition, and detachment. Our results show how inertia, viscosity, capillarity, and elasticity together govern the splashing morphology of hollow non-Newtonian droplets.