Mohammad Reza Daneshvar Garmroodi

Viscoelastic Hollow Droplets

Hollow droplets matter in applications such as coating and spraying, but their impact has been studied far less than that of dense droplets, especially for non-Newtonian fluids. This experimental work, with M.M. Nasiri, looks at the impact of hollow Newtonian (water) and viscoelastic (polymer solution) droplets on a solid surface.

Impact of hollow viscoelastic droplets and the regime map in the Weber-Deborah plane

Hollow droplet flattening has two characteristic features: a central counter-jet forms, and the droplet deposits on the surface. Both come from the rupture of an entrapped air bubble. For Newtonian impacts, the counter-jet grows quickly and breaks up through capillary instabilities. Adding polymer changes this behaviour. Viscoelasticity lowers the counter-jet’s height and velocity through increased viscous dissipation, delays bubble rupture, and prevents the droplet from detaching. We also see beads-on-a-string structures as the filament thins, a sign of competition between elastic and capillary forces.

By varying the polymer concentration and impact velocity, we built a regime map (deposition, partial deposition, rebound) in the Weber–Deborah number plane. The results show how inertia, viscosity, capillarity, and elasticity together set the splashing morphology of hollow non-Newtonian droplets.

  • M.M. Nasiri, M.R. Daneshvar Garmroodi, D.C. Vadillo, and M. Tembely. Experimental study of the impact of a hollow polymer droplet. Submitted to Physics of Fluids.