Collision of viscoelastic droplets against hydrophobic surfaces

The spreading of liquid droplets upon impact is observed in many industrial and natural processes such as inkjet printing, spray coating, and firefighting. In this study, we investigate how the rheological properties of a fluid droplet, when impacted onto a partially hydrophobic surface, can influence its spreading and rebound. A single droplet, generated at the tip of a needle, is allowed to detach and fall onto the hydrophobic surface. The viscoelastic fluids used in the study are prepared from water-glycerin mixtures and polyacrylamide. The impact and spreading behavior of the droplet are analyzed for a range of Reynolds, Weber, and Weissenberg numbers using high-speed imaging techniques, highlighting the interplay between inertial, capillary, viscous, and elastic forces. We find that when the elastic effects are significant, there is a reduction in droplet spreading due to the high elongational viscosity of the fluids. This feature can be used to improve droplet retention in vegetation-like surfaces in forest firefighting applications.