An experimental study on the impact of Boger and Newtonian droplets on spherical surfaces

Abstract

This study examined the impact of Boger droplets on spherical surfaces, considering the various properties of liquids and surfaces for the first time. The experimental setup involved spheres with three different diameters composed of hydrophilic and hydrophobic surfaces. The main innovative aspect of the present study lies in the examination of the effects of rheological properties including elasticity, elongational viscosity, and the first normal stress difference on the impact of Boger droplets on solid surfaces via stress analysis. For this purpose, the results of the impact of Boger droplets are compared to the impact of equivalent Newtonian droplets with the same viscosity and surface tension coefficient. The research also explored the influences of Weber number, diameter ratio, and surface wettability on the dynamic behaviors of viscoelastic droplets during the spreading and receding stages. The results suggested that increasing the Weber number increases the maximum spreading factor of the droplets. A smaller diameter ratio required more time to reach this factor. The surface wettability significantly affected the receding phase, while its effect on the spreading stage was minimal. Droplets exhibited more retraction on surfaces with higher contact angles. The Newtonian fluids showed greater spreading and retraction than viscoelastic fluids with the same viscosity, highlighting the influence of fluid elasticity. It is shown that the normal stress caused by extensional viscosity during droplet spreading is significantly higher than the normal stress difference resulting from shear deformation, showing different behaviors between these two types of stress concerning droplets.