Phenomenology of droplet collision hydrodynamics on wetting and nonwetting spheres

Abstract

In this study, the spreading characteristics of water droplets impacted on a solid spherical target have been investigated experimentally and theoretically. Droplet impact and postimpact feature studies have been conducted on hydrophilic and superhydrophobic spherical surfaces. Effects of the impact Weber number and target-to-drop diameter ratio on the spreading hydrodynamics have been discussed. Postcollision dynamics are explored with side and top views of impaction phenomenon using a high speed imaging technique. The morphological outcome of this impingement process has been quantitatively discussed with three geometric parameters, namely, liquid film thickness at the northpole of the target surface, spread factor, and the maximum spread angle. Observations revel that spread factor and the maximum spread angle increases with the decrease in the size of the spherical target, whereas opposite of this is true for liquid film thickness at the north-pole of the target surface. Temporal variations of liquid film thickness at the north pole of the target have been plotted and found in agreement with the theoretical predictions made in the earlier studies. Finally, a mathematical model based on the energy balance principle has been proposed to predict the maximum spread angle on spherical targets. The theoretical values are found in good agreement with the experimental results for a wide range of spherical diameters studied. The findings may have implications toward a better understanding of fluid wetting, spraying, and coating behavior of complex shapes and geometries.