Post-collision hydrodynamics of droplets on cylindrical bodies of variant convexity and wettability

Abstract

Droplet impact, dynamics, wetting, and spreading behavior on surfaces impose rich and interesting physics, in addition to extensive understanding of processed employing droplets and sprays. The physics and mechanisms become more interesting and insightful when the geometry and wettability of the surface provide additional constraints to the fluid dynamics. Post-impingement morphology and dynamics of water droplets on convex cylindrical surfaces, having diameters similar to that of the droplet, have been explored experimentally. Droplet impact and post-impact feature studies have been conducted on hydrophilic and superhydrophobic (SH) cylindrical surfaces. Effects of the impact Weber number (We) and target-to-drop diameter ratio on the wetting and spreading hydrodynamics have been studied and discussed. The post-impact hydrodynamics have been quantified employing dedicated non-dimensional variables, such as the wetting fraction, the spreading factor, and the non-dimensional film thickness at the north pole of the target. The observations reveal that the wetting fraction and spread factor increase with an increase in the impact We and a decrease in the target-to-drop diameter ratio. An opposite trend is noted for the non-dimensional film thickness at the target’s north pole. It is also deduced that the spread factor is independent of the target wettability, whereas the wetting fraction is remarkably low for SH targets. The lamella dynamics post spreading has also been observed to be a strong function of the wettability, the impact We, and the diameter ratio, and the same has been explained based on wetting and inertial principles. An analytical expression for temporal evolution of film thickness at the north pole of the cylindrical target is derived from first principles. The article also proposes a theoretical model based on energy conservation for predicting the maximum wetting fraction for variant cylindrical targets in terms of the governing We and Capillary number. It is observed that the experimental measurements are in good agreement with the theoretical predictions.