Dynamics of water drop-drop impacts on a hydrophobic surface

Abstract

The dynamic behavior of drops impacting a dry surface, thin film, or moving drop have been previously investigated; however, the behavior of falling drops impacting a sessile drop at rest has been explored to a lesser degree. In this work the spreading and receding of a water drop concentrically impacting a sessile water drop of equal volume and a sessile drop with twice the volume of the impacting drop are studied on a Teflon substrate (hydrophobic). Experimental observations are compared with modeling predictions based on conservation of energy considerations. The influence of the Weber number, We, on the spreading of the coalesced drop is the primary focus of present study. High-speed images were captured during drop impact, spreading, and recoil. For experiments at We=219, the impact results in a coalesced drop that spreads to a final diameter without recoil. At lower Weber numbers (We≤141) the coalesced drop spreads to a maximum diameter, then recoils, and then undergoes a second spreading event. These observations indicate that at high Weber numbers more energy is dissipated to the substrate. A semi-empirical model, developed based on energy conservation from the time of impact to the time of maximum spread, predicts the ratio of maximum spread diameter to the initial diameter of the impacting drop, dmax/Di, to approximately 15% of the experimental results for We≥39 and captures the measured trend in increased spreading with Weber number. The maximum deviation between model and experiment occurs at collisions with the lowest Weber number and subsequent smallest and most uncertain experimental spreading ratio.