On the Impact of High-Speed Drops on Dry and Wetted Surfaces

Abstract

This thesis focuses on the high-speed impact of liquid drops on dry smooth surfaces and thin liquid films in the presence of a strong gas flow using experimental and numerical methods. A flywheel experiment was designed to study such challenging conditions by analyzing the impact with different drop diameters, velocities, liquids and gas mixtures. A sophisticated algorithm was also developed for image analysis and to handle a large number of experimental data. Selected experimental cases were complemented with numerical simulations, which provided insights on the flow around and inside the impacting drop. The results of the drop impact on dry surfaces indicate that the splashing scenario depends strongly on the physical properties of the liquids and not on the surrounding gas or the kinematic impact conditions. It was also demonstrated that the mechanism of gas entrapment at the early stage of the impact is not responsible for the splashing of drops; however, the physical properties of the surrounding gas influenced the spreading lamella and the ejection of secondary droplets. The results demonstrated that the prompt splash is indeed well described by the Rayleigh-Taylor instability in the spreading lamella. The drop impact on wetted surfaces always generates a chaotic and thin corona, which bends and deforms itself during the entire splashing process. Contrary to the splashing on dry surfaces, the break-up of the corona can be attributed to at least three different instabilities: rim-bending, Rayleigh-Taylor, and Rayleigh-Plateau instabilities. Those instabilities generate a host of small secondary droplets in a wide range of sizes and velocities. It was found that the thin ejected corona leads to hole formation and the eventual detachment from its base at the last stage of impact. The thickness of the corona was also estimated using two different theoretical methods. The outcome of high-speed impact on dry and wetted surfaces were quantified in this thesis. The results of the theoretical, experimental, and numerical analyses were combined to describe completely the formation of the secondary droplets. The presented methods allow the prediction of the splashing phenomenon for a drop impacting dry or wetted surfaces at low and high speed.