Spreading behavior of droplets impacting over substrates with varying surface topographies

Droplet interaction with solid surface plays an important role in a number of practical applications such as thermal management systems, steel production, painting, prevention of impurit deposition, and formation of corrosion. In this study, droplet impact on different surface topographies was experimentally investigated. A major part of this work was to devoted to developing supherhydrophobic surfaces using a combination of two conventional manufacturing techniques in order to study the droplet dynamics with the aim of preventing liquid attachement at the wall. Five surfaces were manufactured for which the methods such as mechanical polishing, laser-ablation, anodization, superhydrophobic spray-coating, and combination of laser-ablation and anodization were applied. Deionized water was used as the working liquid. The effects of velocity and surface temperature on spreading dynamics were investigated by impacting single droplets for Weber numbers between 67 and 565. Experiments are performed at 25 °C ambient temperature with a constant droplet temperature of 25 °C, while the effect of surface temperature has been studied for 25 °C and 2 °C. Maximum spreading factor data was obtained and compared with theoretical models and experimental data found in the literature. Through the combination of laser-ablation and anodization methods, superhydrophobicty is obtained with static contact angles similar to that measured on the superhydrophobic coating. Experiments at 25 °C surface temperature show that the droplet impacting on the combined surface had greater maximum spreading factor values than only laser-ablated and anodized surfaces and lower than those determined at the cotaed surface. At low surface temperature, the smallest maximum spreading factor was measured at the substrate with its surface treated by the combined method. The mathematical models found in literature show a good agreement concerning the maximum spreading factor values determined at the polished, anodized and laser-ablated surfaces. However, the maximum spreading factor at both spray-coated and combined surfaces is larger than the model predictions.