Effect of Stagnation Flow on an Impacting Water Droplet on a Superhydrophobic Surface

One of the relevant applications of this study is related to designing anti-icing surfaces. Supercooled water droplet impact at high Weber number on a wing of airplane is one of the main concerns in aircraft ice accumulation. In order to address this issue, an experimental setup which generates co-flow is designed to mimic the real scenario of droplet impact in practical icing condition. This process is observed using a high-speed camera to capture the correct moment of sliding at 10000 frames per second and 120000 1/s shutter speed. Different air stream velocities are generated by a convergent nozzle with a maximum Mach number of 0.1. Two different cases are considered. First, droplet impact in still air with an impact velocity of 2 m/s is performed as the base case. Then droplet impact accompanied with 10, 18 and 20 m/s air stream velocities having the same impact velocity are conducted. Droplet impact velocity will change either by air flow or gravity. So, it means that an increase in air speed must be accompanied with a decrease in needle height to substrate. It is clearly demonstrated that for Weber number around 140 (e.g. impact velocity equal to 2.1 m/s and droplet size equal to 2.3 mm) in still air there is no droplet break up either in spreading or in receding stages. On the other hand, violent splashing of water droplet impacting with the same Weber number accompanied with 18 and 20 m/s airstream velocities were observed during the recoiling stage.