Oblique impact of droplets on heated supherydrophobic surfaces*

Superhydrophobic surfaces have been a topic of much recent interest in fluid dynamics and heat transfer applications. Many studies addressing droplet impact dynamics have focused on the fluid flow physics and thermal transport behavior for drops impacting the surface in the normal direction. However, in practice, the impact of fluid droplets on surfaces occurs across a full range of incident angles leading to different impact dynamics and heat transfer. Experiments were conducted that utilized high-speed IR imaging to measure the average temperature difference between pre- and post-impact droplets impinging on superhydrophobic surfaces. The total heat transfer to an impacting droplet is a function of the surface temperature, surface micro-structure cavity fraction, drop impact Weber number, and the angle at which the droplet impacts. There has been little prior work addressing the impact angle on the overall heat transfer and this is the primary focus of this work. Results show a dramatic reduction in heat transfer as the impact angle increases and as the level of superhydrophobicity of the surface increases. Furthermore, the formation of coherent Worthington jets was observed during the impact process in a narrow range of the influencing parameters.