Low Weber number droplet impact on heated hydrophobic surfaces

Using synchronized high-speed optical and infrared (IR) imaging, we correlate droplet dynamics during low-We impact on heated hydrophobic surfaces to the spatial distribution of the solid-liquid interfacial temperature, heat flux, and the total heat transfer to the droplet. Denoting the droplet diameter and impact velocity as D and v, respectively, based on scaling arguments we find that the total transferred heat Q scales as D^1.25v, which is validated experimentally. A unique feature of low-We droplet impact on non-wetting surfaces is the formation of a sub-millimetric entrapped bubble that forms during receding. The substrate temperature underneath the bubble is significantly higher than for the surrounding droplet due to the low thermal conductivity of air. The local heat flux at the inner droplet-bubble contact line becomes increasingly important as the droplet recedes. Nonetheless, the overall heat transfer is reduced by 5.6% and 7.1% at surface temperatures of 50°C and 65°C, respectively, as the bubble reduces the liquid-solid interface area. On a rough hydrophobic surface, the average heat flux is lower compared to the smooth surface, indicating a larger interfacial thermal resistance on the rough surface.