Droplet impingement and vapor layer formation on hot hydrophobic surfaces

We use pump-probe thermal transport measurements and high speed imaging to study the residence time and heat transfer of small (360 μm diameter) water droplets that bounce from hydrophobic surfaces whose temperature exceeds the boiling point. The structure of the hydrophobic surface is a 10 nm thick fluorocarbon coating on a Si substrate; the Si substrate is also patterned with micron-scale ridges using photolithography to further increase the contact angle. The residence time determined by high-speed imaging is constant at 1ms over the temperature range of our study, 110<T<210°C. Measurements of the thermal conductance of the interface show that the time of intimate contact between liquid water and the hydrophobic surface is reduced by the rapid formation of a vapor layer and reaches a minimum value of 0.025 ms at T>190°C. We tentatively associate this time-scale with a ̃1m s ^-1 velocity of the liquid/vapor/solid contact line. The amount of heat transferred during the impact, normalized by the droplet volume, ranges from 0.028 J mm^-3 to 0.048 J mm^-3 in the temperature range 110<T<210°C. This amount of heat transfer is 1-2% of the latent heat of evaporation.