Residence Time and Heat Transfer When Water Droplets Hit a Scalding Surface

We study, using pump-probe optical methods with a time resolution of 1 ms, heat transfer when a series of water droplets impact a smooth surface whose temperature exceeds the boiling point. The volume of the individual water droplets is ≈10 nl, the time between droplets is ≈0.3 ms, and the number of water droplets in the series of droplets is 3, 20, or 100. In the temperature range 100 °C < T < 150 °C, our measurements of the heat transfer, and the residence time of water in contact with the surface, show that nearly all of the dispensed water vaporizes, but more rapidly, the higher the temperature. At higher temperatures, 150 °C < T < 220 °C, droplet shattering plays an increasingly important role in limiting heat transfer and, as a result, the volume of water evaporated and residence time decrease strongly with increasing temperature.