Experiments on adding a surfactant to water drops boiling on a hot surface

The effect of adding a surfactant (sodium dodecyl sulphate) to droplets boiling on a hot stainless steel surface was studied. Experiments were done using solutions containing 100 ppm and 1000 ppm by weight of surfactant and the results compared with those for droplets of pure water. Surface temperature was varied from 60 degrees C to 340 degrees C. Droplet impact and evaporation was photographed using both video and 35 mm cameras. Addition of the surfactant significantly reduced lifetimes of droplets in a state of evaporation or nucleate boiling. For surface temperatures below those required to initiate nucleate boiling, the principal effect of the surfactant was to reduce liquid-solid contact angle, increasing the surface area wetted by the drop. At higher surface temperatures, the surfactant promoted vapour bubble nucleation and foaming in the liquid, greatly enhancing heat transfer. Increasing surfactant concentration reduced the Leidenfrost temperature, above which droplets were levitated above the surface on a thin film of their own vapour. The surfactant had no effect on evaporation time of droplets in film boiling.