Surface Coating Tames Explosive Boiling

A highly water-repellent silicon-based coating wards off the effects of a violent boiling process that can occur when hot solids contact water, according to a study published in Nature (DOI: 10.1038/nature​11418). The finding may lead to chemical treatments for equipment used with water in high-temperature settings, such as nuclear power reactors. The familiar way in which water droplets dance across the surface of a hot iron or frying pan results from a levitating vapor film that remains stable when the surface temperature is above a critical value. Ivan U. Vakarelski of King Abdullah University of Science & Technology, in Saudi Arabia; Neelesh A. Patankar of Northwestern University; and coworkers used high-speed photography to monitor boiling processes on steel balls chemically treated to make their surfaces vary from hydrophilic to superhydrophobic. In one case, they found that immersing 20-mm hydrophilic balls heated to more than 400 °C in water caused the water to boil at the steel surface gently in a nearly bubble-free manner known as film boiling (shown, left). As the surface temperature cooled to 275 °C, the protective vapor layer collapsed, leading to an explosive transition to the “nucleate boiling” regime (right). A textured superhydrophobic coating completely suppresses the violent transition, they report.