Boiling water without bubbles: that's just our cup of tea

Imagine a specially-engineered surface that could allow liquids to boil without bubbling. This sounds counter-intuitive and, in a way, it is. But consider the following. When a small drop of water is dropped onto a very hot frying pan, it skitters around and takes up to a minute to evaporate. On initial contact, the hot surface vaporises part of the drop and creates an insulating vapour layer between the drop and the hot surface, much like the air gap in a double-glazed window. This vapour layer can only be sustained if the hot surface is above the so-called Leidenfrost temperature. It also acts as an efficient lubricant and can reduce the drag on a hot sphere travelling through water by up to 85%. The Leidenfrost vapour layer also plays an important part in boiling and cooling. If in place of small drops of water in a hot frying pan, we have a hot kettle filled with water, the Leidenfrost vapour layer will collapse when the kettle cools below the Leidenfrost temperature, resulting in an explosion of vapour bubbles as the water makes direct contact with the (still) hot surface. Until recently, this rather violent explosive ending for vapour, reminiscent of the death throes of a star, was thought to be inevitable when a hot surface in contact with water cooled below the Leidenfrost temperature. In a report published yesterday by my colleagues and I in the journal Nature, it was found that by modifying the heating surface to give it nano-scale roughness and a coating to render it highly water repellent---or super-hydrophobic---the Leidenfrost vapour layer can be sustained at all surface temperatures, thereby eliminating the vapour explosion.