Why are there bubbles in boiling water

Boil water without bubbles

Thuwal (Saudi Arabia) - If drops of water fall onto a hot stove, they dance back and forth on a tiny cushion of steam. This phenomenon, which Johann Gottlob Leidenfrost discovered in Duisburg over 250 years ago, is now being used by Arab and American researchers to bring water to the boil without the formation of bubbles. Thanks to a highly water-repellent surface, the otherwise everyday bubbling could be avoided. As the scientists report in the journal “Nature”, their discovery could lead to the safer operation of steam cycles, for example in nuclear power plants.

"Heat transport and boiling water occur in many industrial processes," write Ivan Vakarelski from King Abdullah University of Science and Technology in Thuwal and his colleagues from Northwestern University in Evanston. Today, the sometimes dangerous formation of bubbles is avoided through precise operational controls. In the future, finely structured surfaces of tubes and vessels could further increase safety. On the way to a bubble-inhibiting system, the researchers coated a two-centimeter steel ball with a highly water-repellent, so-called superhydrophobic paint. This consisted of special organic substances and finely divided nanoparticles.

Vakarelski and colleagues immersed this steel ball in hot water, heated to up to 400 degrees Celsius. Immediately a thin film of water vapor (left in the picture) formed around the sphere, analogous to the cushion of vapor under the drops dancing on the stove top. Because of the superhydrophobic and nano-structured coating, no bubbles could form on the surface of the sphere. On the other hand, many small steam bubbles quickly developed on untreated steel balls, which rose bubbling through the water (right in the picture).

Extremely water-repellent surfaces stabilize the evaporation phase of water, which is named after Leidenfrost. With other surfaces, this condition quickly changes into the blistering familiar from everyday life. However, since the bubble-inhibiting steam layers have a heat-insulating effect, heating in specially coated kettles could take longer and require more heating energy.