Scientists in Germany have been making nanoparticles inside tiny water droplets that levitate above a hot plate on a layer of their own steam.

Their technique uses the Leidenfrost effect, which is what makes water hover and whizz around when dropped onto a hot surface such as a frying pan. It happens because the part of the drop which hits the pan rapidly vaporises, creating a cushion of steam for the drop to float on.

Mady Elbahri and colleagues at the University of Kiel in Germany examined the temperature, pH and charge conditions inside Leidenfrost drops and found they provided the perfect environment for nanoscale reactions. This video shows gold nanoparticles being synthesised within a Leidenfrost drop of tetrachloroauric acid. Superheating the drop creates a high concentration of hydroxide ions that reduce AuCl₄^- without the need for other reducing agents. The group has also used their Leidenfrost reactors to make nanoporous metal-polymer hybrid foams and to coat nanoparticles onto a three-dimensional substrate.