The Leidenfrost effect was first named after Johann Gotlob Leidenfrost (1715-1794), who carefully described in his Treatise on the Properties of Common Water, published in 1756, how he used polished iron spoons "heated over glowing coals" and noticed that a drop of water falling into the glowing spoon "does not adhere to the spoon, as water is accustomed to do, when touching colder iron." (Quéré, Annu. Rev. Fluid Mech. 45, 2013). It is familiar to Physicists working with liquid nitrogen whose droplets can roll freely across a floor and to Engineers working on poor heat transfer from hot solids into liquids. The Leidenfrost effect is the instantaneous conversion of a layer of water to vapour upon contact with a solid surface that is substantially hotter than the liquid's boiling point. The vapour layer removes the liquid-solid contact usually observed for a droplet resting on a solid surface and imparts both a thermally insulating barrier and a virtually frictionless motion.

After centuries of curiosity and low intensity study, the Leidenfrost effect has burst into life becoming a rapidly growing field of research, initially, as a model of a perfectly (super) hydrophobic surface. Interest has grown as it has been realized that such surfaces may offer significant drag reduction, that surfaces may be micro-structured to create linear motion and that it is possible to modify surface materials/texture to reduce the transition temperatures. In addition, the scope of the Leidenfrost effect has been widened to include vapour layers created by sublimation so that solid-vapour phase transitions, as well as liquid-vapour phase transitions can be understood using similar ideas. Largely, this recent focus has remained on scientific understanding rather than engineering applications.

In 2015 we published a proof-of-concept in Nature Communications (vol. 6, 2015) - a Leidenfrost Engine - which was both a mechanical engine achieving rotation and the first ever demonstration of a sublimation-based heat engine. This was based on the idea of hot turbine-like substrates allowing vapour to be created and directed, such that rotational motion of discs of dry ice, droplets of water and solid discs-coupled by surface tension to rotating droplets of water was achieved. Once rotation had been achieved, we demonstrated that a small voltage could be generated.

This proposal explores the new concept of a Leidenfrost Engine based on substrates with turbine shaped surface patterns, and will use two types of phase changes: i) thin film boiling (liquid-vapour phase transition), and ii) carbon dioxide sublimation (solid-vapour phase transition). We aim to investigate i) a range of surface texture designs to create effective Leidenfrost turbine surfaces, ii) the use of liquids and solids as "fuels" and "working substances" and iii) designs for batch and continuous mode operation. We aim to investigate both small-scale designs, for use where there is high surface area to volume ratio and friction is a dominant concern, and at larger scales, where gravity is a concern and thus levitation by the vapour is energetically costly. We will therefore integrate controlled-levitation configuration (CLC) designs at small scales and fixed-bearing configuration (FBC) designs at larger scales into engine prototypes. By doing so, we expect this project to establish clear design principles for heat engines based on thin-film boiling and sublimation, thereby translating recent scientific advances into engineering possibilities.