New Engineering Concepts from Phase Transitions: A Leidenfrost Engine

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.

1) Knowledge and Techniques
Our Nature Communications article ("A sublimation heat engine", 2015) was the first heat engine demonstration using a solid-vapour phase transition and so our proposal is at Technology Readiness Levels 1-3. It focuses on micro-scale controlled-levitation and larger-scale fixed bearing configurations, and research underpinning long term translation to industrial TRLs. The project will establish design recipes for new types of heat engines, develop techniques of potential use within the space/energy sectors, and complement existing approaches to microscale systems and energy harvesting.

(2) Economic Impact
A key priority for the UK is high-value and specialist manufacturing. Long-term relevance may relate to extreme/resource constrained environments (e.g. space), and energy harvesting (e.g. at macro- or microscale). A recent NASA report highlighted that transporting fuels/materials into space in bulk is impracticable and so use of local resources is likely. One common space resource is ices, including water and carbon dioxide (e.g. Mars, various moons, asteroids and comets) and ammonia (e.g. Europa), and these are present alongside extreme temperature differences. Our proposal has long term potential as an alternative to solar voltaic and thermoelectric generation of electricity, as an accessible resource which could be developed for large-scale engines. In harvesting ambient energy, industry reports on-demand and off-grid usage is likely to grow from $131.4M in 2012 to $4.2B in 2019. Growth is driven by improvements in energy storage, miniaturisation of electronics, distributed sensors, and a need for low power devices, which do not have to be tethered to electrical outlets. Controlled-levitation in our micro-scale engine research may overcome frictional losses due to high surface area-to-volume ratios.

Intellectual property will be protected for UK benefit by a Collaborative Research Agreement, Invention Disclosures, annual IPR reviews with transfer to industry supported by a Business Development Manager using patents and licences. An industrialist will organise IP workshops and act as an industry translator. As appropriate, demonstrator or industrial R&D projects may be initiated. The University partners have spin-out/incubation facilities and the SMC (Edinburgh) can provide access to fabrication of prototypes. As an early TRL project we have identified specific Industry networks for engagement via talks/other activities at their regular/annual industry days/boards (including, HEXAG, PIN, ESA, KTN Energy Harvesting Group in the UK and I2CNER in Japan). Our Pathways to Impact lists networks and contacts which will be part of a press and industry communications plan. SMC marketing activities, attendance at trade shows and an industry day (month 36) will benefit industrial awareness.

(3) People Pipeline and Public Engagement
The UK skills base would benefit from the training of two multidisciplinary postdoctoral researchers having skills relevant to high value manufacturing via work in (i) heat and mass transfer, (ii) measurement and instrumentation, (iii) test-rig construction and operation, (iv) microfabrication techniques and (v) computational modelling. They will receive public communication training (The Royal Society courses) and take part in our Natures Raincoats (www.naturesraincoats.com) Outreach to enhance their skills, including at national exhibitions (e.g. British Science Festival). Undergraduate summer student placements with the HEFCE-funded "Think Physics" team will develop experiments for schools, assist Reece Foundation Summer Schools and create a "Kit-in-a-Kase" (portable outreach demonstrator experiment) to promote interest and develop a pipeline of future PhD students. General public/industry will have access to results via University repositories, the Natures Raincoats website with its "Work with Us" industry section and popular articles (e.g. "The Conversation").