From Thermal Energy to Kinetic Energy : Droplet Motion Triggered by the Leidenfrost Effect

When a liquid is dropped on a surface significantly hotter than the liquid's boiling point, a vapor film forms beneath the droplet creating an insulation layer sufficient enough to prevent the droplet from rapidly boiling. This phenomenon is known as the Leidenfrost effect, and enables droplets to survive for up to several minutes before fully evaporating. Solids are similarly able to levitate due to sublimation. Furthermore, a liquid droplet placed on a heated flat surface moves randomly, but on a ratcheted substrate, will self-propel and move unidirectionally along the ratchets. Such a system with no other external energy fields applied is designated a Leidenfrost self-propulsion device, first introduced by Linke et al. Given the ability of such an arrangement to effectively convert thermal energy into kinetic energy, numerous studies have subsequently attempted to understand and refine the control of motion of the levitated droplets/solids. This review addresses the fundamental understanding of this “heat-to-motion” mechanism, where the main focus is conversion of thermal energy into kinetic energy through the unique Leidenfrost self-propulsion mechanism. Potential applications of Leidenfrost self-propulsion devices are also discussed, including a brief outlook for the future of this research field.