A Leidenfrost Thermostat

A simple thermostat based on self-propelled Leidenfrost droplets is proposed and demonstrated. The proof-of-principle device sits on a heated hotplate, which provides the heat, but under dripping water which cools it. Using temperature dependent directionality of droplets on a substructured sawteeth surface, droplets are either discarded or fed into a region with high Leidenfrost temperature and enhanced heat-loss. The system can therefore adjust how much of the droplets' cooling power it uses depending on its own temperature, and this feedback enables it to maintain a constant set temperature and act as a thermostat.