Freezing droplet ejection by spring-like elastic pillars

Abstract

Preventing water droplet accretion on surfaces is fundamentally interesting and practically important. Water droplets at room temperature can spontaneously detach from surfaces through texture design or coalescence-induced surface-to-kinetic energy transformation. However, under freezing conditions, these strategies become ineffective owing to the stronger droplet–surface interaction and the lack of an energy transformation pathway. Leveraging water volume expansion during freezing, we report a structured elastic surface with spring-like pillars and wetting contrast that renders the spontaneous ejection of freezing water droplets, regardless of their impacting locations. The spring-like pillars can store the work done by the seconds-long volume expansion of freezing droplets as elastic energy and then rapidly release it as kinetic energy within milliseconds. The three-orders-of-magnitude reduction in timescales leads to sufficient kinetic energy to drive freezing droplet ejection. We develop a theoretical model to elucidate the factors determining the successful onset of this phenomenon. Our design is potentially scalable in manufacturing through a numbering-up strategy, opening up applications in deicing, soft robotics and power generation.