Cold Leidenfrost Droplet Jumping on Hot Microstructured Surfaces

It is well known that the Leidenfrost effect, namely the levitation and trampolining of liquid drops on hot solids, occurs only if the surface temperature exceeds a critical Leidenfrost point to initiate the transition from the nucleate boiling to the film boiling regime. Here we report that the levitation and prompt jumping of a Wenzel-state water drop can be achieved way below the Leidendrost point by tuning the vapor bubble growth modes using micro-pillars. In our design, the inclusion of micropillars effectively penetrates the droplet base and increases the effective thickness of the thermal boundary layer. At lower superheat conditions, as the pillar height increases, the thickening of the thermal boundary layer ensures that the vapor bubble expands rapidly in an inertia-controlled mode to cover the entire droplet base. The explosive momentum associated with the violent bubble expansion ignites the levitation and jumping-off of an originally pinned Wenzel-state drop in only 1.33 ms. We demonstrate that the ability to tune bubble dynamics in the “cold Leidenfrost region” with surface topography paves a new path for the facile removal of liquid drops on hot structured surfaces in a controlled manner and inspires potential applications in anti-fouling, self-cleaning and spray cooling.