Directional propulsion of transition boiling droplets on high-temperature surfaces induced by tilted nanoforests with asymmetric wettability

Directional propulsion of droplets on asymmetric surfaces with nanoscale structures at a high temperature approaching the Leidenfrost point has attracted attention for potential applications in various fields. In this work, tilted nanoforests (TNFs) with superhydrophilicity are prepared by using a mask-free method involving only subsequential oxygen and argon plasma treatment. On a surface composed of such TNFs, droplets exhibit asymmetric spreading at room temperature, however, by heating the surface to a temperature lower than the Leidenfrost point and by impacting a droplet on the surface, directional propulsion of the droplet can be realized in a transition boiling state due to the asymmetric surface tension introduced by the TNFs. In addition, such a surface is also able to achieve uphill propulsion of the droplet against gravity on a slope. Moreover, in this work, a relationship between the dynamic behavior of droplets and the precise nanostructures at high temperatures is established, which is expected to be applied to the fields of microfluidics and heat transfer. Tilted nanoforest structures with a controlled inclination angle are achieved by a mask-free plasma treatment. Due to the asymmetric wettability of TNFs, directional propulsion of droplets is achieved on hot surfaces at temperatures approaching Leidenfrost, and it is even possible to achieve uphill movement of droplets against gravity on a slope inclined at 10° This is prospective for applications in thermal management, microfluidics and energy harvesting. [Display omitted]