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Wicking, defined as absorption and passive spreading of liquid into a porous medium, has been identified as a key mechanism to enhance the heat transfer and prevent the thermal crisis. Reducing the evaporation time and increasing the Leidenfrost point (LFP) are important for an efficient and safe design of thermal management applications, such as electronics, nuclear, and aeronautics industry. Here, we report the effect of the wicking of superhydrophilic nanowires (NWs) on the droplet vaporization from low temperatures to temperatures above the Leidenfrost transition. By tuning the wicking capability of the surface, we show that the most wickable NW results in the fastest evaporation time (reduction of 82, 76, and 68% compared with a bare surface at, respectively, 51, 69, and 92 °C) and in one of the highest shifts of the LFP of a water droplet (5 μL) in the literature (about 260 °C).

When a liquid droplet is deposited onto a heated surface, evaporation occurs. If the temperature of the surface is sufficiently high, bubbles are released from activated nucleation sites, making the heat transfer more efficient. However, if the temperature of the surface is further increased above the Leidenfrost point, a vapour cushion will form underneath the droplet, deteriorating the heat transfer between the surface and the droplet. In this work, we show that patterned Si nanowires can allow shifting the Leidenfrost temperature while maintaining a minimum droplet evaporation lifetime. In particular, it is observed that the Leidenfrost point is reached when the phase-change time scale compared to the wicking time scale becomes dominant. In this situation, the energy of the lift-off process is not sufficient for allowing the droplet to reach a sufficient height from where the droplet can penetrate in the porous surface.

In this paper, the effect of Si nanowires on the Leidenfrost point on impacting water droplet is presented. In the Leidenfrost regime, the low thermal conductivity of the vapor layer hinders the heat transfer from the hot surface. Nanostructured surfaces can dramatically increase the Leidenfrost temperature improving heat transfer at high temperature. To determine the point of the minimum efficient heat transfer, the droplet lifetime method was employed for both the polished and processed surfaces. The cooling performance was discussed in terms of the droplet evaporation time. The surface with the tallest NWs structure yielded the highest shift in the Leideinfrost point, about 156 % higher than a plain Si surface.

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