Dynamic behavior of droplet impact on micro-nano structures with extreme wetting properties

Extreme wetting surfaces represented by superhydrophilic and superhydrophobic surfaces have important applications in heat transfer engineering such as boiling and condensation. The droplet impact behavior is helpful to deepen the understanding of extreme wetting characteristics and guide practical engineering applications. In this study, superhydrophilic and superhydrophobic micro-nano structures were fabricated by micro embossing and subsequent chemical treatment, and the droplet impact dynamics of microstructures, nanostructures, and micro-nano structures were analyzed using water and alcohol. Alcohol has the lower surface tension and wets the fabricated surfaces more easily than water. Among all fabricated structures, micro-nano multiscale structures exhibit the most extreme droplet impact dynamics. Whether it is water or alcohol droplets, the superhydrophilic micro-nano structures show the best spreading properties, and the maximum spreading factor (the ratio of the droplet wetting diameter to the initial diameter) of water is 134.1 % higher than the flat surface, which is also higher than both the microstructures and the nanostructures. Water droplets are more likely to bounce after impacting the superhydrophobic micro-nano structures than flat and nanostructures. However, due to the pinning effect of the larger-scale microstructures, the droplets impact on micro-nano structures would break and splash while the Weber number exceeds 43.41. The high-temperature impact dynamics show that the superhydrophobic micro-nano structures has excellent high-temperature resistance and can fully bounce within the tested range. The superior bouncing properties indicates the excellent condensation potential of the superhydrophobic micro-nano structures. The superhydrophilic micro-nano structures will be beneficial to enhance boiling heat transfer engineering applications, and the boiling heat transfer of droplets on the superhydrophilic micro-nano structures is the most severe during the boiling temperature range, and the drying time after alcohol impact is similar to 63.4 % shorter than that of the superhydrophobic micro-nano structures.