Dynamic behavior and maximum width of impact droplets on single-pillar superhydrophobic surfaces

Superhydrophobic surfaces with excellent water repellence show huge potential in diverse application fields such as self-cleaning and energy harvesting. This potential of superhydrophobic surfaces can be further improved by adding various micro- or macro- structures on the superhydrophobic surfaces. Pillar is a typical structure, while the droplet impact dynamics on the surfaces with a single pillar remain unclear due to the lack of related studies. Here, we experimentally explore the droplet impact behaviors on the superhydrophobic surfaces with a single pillar of different sizes. The effects of Weber number and the pillar-to-droplet diameter and height ratios on the contact state between the droplet and solid surface (including the pillar and the substrate) and the maximum width are investigated in detail. As the Weber number increases, the impact droplets exhibit three outcomes in sequence: failing to touch the substrate, touching the substrate without liquid fingers, and droplet breakup with liquid fingers. The critical Weber number for touching the substrate and forming liquid fingers is affected by the pillar-to-droplet diameter and height ratios. The increasing pillar size decreases the maximum width factor of impact droplets, which is smaller than that on the flat superhydrophobic surface under the same Weber number. A theoretical model based on energy conversation is developed for calculating the maximum width factor on the single-pillar superhydrophobic surfaces, which agrees well with experimental results and yields a ±10 % prediction deviation. This work provides useful guidance for the design and application of structures on superhydrophobic surfaces.