Effects of the Surface Roughness on Sliding Angles of Water Droplets on Superhydrophobic Surfaces

TLDR

The authors fabricated superhydrophobic films with varying roughness, examined how sliding angle, contact angle, and surface structure interrelate, and derived an equation linking sliding and contact angles on rough surfaces. They found that in the highly hydrophobic regime sliding angles decrease as contact angles increase, that air‑trapping microstructures are essential for low sliding angles, that the equation’s predictions match experiments, and that they produced a transparent film with a ~1° sliding angle and negligible resistance.

Abstract

Various superhydrophobic films having different surface roughnesses were prepared, and the relationships between the sliding angle, the contact angle, and the surface structure were investigated. In the highly hydrophobic region, the sliding angles of water droplets decreased with increasing contact angles. Microstructural observation revealed that surface structures that can trap air are important for the preparation of low-sliding-angle surfaces. We have also derived an equation that describes the relationship between sliding angles and contact angles on superhydrophobic surfaces with roughness. The results calculated on the basis of this equation agreed well with the experimental ones. Moreover, we have successfully prepared a transparent superhydrophobic film whose sliding angle is ∼1° for a 7 mg water droplet. On this film, there was almost no resistance to the sliding of water droplets. The film obtained satisfies the requirements of superhydrophobicity, transparency, and a low water sliding angle.