Relationship between surface hydrophobicity and water bounces – a dynamic method for accessing surface hydrophobicity

Abstract

Video camera analysis was used to measure the number of bounces of water droplets and modified water droplets on a range of superhydrophobic surfaces with different surface microstructures from rounded to ridge-like to needle-like. It was found that the number of bounces could be related to the static water contact angle on the surface. The initiation point for water bouncing occurred at a static water contact measurement of 151° for rounded surfaces, 156° for sharp surfaces and 171° for extremely sharp (needle-like) surfaces. The number of bounces observed on the same superhydrophobic surface was directly probed by the addition of a surfactant (sodium dodecyl sulfate), salts (NaCl, CaCl₂, LaCl₃) and addition of methanol. This showed the number of bounces was related to droplet size, droplet density, droplet surface tension, surface microstructure and surface free energy of interaction with water.