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

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.