Ultrahydrophobic surfaces: Relation between roughness and contact angle hysteresis

The effect of roughness on the wetting properties of anodically oxidized and subsequently hydrophobized aluminium surfaces has been investigated using scanning force microscopy, scanning electron microscopy and contact angle measurements. It was found that electrolyte temperature has an influence on surface roughness. The wetting characteristics range from "noncomposite" to composite" surfaces. The transition correlates with characteristic roughness features. Ultrahydrophobicity (theta(a) = 154degrees; theta(r)= 151degrees) was obtained for aluminium plates having root mean square roughness values of S-q = 0.2 mum, a dominant wavelength of lambda = 0.8 mum and a roughness coefficient of r(S) = 2.3. Water droplets roll freely on these surfaces. it could be shown that a further increase in roughness does not have any influence on wetting behaviour.