When the surface chemistry makes a surface more hydrophilic at the centre than the edge, droplets tend to accumulate in the centre, but what happens if the surface chemistry is the same everywhere and only the roughness changes? In this experiment a flat copper plate has a fractal roughness that increases from centre to edge. When this surface is made hydrophobic, the surface has a gradient from hydrophobic in the centre to superhydrophobic at the edge – see image.
Deposited droplets released at the edge roll to the centre, whilst those which shoot past the centre turn around and come back.
Superhydrophobicity: Localized parameters and gradient surfaces
G. McHale, S.J. Elliot, M.I. Newton and N.J. Shirtcliffe
in Mittal, K.L., ed, ‘Contact Angle, Wettability and Adhesion, Koninklijke Brill NV, Vol. 6 219-233 (2009).