Wetting against the nap - how asperity inclination determines unidirectional spreading.

We have carried out wetting experiments on textured surfaces with high aspect ratio asperities in the Wenzel state. When inclination is imparted to the asperities, we observe a strictly unidirectional spreading opposite to the direction in which the asperities point. The advancing contact angle decreases markedly as inclination increases. A crude numerical analysis successfully accounts for this behaviour, highlighting the interplay between Gibbs pinning at the top of the structures and imbibition along the valleys between them. In Gibbs pinning non-linearities play a major role and we find that simple line averaging - i.e. a rule of mixture - cannot account for this evolution except for weak surface perturbations, i.e. large inclinations.