Wetting against the nap – how asperity inclination determines unidirectional spreading

Abstract

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.

The effect of the orientation and the height of periodic sub-micrometric texturing on dropwise condensation

“…These results broaden the scope of the numerous studies made on the effect of periodic anisotropic surface pattern [2,3,4,5]. Indeed, most of investigations were made on sessile drops, which differs from the study of a dense pack of droplets (breath figure) [15,16,17,18,46,47,48].…”

“…The development of surface texturing techniques over the last 20 years has significantly advanced our understanding of surface wetting properties. Numerous studies have shown that the direction of a given surface pattern influences the orientation of sessile droplets by forming anisotropic droplets, oriented droplets or even self-propelled droplets [1,2,3,4,5,6,7,8]. Oriented structures favor droplet deformation and in effect, create anisotropic wetting as opposed to isotropic wetting [9].…”

Mechanically Switchable Wetting Petal Effect in Self-Patterned Nanocolumnar Films on Poly(dimethylsiloxane)

“…Thus topographies inspired in hierarchical natural structures [5,6] and lithographic surfaces consisting of patterned lines [7,8] are commonly utilized to induce high water repellency [9] and low flow resistance [10][11][12]. Patterning using nanoparticles, [13] laser [14,15], etching [16,17] or plasma treatments [18] have been also used to tailor droplet sliding on surfaces. Similarly, a pin-release droplet effect has been reported on nanostructured thin films grown by physical vapor deposition at glancing angles (GLAD) [2].…”

Fabrication of a flexible biomimetic film with spontaneously unidirectional water‐spreading property

“…Unidirectional liquid spreading is an extreme anisotropic wetting phenomenon, which means the liquid only spreads in one unique direction on the surface, but pins in all others. Anisotropic wetting properties can be achieved either by chemical modification [9][10][11][12][13][14] or surface topography [15][16][17][18][19][20]. Chemically modifying of surface is relatively simple and can achieve excellent anisotropic wetting properties, but lack of long-term stability.…”

“…Chemically modifying of surface is relatively simple and can achieve excellent anisotropic wetting properties, but lack of long-term stability. In recent years, some literatures have successfully achieved unidirectional spreading property solely based on the microstructure of bent micro or nanopillars [15][16][17][18][19][20]. The wetting anisotropy of this type of structure relies on the asymmetry of the microstructure, as well as the contact angle (CA), which should be larger than a critical value (∼30°), such that the liquid does not spread reversely.…”