Uni-, Bi-, and Tri-Directional Wetting Caused by Nanostructures with Anisotropic Surface Energies

Abstract

Wetting is a pervasive phenomenon that governs many natural and artificial processes. Asymmetric wetting along a single axis, in particular, has generated considerable interest but has thus far been achieved only by the creation of structural anisotropy. In this paper, we report that such directional wetting can also be achieved by anisotropically coating nanostructure surfaces with materials that modify the nanostructure surface energy, a phenomenon that has not been observed in natural or artificial systems thus far. Moreover, by combining this newfound chemical influence on wetting with topographic features, we are able to restrict wetting in one, two and three directions. A model that explains these findings in terms of anisotropy of the pinning forces at the triple phase contact line is presented. Through the resulting insights, a flexible method for precise control of wetting is created.

Effects of structural and chemical anisotropy of nanostructures on droplet spreading on a two dimensional wicking surface

“…Consistent with the analysis for nanofins above, the different extents of wetting (a(þY) > a(þX) ¼ a(ÀX) > a(ÀY)) in the various directions in the first regime is caused by the differences in pinning strength on the contact line with the most hydrophilic side (Al coated) having the least strength and the most hydrophobic side (PS) having the most. 20 Interestingly, as the droplet became more isotropic, the anisotropy in wetting lengths appeared to remain intact ( Fig. 4(c)).…”

“…The details of the fabrication procedure can be found in our previous paper. 20 To examine the droplet spreading behaviour, deionized water (q ¼ 1000 kg/m 3 , c ¼ 0.072 N/m, l ¼ 8.9 Â 10 À4 Pas, h ¼ 56.3 ) or silicone oil (q ¼ 1065 kg/m 3 , c ¼ 3.40 Â 10 À2 N/m, l ¼ 3.94 Â 10 À2 Pas, h ¼ 18 ) was deposited quasistatically ($ 1 mm/s) onto the center of the samples by means of a micropipette. Here, q, c and l refer to the density, surface tension and viscosity of the liquid respectively.…”

“…20 As this Wenzel spreading regime transits into the next regime, however, the emergence of the wicking film ahead of the droplet edge changes the rough, chemically homogeneous surface that the droplet was spreading on to a flat, solid-liquid composite surface made up of the top faces of the nanostructures and the top of the wicking film. Note that the wicking film wets the entire height of the nanostructures except for their top faces.…”

“…This is a unique result that is in marked contrast to Wenzel and CassieÀBaxter states of wetting, where droplets come to rest in metastable shapes as a result of contact line pinning at the top of the nanostructures. 13,19,20 Using the example of Fig. 3(c), which shows a droplet in Wenzel state wetting a row of nanostructures, it can be seen that the driving force for wetting must be able to rotate the droplet edge by 90 to wet the nanostructure sidewall before the triple phase contact line can advance.…”

“…Unlike Wenzel and Cassie-Baxter states, however, the dynamics of droplet wetting on 2D wicking surfaces is not well-studied. In particular, the effect of surface roughness anisotropy on droplet spreading for 2D wicking surfaces has yet to be reported despite the fact that such surface roughness anisotropy has been shown to have a significant influence on droplet spreading for Wenzel 19,20 and Cassie-Baxter states. 13 The motivation of this paper, therefore, is to conduct a systematic study on how structural and chemical anisotropy of a)…”