Droplet spreading on a two-dimensional wicking surface

Abstract

The dynamics of droplet spreading on two-dimensional wicking surfaces were studied using square arrays of Si nanopillars. It was observed that the wicking film always precedes the droplet edge during the spreading process causing the droplet to effectively spread on a Cassie-Baxter surface composed of solid and liquid phases. Unlike the continual spreading of the wicking film, however, the droplet will eventually reach a shape where further spreading becomes energetically unfavorable. In addition, we found that the displacement-time relationship for droplet spreading follows a power law that is different from that of the wicking film. A quantitative model was put forth to derive this displacement-time relationship and predict the contact angle at which the droplet will stop spreading. The predictions of our model were validated with experimental data and results published in the literature.

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

“…Moreover, the a-t plots of both the X-and Y-axes for t > 0.05 s fit closely to the displacement-time equation derived for an isotropic droplet spreading on nanostructures imbibed with a wicking film (Figs. 2(a) and 2(b)), 18 t % 3l dc…”

“…18 for a more detailed description). According to de Gennes' analysis, 1,7 viscous dissipation during droplet spreading takes place mainly within this wedge.…”

“…Since Eq. (2) was originally derived for an isotropic droplet (spherical cap shape), 18 the good fit between the experimental and calculated a-t trends is rather unexpected. This is most likely due to the relatively low anisotropy of the droplets exhibited on nanofins, which makes Eq.…”

“…In the first regime (t < 0.05 s), the droplet rapidly adopts an elliptical cap shape upon contact with the nanofins. Recent evidences 18 have suggested that during the early stages of droplet spreading on a 2D wicking surface, the wicking film has yet to advance ahead of the droplet edge, which basically means that the droplet is spreading in the Wenzel state. Indeed, a comparison between the elliptical cap shape shown in Fig.…”

“…This involves the droplet liquid seeping into the roughness of the surface forming a wicking film that advances ahead of the droplet edge. [14][15][16][17][18] The wicking film wets the sides but not the tips of the roughness. This causes the droplet to spread on a flat composite surface made up of solid and liquid phases and can be considered as a combination of Wenzel and CassieBaxter states of wetting.…”