Wetting transitions on textured hydrophilic surfaces

Abstract

We consider the quasi-static energy of a drop on a textured hydrophilic surface, with taking the contact angle hysteresis (CAH) into account. We demonstrate how energy varies as the contact state changes from the Cassie state (in which air is trapped at the drop bottom) to the Wenzel state (in which liquid fills the texture at the drop bottom) assuming that the latter state nucleates from the center of the drop bottom. When the textured substrate is hydrophilic enough to allow spontaneous penetration of liquid film of the texture thickness, the present theory asserts that the drop develops into an experimentally observed state in which a drop looks like an egg fried without flipped over (sunny-side up) with a well-defined radius of "the egg yolk." Otherwise, the final contact state of the drop becomes like a Wenzel state, but with the contact circle smaller than the original Wenzel state due to the CAH. We provide simple analytical estimations for the yolk radius of the "sunny-side-up" state and for the final radius of the contact circle of the pseudo-Wenzel state.

Physics and applications of superhydrophobic and superhydrophilic surfaces and coatings

“…64,[66][67][68] An energy barrier unfavorable to liquid wicking may be maximized in this manner. 59,[69][70][71][72][73] This energetic barrier, if it is larger than the inherent thermal energy 7 needs to be overcome by mechanical means such as vibrations, 74,75 impact 76,77 or load imposed on the drop.…”

Wetting state on hydrophilic and hydrophobic micro-textured surfaces: Thermodynamic analysis and X-ray visualization

“…13 In this case, if a droplet has a partial spherical shape, the wetting state of the droplet is defined as the Wenzel state. 1 However, if a droplet has a precursor (liquid film) that stretches out from the liquid drop along the surface, the wetting state is called the hemi-wicking state ("sunny-side-up" in other literature 14 ). Hydrophobic textured surfaces are generally only partially wetted, i.e., the droplet is in the Cassie-Baxter state.…”

“…Although wetting state has been extensively explored in previous reports, 13,14,16,[18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] the majority of these previous reports have focused on hydrophobic textured surfaces. Besides the wetting state was inferred from visible-wavelength images, making it difficult to identify the physical behavior at the vicinity of the micro-pillars on the test surfaces.…”

Direct Observation of the Wetting Mode Transition during Evaporation of Water Droplets on Superhydrophobic Surfaces with Random Roughness Structure

“…Ishino et al compared the energetic stability for Cassie's and Wenzel's modes. It indicated the possibility of a "sunny-side-up state" 26,29) (Fig. 4 (b)) as an additional wetting state, which is the droplet on the liquid film.…”

“…This transition commonly accompanies an increase of contact angle hysteresis 8,11) . The stability on each wetting mode has been evaluated from the perspective of surface structure and energy balance [14][15][16][17][18][19][20][21][22][23][24][25][26] .…”