Evaporation-Triggered Wetting Transition for Water Droplets upon Hydrophobic Microstructures

Abstract

When placed on rough hydrophobic surfaces, water droplets of diameter larger than a few millimeters can easily form pearls, as they are in the Cassie-Baxter state with air pockets trapped underneath the droplet. Intriguingly, a natural evaporating process can drive such a Fakir drop into a completely wetting (Wenzel) state. Our microscopic observations with simultaneous side and bottom views of evaporating droplets upon transparent hydrophobic microstructures elucidate the water-filling dynamics and suggest the mechanism of this evaporation-triggered transition. For the present material the wetting transition occurs when the water droplet size decreases to a few hundreds of micrometers in radius. We present a general global energy argument which estimates the interfacial energies depending on the drop size and can account for the critical radius for the transition.

How superhydrophobicity breaks down

“…Different impalement scenarios have been discussed. For situations in which inertia is negligible, the two most important ones are depinning and sagging (31,(34)(35)(36)(38)(39)(40)(41)(42)(43). In depinning, the three phase contact line, or briefly contact line, unpins from the edge of the asperity (37).…”

“…In the sag mechanism, the underside of the liquid surface sags until it reaches the substrate (36). The Cassie-to-Wenzel transition takes place as soon as the lowest point of the meniscus touches the bottom surface (31,40). To simplify the discussion, we discriminate between the "contact line" on the microscopic and the "rim" on the macroscopic length scale.…”

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

“…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.…”

Regimes of wetting transitions on superhydrophobic textures conditioned by energy of receding contact lines

“…[24][25][26] We study a wetting transition by monitoring the evaporation of a drop. An evaporating drop is known to produce various intriguing phenomena, such as coffee stains 27 or wine tears, 28 and has been already studied with isotropic SH surfaces, 7,29,30 although the quantitative understanding is still at its infancy. During the evaporation the drop becomes smaller and the contact angle retracts.…”