From Hygrophilic to Superhygrophobic: Theoretical Conditions for Making High-Contact-Angle Surfaces from Low-Contact-Angle Materials

Abstract

The possibility of making high-contact-angle, rough surfaces from low-contact-angle materials has recently been suggested and demonstrated. A thermodynamic analysis of this possibility in terms of feasibility and stability is presented. It turns out that only roughness topographies that conform to a feasibility condition which is developed in the present paper can support this phenomenon. Even under conditions that support the phenomenon, the high-contact-angle state may not be stable, and transition from the heterogeneous (Cassie-Baxter) wetting regime to the homogeneous (Wenzel) regime with a lower contact angle may occur. In addition, it is suggested to use the general terms hygrophilic and hygrophobic (based on the Greek prefix hygro- that means liquid) to describe low- and high-contact-angle surfaces, respectively.

Hygro-responsive membranes for effective oil–water separation

“…Recent work 15,18,19,[27][28][29][30] has explained how 're-entrant surface texture' , in conjunction with surface chemistry and roughness, can be used to design superoleophobic surfaces. We have previously discussed the spacing ratio, D*, which is a dimensionless measure of surface porosity 19 .…”

“…However, that methodology requires continuous application of a very high voltage and is limited to separation of a few millilitres of emulsions. Guided by the design parameters discussed above, in this paper we develop novel membranes with hygroresponsive surfaces 30 , which are both superhydrophilic (θ * water ≈0°) and superoleophobic (θ * oil > 150°) both in air and under water. Our membranes can separate, for the first time, several litres of oil-water mixtures, including surfactant-stabilized emulsions, solely using gravity, in a single-unit operation, with >99.9% separation efficiency, by using the difference in capillary forces acting on the two phases.…”

Quantitative Testing of Robustness on Superomniphobic Surfaces by Drop Impact

“…This is related to the meta-stable character of the Cassie-Baxter state: in a free-energy landscape, this state can be located at local minimum, whereas the free-energy is often at a global minimum in the Wenzel state. Therefore, a perturbation of finite strength is required to jump over the energy barrier between the Cassie-Baxter and Wenzel states [6,17]. Furthermore, it has been recently evidenced [10,17] that a meta-stable Cassie-Baxter state can be achieved even if θ 0 ≤ π 2 , providing that the roughness elements have an overhanging shape.…”

“…Therefore, a perturbation of finite strength is required to jump over the energy barrier between the Cassie-Baxter and Wenzel states [6,17]. Furthermore, it has been recently evidenced [10,17] that a meta-stable Cassie-Baxter state can be achieved even if θ 0 ≤ π 2 , providing that the roughness elements have an overhanging shape. This peculiar geometry allowed for low-γ liquids repellent surfaces.…”

A superoleophobic textile repellent towards impacting drops of alkanes

“…The liquids of interest are common oils but also pure van-der-Waals liquids with surface tensions ranging from 35 mNm -1 down to 20 mNm -1 . Liquids which such a low surface tension wet all common materials making the design of highly oil-repellent surfaces much more demanding compared to superhydrophobic surfaces [7]. Only recently the requirements for the design of highly oleophobic surfaces have emerged.…”