http://web.mit.edu/nnf/publications/GHM137.pdf

A modified Cassie-Baxter relationship to explain contact angle hysteresis and anisotropy on non-wetting textured surfaces.

Their Applications Kesong Liu,†,∥ Moyuan Cao,† Akira Fujishima, and Lei Jiang*,†,‡ †Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry and Environment, Beihang University, Beijing 100191, PR China ‡Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China Research Institute for Science and Technology, Photocatalysis International Research Center, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan Institute for Superconducting and Electronic Materials, University of Wollongong, Innovation Campus, Squires Way, North Wollongong, NSW 2500, Australia

Bio-Inspired Titanium Dioxide Materials with Special Wettability and Their Applications

Most of the artificial superhydrophobic surfaces that have been fabricated to date are not biodegradable, renewable, or mechanically flexible and are often expensive, which limits their potential applications. In contrast, cellulose, a biodegradable, renewable, flexible, inexpensive, biopolymer which is abundantly present in nature, satisfies all the above requirements, but it is not superhydrophobic. Superhydrophobicity on cellulose paper was obtained by domain-selective etching of amorphous portions of the cellulose in an oxygen plasma and subsequently coating the etched surface with a thin fluorocarbon film deposited via plasma-enhanced chemical vapor deposition using pentafluoroethane as a precursor. Variation of plasma treatment yielded two types of superhydrophobicity : "roll-off" (contact angle (CA), 166.7 degrees +/- 0.9 degrees ; CA hysteresis, 3.4 degrees +/- 0.1 degrees ) and "sticky" (CA, 144.8 degrees +/- 5.7 degrees ; CA hysteresis, 79.1 degrees +/- 15.8 degrees ) near superhydrophobicity. The nanometer scale roughness obtained by delineating the internal roughness of each fiber and the micrometer scale roughness which is inherent to a cellulose paper surface are robust when compared to roughened structures created by traditional polymer grafting, nanoparticle deposition, or other artificial means.

Fabrication of “Roll-off” and “Sticky” Superhydrophobic Cellulose Surfaces via Plasma Processing

Surface roughness has a profound influence on the wetting properties of a material. This fact is especially true with respect to the wetting of superhydrophobic surfaces. As a result of a special surface structure, a drop of water brought into contact with such a material forms an almost perfect sphere and even a very slight tilting of the superhydrophobic object is sufficient to cause the drop to roll off. For the description of the behavior of drops on rough surfaces two theories, namely those of Cassie and Wenzel, are often employed. However, it is currently becoming well established that both models explain the wetting behavior more from a qualitative or practical point of view rather than giving a quantitative description. For a prediction of actual contact angles, a more complex description is required that, next to thermodynamics, takes into account the kinetics of wetting. In this article, we focus on a few key aspects of superhydrophobic wetting, namely the characterization of superhydrophobic surfaces, models for the movement of drops, transitions between the Cassie and Wenzel states, and the behavior of superhydrophobic materials under condensation.

Some thoughts on superhydrophobic wetting

The Wenzel and Cassie−Baxter equations depending on the extent of liquid/solid interfacial contact area were generally used to estimate water contact angles on superhydrophobic surfaces. In this study, a simple method is proposed on the criterion to use the Wenzel and Cassie−Baxter equations to evaluate the contact angle results on superhydrophobic surfaces. In this method, the difference between the theoretical (geometric) and experimental contact angle-dependent Wenzel roughness parameter, Δrw, and Cassie−Baxter solid/liquid contact area fraction, ΔfsCB were determined, and the validity of these equations was evaluated. We used the data of eight recent publications where the water drop sits on square and cylindrical pillar structured superhydrophobic model surfaces. We evaluated the contact angle results of 166 patterned samples with our method. We also found that the effect of contact angle error margins was low to vary these parameters. In general, the use of the Wenzel equation was found to be wrong ...

Range of Applicability of the Wenzel and Cassie−Baxter Equations for Superhydrophobic Surfaces†

Cassie-Baxter theory has traditionally been used to study liquid drops in contact with microstructured surfaces. The Cassie-Baxter theory arises from a minimization of the global Gibbs free energy of the system but does not account for the topology of the three-phase contact line. We experimentally compare two situations differing only in the microstructure of the roughness, which causes differences in contact line topology. We report that the contact angle is independent of area void fraction for surfaces with microcavities, which correspond to situations when the advancing contact line is continuous. This result is in contrast with Cassie-Baxter theory, which uses area void fraction as the determining parameter, regardless of the type of roughness.

/pdf/effect-of-three-phase-contact-line-topology-on-dynamic-58v2v9l8ac.pdf

Svetlana Tatic-Lucic

Papers

Effect of three-phase contact line topology on dynamic contact angles on heterogeneous surfaces.