Equilibrium wetting on rough surfaces is discussed in terms of the “competition” between complete liquid penetration into the roughness grooves and entrapment of air bubbles inside the grooves underneath the liquid. The former is the homogeneous wetting regime, usually described by the Wenzel equation. The latter is the heterogeneous wetting regime that is described by the Cassie−Baxter equation. Understanding this “competition” is essential for the design of ultrahydrophobic surfaces. The present discussion puts the Wenzel and Cassie−Baxter equations into proper mathematical−thermodynamic perspective and defines the conditions for determining the transition between the homogeneous and heterogeneous wetting regimes. In particular, a new condition that is necessary for the existence of the heterogeneous wetting regime is added. It is demonstrated that when this condition is violated, the homogeneous wetting regime is in effect, even though the Cassie−Baxter equation may be satisfied.

Wetting on Hydrophobic Rough Surfaces: To Be Heterogeneous or Not To Be?

With trimming of the diameter of the well-ordered polystyrene arrays using oxygen plasma treatment and modification of these surfaces with gold and alkylthiols, the water contact angles on these su...

Fabrication of Tunable Superhydrophobic Surfaces by Nanosphere Lithography

Millions of years before we began to generate functional nanostructures, biological systems were using nanometer-scale architectures to produce unique functionalities. For instance, moths use hexagonal arrays of nonclose-packed (ncp) nipples as antireflection coatings (ARCs) to reduce reflectivity from their compound eyes. The outer surface of the corneal lenses of moths consists of ncp arrays of conical protuberances, termed corneal nipples, typically of sub-300 nm height and spacing. These arrays of subwavelength nipples generate a graded transition of refractive index, leading to minimized reflection over a broad range of wavelengths and angles of incidence. Similar periodic arrays of ncp pillars have also been observed on the wings of cicada to render superhydrophobic surfaces for self-cleaning functionality. In this Communication, we report a simple and scalable bioinspired templating technique for fabricating broadband and superhydrophobic ARCs on technologically important silicon and glass substrates. Crystalline silicon is the most important material for solar cells. Unfortunately, due to the high refractive index of silicon, more than 30% of incident light is reflected back from the surface of crystalline silicon. ARCs are therefore widely utilized to reduce the unwanted reflective losses. Quarterwavelength silicon nitride (SiNx) films deposited by plasmaenhanced chemical vapor deposition (PECVD) are the industrial standard for ARCs on crystalline silicon substrates. However, the PECVD-deposited SiNx films are expensive to fabricate. Additionally, commercial SiNx ARCs are typically designed to suppress reflection efficiently at wavelengths around 600 nm. The reflective loss is rapidly increased for near-infrared and other visible wavelengths, which contain a large portion of the incident solar energy. In contrast, subwavelength-structured moth-eye ARCs directly patterned in the substrates are broadband and intrinsically more stable and durable than multilayer ARCs since no foreign material is involved. Nevertheless, current topdown lithographic technologies in creating subwavelength

Bioinspired Self‐Cleaning Antireflection Coatings

https://hal.archives-ouvertes.fr/hal-01087663/document

Ryuichi Inui

Papers

Effects of surface roughness on wettability