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

Structuration, selective dispersion and compatibilizing effect of (nano)fillers in polymer blends

This review covers several basic methodologies of surface treatment and their effects on titanium (Ti) implants. The importance of each treatment and its effects will be discussed in detail in order to compare their effectiveness in promoting osseointegration. Published literature for the last 18 years was selected with the use of keywords like titanium dental implant, surface roughness, coating, and osseointegration. Significant surface roughness played an important role in providing effective surface for bone implant contact, cell proliferation, and removal torque, despite having good mechanical properties. Overall, published studies indicated that an acid etched surface-modified and a coating application on commercial pure titanium implant was most preferable in producing the good surface roughness. Thus, a combination of a good surface roughness and mechanical properties of titanium could lead to successful dental implants.

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Surface Modifications and Their Effects on Titanium Dental Implants

Effects of surface roughness on wettability

The surface roughness factors, such as the Wenzel roughness factor and so on, are very interrelated to each other. Therefore, it makes a precise discussion difficult on how the surface roughness affects the wettability. We already reported the effect of the surface roughness on the wettability at a constant Wenzel roughness factor using two kinds of models, the hemisphere close packing model and the hemiround rod close lining model. Nevertheless, the pitch is proportional to the height in these models. Therefore, we could not independently discuss the influence of roughness height and roughness pitch on the wettability. We developed our new models which can independently describe the influence of the surface roughness height and the roughness pitch on the wettability. We simulated loose packing sphere models by periodically placing small ball bearings and the loose lining round rod models by winding fine wires. The wettability was measured by the sessile drop method for the non-wetting system using paraffin coated samples and aqueous solutions. These results show that there is a critical pitch which determines the maximum contact angle in both systems. These results can be explained by the ratio of the solid/liquid/vapor and liquid/vapor line length at the three phase line.

Effects of roughness pitch of surfaces on their wettability

http://www.jwri.osaka-u.ac.jp/~dpt9/Acta(1996)MgOAl.pdf

Equilibrium contact angle in the magnesium oxide/aluminium system

Through the analysis of forces acting on the particles at the solid/liquid interface based on the interfacial energies, we propose a new theoretical particle transfer model. This model can predict the behavior of the particles at the interface. The main concept of this model is proved by the mutual wetting behavior among the solid, liquid and particle phases. If the contact angle at a solid/liquid interface and a particle is 90°, the particle would be pushed. This contact angle is measured by unidirectional solidification (UDS) experiments. But if solidification experiments were carried out for particle dispersed composites in a conventional UDS, the particles completely descended before freezing, therefore, a zone-UDS was used in the experiment. In Al–Sr, Al–Ca and Al–Ce matrix composites, Al 2 O 3 particles are pushed by the solid because the contact angles are >90°. On the other hand, in Al–12.6wt.%Si–Sr–Al 2 O 3 composites alloyed with Ca, the Al 2 O 3 particles are engulfed into the solid because the contact angle is

Engulfment of Al2O3 particles during solidification of aluminum matrix composites

The contact angles between molten aluminum and samples of A1N and A1N composites were measured to establish a way of estimating the wetting of nonreactive systems, reactive systems, and composite systems. The experiments were conducted using an improved sessile drop technique which prevented the oxidation of the aluminum. By plotting the results on a logarithmic time scale, it was found that the contact angle value for A1N containing Y2O3 as a sintering agent progressed through four phases (I: original-wetting phase, II: quasiequilibrium phase, III: interfacial-reaction-wetting phase, and IV: equilibrium phase), while the contact angle of purified A1N progressed through only two phases, I and IV. Phase III in the sample containing Y2O3 was caused by the interfacial reactions; the wetting speed in phase III (interfacial-reaction-wetting speed) and the value of the contact angle in phase IV (equilibrium wettability) depend on the interfacial reactions. The contact angle for A1N composite also progressed through the four phases. The contact angle of phase II (original wettability) conformed to Cassie’s law, whereas the contact angle of phase IV depended on the condition of the interface. However, Cassie’s equation should be applicable throughout phases II through IV when the condition of the interface is understood. Also, the mechanism of the interfacial reaction wetting was explained using Cassie’s equation by taking into account the ratio of area of components at the interface in phase III.

Hideo Nakae

Papers

Effects of surface roughness on wettability

Effects of roughness pitch of surfaces on their wettability

Equilibrium contact angle in the magnesium oxide/aluminium system

Engulfment of Al2O3 particles during solidification of aluminum matrix composites

Four wetting phases in AIN/AI and AIN Composites/AI systems, models of nonreactive, reactive, and composite systems