Introduction

Contact angle is the main parameter that characterizes wetting of solid surfaces by liquids. Water droplets on smooth hydrophobic surfaces do not usually form contact angles with the solid surface greater than 120°. When the angle achieved exceeds 150° this is termed superhydrophobicity. Superhydrophobic surfaces also usually have low contact angle hysteresis, show self-cleaning properties, and have low drag for fluid flow. The range of actual and potential application of self-cleaning surfaces is diverse including optical (e.g. self-cleaning lenses), building and architecture (windows, exterior paints, roof tiles), textiles, solar panels, microdevices (where the reduction of adhesion is crucial), applications requiring antifouling from biological and organic contaminants, etc [1].

Two models of superhydrophobic behavior are used known as Wenzel [2] and Cassie-Baxter [3] models. The main difference between the two models is whether the liquid droplet retains contact with the solid surface at all points or whether the liquid bridges only across surface protrusions, thus, resulting in a droplet suspended on a composite solid and vapor surface [4]. In the Wenzel model (Fig. 7.1a), the solid-liquid contact occurs at all points below the droplet and the observed equilibrium contact angle with the rough surface, h, is given by where the roughness factor Rf = ASL/AF > 1 is the ratio of the real substrate area ASL to the projected area AF, and h0 is the contact angle on a smooth surface of the same material. In the Cassie-Baxter model (Fig. 7.1b) the droplet suspends itself across surface protrusions, and an average of the cosines of the angle on the solid (i.e. cos h0) and on the air (i.e. cos 180° = — 1) below the drop is used. IffSL is the fraction of the solid surface upon which the drop sits and (1 — fSL) is the fraction below the drop that is air, then the Cassie-Baxter equation applies. When solidliquid interface is rough, the roughness factor should also be included as

The overall conclusion is that two factors are needed to produce a superhy-drophobic surface: roughness (providing high Rf and fSL) and a certain extent of initial hydrophobicity (e.g. a coating), such that cosh0 < 0. Surface roughness magnifies the hydrophobicity bringing the contact area into the superhydrophobic region, 150° < h < 180°. Furthermore, proper surface roughness is more critical than the initial superhydrophobicity, since under certain conditions even initially hydrophilic surface can show superhydrophobic properties [5]. There is also evidence that surfaces with dual-scale roughness (nanoroughness superimposed on microroughness) makes hydrophobic properties much more sustainable [6].

Since the 1990s, when new technologies emerged to produce microstructured surfaces, a huge amount of research work was done on design, fabrication, and characterization of superhydrophobic surfaces from various materials, ranging cos h = Rf cos h0

Fig. 7.1 Wetting of a microstructured surface in the

(a) homogeneous (Wenzel, solid-liquid) and

(b) composite (Cassie-Baxter, solid-liquid-air) regime

Fig. 7.1 Wetting of a microstructured surface in the

(a) homogeneous (Wenzel, solid-liquid) and

(b) composite (Cassie-Baxter, solid-liquid-air) regime

from polymers and ceramics to textiles, etc. A significant limitation on the practical application of the Lotus effect for self-cleaning is the sustainability of superhydrophobic microstructured coatings, which is often extremely vulnerable even to small wear rates and contamination [7].

It is much more difficult to produce a superhydrophobic metallic material than a polymer- or ceramic-based one, because metals tend to have higher surface energies [8-11]. In the area of metallic superhydrophobic materials a number of advances have been made. Yet in the 1950s, Bikerman [12] investigated wetting of stainless steel plates with different finishes with the contact angles around 90° and proposed that the surface roughness provides resistance for the sliding of water droplets. Since then, few studies of non-wetting metallic materials have been conducted. Baitai et al. [13] studied the effect of the surface roughness induced by the chemical etching on metallic composites super-hydrophobicity. They used Al, Cu, and Zn specimens immersed into an etchant (a mixture of HCl, H2O, and HF) at room temperature for time periods from 5 to 15 s. Shirtcliffe and McHale [14] studied the wettability of Cu-base superhydrophobic surfaces. They used Cu to form the base material and a coating to hydrophobize it. The removal or addition of material roughened the surface to control wetting by combining roughness with surface patterning. Sommers and Jacobi [15] achieved anisotropic wettability on an Al surface by controlling its surface micro-topography.

Metal matrix composites (MMCs) are composite materials which have a metallic matrix and a reinforcement of another metallic or nonmetallic (ceramics, polymer, etc.) material [16]. MMCs with hydrophobic reinforcement can provide much broader opportunities than pure metals for design and fabrication of composite surfaces and readily supply the reinforcement hydrophobic fraction and surface roughness due to the reinforcement. However, superhydrophobic MMCs have not yet been explored in the literature. Furthermore, in a composite material the hydrophobic reinforcement is in the bulk of the material rather than at the surface and thus wear does not necessary lead do the deterioration of the hydro-phobic coatings making these materials appropriate to the situations where traditional Lotus-effect coatings cannot be used. The use of composite materials with hydrophobic reinforcement in the bulk has already been suggested for concretes to prevent water penetration [17]. In this paper we investigate wetting of MMCs with the potential for various applications where self-cleaning sustainable surfaces are needed ranging from antifouling for the water industry to magnetic tape-head interfaces [18].

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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