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a phenomenon arising upon contact of a liquid with the surface of a solid or other liquid. In particular, wetting is seen in the spreading of a liquid over a solid surface in contact with a gas (vapor) or other liquid, in the impregnation of porous bodies and powders, and in the change in shape of the surface of a liquid at the surface of a solid. Thus, wetting results in the formation of a spherical meniscus in a capillary tube and determines the shape of a droplet on a solid surface or the shape of a gas bubble on the surface of an object immersed in a liquid. Wetting is often regarded as a result of intermolecular (van der Waals’) interactions in the zone of contact of three phases (bodies, media). However, in many cases, for example, in the contact of liquid metals with solid metals, oxides, diamond, and graphite, wetting is caused more by the formation of chemical bonds, solid and liquid solutions, and diffusion processes in the surface layer of the wetted body than by intermolecular interactions. The heat effect accompanying the contact of a liquid with the surface being wetted is called the heat of wetting.
The contact angle θ between the surface being wetted and the surface of the liquid at the periphery usually serves as a criterion of wetting (Figure 1). The angle θ is measured through the liquid. For steady-state (equilibrium) wetting, this angle is related to the surface tension of the liquid (σL), the surface tension of the solid (σs), and the interphase tension at the solid-liquid boundary (σSL) by the Young equation: cos θ = (σs – σSL)/σL. The lyophily and lyophoby of surfaces relative to various liquids are determined by the magnitude of θ. On a lyophilic surface, a liquid spreads; that is, there is incomplete (0° < θ < 90°) or complete (θ → 0°) wetting. Spreading does not occur (θ > 90°) on a lyophobic surface (Figure 2).
The contact angle depends on the relationship between the forces binding the molecules of the liquid to the molecules or atoms of the body being wetted (adhesion) and the forces binding the molecules of the liquid to each other (cohesion). The reversible work of adhesion and work of cohesion, respectively, are calculated from the equations WAd = σL(I + cos θ) and WCo= 2σL. Since θ is always > 0° when WAd < WCo, wetting will improve with an increase in the ratio WAd/WCo. The term S = WAd/WCo is called the spreading coefficient. The delay often observed in establishing equilibrium contact angles is called wetting hysteresis. A distinction is made between kinetic (dynamic) and static hysteresis. Possible causes of hysteresis include roughness of the surface, structural features of the surface layer, and relaxation processes in the liquid phase. Selective wetting occurs when a solid comes into contact simultaneously with two immiscible liquids. Surfactants are efficient regulators of wetting, being able to either improve or impede the process.
Wetting has great significance in nature, technology, and daily life. Good wetting is necessary in dyeing and washing, in processing photographic materials, and in applying paints and varnishes. Wetting also figures in the impregnation of fiber materials and in gluing, soldering, and amalgamation. An attempt to reduce wetting to a minimum is made in producing hydrophobic coatings and waterproofing materials. In some cases, for example, in flotation and emulsification using solid emulsifying agents, the contact angle must be kept within a set range of values. Wetting plays a vital role in metallurgical processes and in the dispersion of solids in a liquid medium. It affects the distribution of groundwater, the moistening of soils, and a variety of biological and other natural processes. P. A. Rebinder, A. N. Frumkin, and B. V. Deriagin made significant contributions to the theory and applications of wetting.
REFERENCESGoriunov, Iu. V., and B. D. Summ. Smachivanie. Moscow, 1972.
Fridrikhsberg, D. A. Kurs kolloidnoi khimii. Leningrad, 1974. Page 60.
Naidich, Iu. V. Kontaktnye iavleniia v metallicheskikh rasplavakh. Kiev, 1972.
Zimon, A. D. Adgeziia zhidkosteii smachivanie. Moscow, 1974.
L. A. SHITS