Wetting


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wetting

[′wed·iŋ]
(electronics)
The coating of a contact surface with an adherent film of mercury.
(metallurgy)
Spreading liquid filler metal or flux on a solid base metal.

Wetting

 

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).

Figure 1. Droplet on the surface of a solid

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.

Figure 2. Position of a droplet (bubble) on a solid under various conditions of wetting: (G) gas, (L) liquid, and (S) solid

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.

REFERENCES

Goriunov, 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

wetting

In soldering or brazing, the spreading of a liquid filler metal or flux on a solid base metal.
References in classic literature ?
I cared nothing for the wetting, but I obeyed him without knowing why.
I wanted to run out through the long pasture grass, wetting my legs with the dew of it, and to climb the rail fence, and keep on through the timber and up and up over the divide so as to get a look around.
I was rather in want of amusement, and so rare an opportunity was not to be neglected; so, leaving both meadow and hedge, I quickly repaired to the spot, but not before Sancho, who, immediately upon perceiving his young friend, scoured at full gallop the intervening space, and pounced upon him with an impetuous mirth that precipitated the child almost into the middle of the beck; but, happily, the stones preserved him from any serious wetting, while their smoothness prevented his being too much hurt to laugh at the untoward event.
I protest I was never more affected in my life than when I heard the little wretch, who is hardly yet seven years old, while his mother was wetting him with her tears, beg her to be comforted.
The lee rail, where the dead man lay, was buried in the sea, and as the schooner lifted and righted the water swept across the deck wetting us above our shoe-tops.
No," said he, wetting his lips, "but you'll allow, yourself, it's a cold ending.
But first of all, as an opener and just as a matter of course, start wetting the towel over her eyes.
Wet Wipe Having A Stratified Wetting Composition Therein And Process For Preparing Same: No.
Cognis Corporation has announced the addition of Starfactant[TM] wetting agents to its Star[TM] product line.
Cognis Corporation's industrial coatings and adhesives division has expanded its Star product line with the introduction of a new range of wetting agents.
Wetting balance has long been a useful laboratory test for evaluating solder wetting properties as a pre-screen for what can be expected in a printed circuit assembly process on the production floor.
Using screw conveyors, dust is transported from the baghouse into a small silo, which feeds a high-speed pin mixer for wetting the dust prior to placement in a landfill.