Water adhesion tension, spreading

In order to spread on a water surface and form a stable film a substance has to have hydrophilic and hydrophobic groups in it. The bottom side of a spread film of protein must be predominantly hydrophilic, while the side directed towards the air must be predominantly hydrophobic, otherwise a protein monolayer would not be stable on the water surface. That the air side of a protein monolayer is predominantly hydrophobic is shown by the experiments of Bull (70) who deposited single monolayers of egg albumin at various pressures on glass slides and measured the contact angle between water and the deposited film. The film deposited at 15 dynes pressure showed an adhesion tension of about 100 ergs per sq. cm. The adhesion tension of a pure hydro-... 

The surface of PTFE articles is sHppery and smooth. Liquids with surface tensions below 18 mN/m(=dyn/cm) are spread completely on the PTFE surface hence, solutions of various perfluorocarbon acids in water wet the polymer (78). Treatment with alkafl metals promotes the adhesion between PTFE and other substances (79) but increases the coefficient of friction (80). 

PDMS based siloxane polymers wet and spread easily on most surfaces as their surface tensions are less than the critical surface tensions of most substrates. This thermodynamically driven property ensures that surface irregularities and pores are filled with adhesive, giving an interfacial phase that is continuous and without voids. The gas permeability of the silicone will allow any gases trapped at the interface to be displaced. Thus, maximum van der Waals and London dispersion intermolecular interactions are obtained at the silicone-substrate interface. It must be noted that suitable liquids reaching the adhesive-substrate interface would immediately interfere with these intermolecular interactions and displace the adhesive from the surface. For example, a study that involved curing a one-part alkoxy terminated silicone adhesive against a wafer of alumina, has shown that water will theoretically displace the cured silicone from the surface of the wafer if physisorption was the sole interaction between the surfaces [38]. Moreover, all these low energy bonds would be thermally sensitive and reversible. 

Certain chemicals have the ability to lower the surface tension of water. This allows water to spread out over a surface rather than bead up. Wetting agents decrease the cohesive forces between water molecules, and this helps water to spread over the surface of an object by adhesive force. 

When multilamellar lipid vesicles are placed on a SU-8 substrate, the contained lipid rapidly spreads out over time as a monolayer on the entire available surface (Fig. 23.8a). The contact angle of water on SU-8 was determined to be 91.4° 1.5°. The formed lipid patches are perfectly circular as shown in Fig. 23.8b. The multilamellar vesicles are eventually entirely depleted in the process. The tension induced by SU-8 is sufficiently large to disrupt the structure of the multilamellar vesicle, and the surface adhesion energy of lipids on SU-8, S, equals the tension at the spreading edge, and is therefore also equal to the lipid/SU-8 adhesion energy. 

RELATION OF INTIAL SPREADING PRESSURE OF POLAR COMPOUNDS ON WATER TO INTERFACIAL TENSION, WORK OF ADHESION, AND SOLUBILITY. 

To explain the great tendency of zinc particles to corrode at the film surface of water-based nano lithium silicate primers as comparing with those solvent-based, partially hydrolyzed tetraethyl orthosilicate, it is necessary to consider that the first ones are based on binders, as mentioned, with a higher superficial tension. The last one implies inferior wetting, that means lower adhesion, penetration and spreading during metal zinc incorporation previous to application consequently, they wet with more difficult the zinc particles while the second ones do it in a better way (more reduced interfacial tension). 

The study of surface tension is really a branch of surface chemistry, and its development has been exceedingly rapid in the last decade. Thus, adhesion can be considered to be partially an exercise in wetting and spreading of liquids on solid surfaces. The flotation of ores is accomplished by gravity differences as well as by the adhesion of the solid particle to an air bubble, and it involves solid-liquid-gas interfaces. It is possible to reduce the vaporization of water from bodies of water with large surfaces such as reservoirs and lakes, by adding a monolayer of a substance such as hexadecanol or other surface-active agents. The action of soaps produces a decrease in surface tension on water. Many other applications in our modem environment can be readily identified. 

For an adhesive to wet a surface, it requires a lower surface tension than the surface energy of the solid. If this condition is not met, the liquid does not spread across the surface but forms spherical droplets on the surface. Water has a relatively high surface tension (70 mN/m) and so on a highly polished car bonnet, the water will form droplets (Figure 6.3) because the waxed surface of the metal bonnet will have a lower surface energy than the water and so prevents wetting. 

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