Wetting equilibria and contact angles

Clearly the adhesive must wet the adherend, implying the common-sense idea of a thin film of liquid spreading uniformly without breaking into droplets on the surface (Fig. 3.3). The water break-free test is the simplest approach to a qualitative visual assessment the thermodynamic approach to wetting allows quantitative study. 

7sv represents the surface free energy of the solid substrate resulting from adsorption of vapour from the liquid, and will be lower than the surface free energy of the clean bare solid surface in vacuo, 75. Duprd considered the work needed to separate a layer of liquid from a solid surface, viz 

The difference between 75 and 7sv is known as the spreading pressure, its, of the liquid s vapour on the solid surface. These two classic equations may then be combined so that 

Intuitively, optimum performance is expected if the liquid adhesive exhibits a zero or near zero contact angle, 0, giving a high value of W A. 

Real solid surfaces are not flat, so that observed contact angles 

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