Contact angle and wettability of solid surfaces

A representation of the several forces acting on a drop of liquid placed on a flat, solid surface is shown in Fig. 1.17(a). The surface tension of the solid, will favour spreading of the liquid, but this is opposed hy the solid-liquid interfacial tension, ygi, and the horizontal component of the surface tension of the liquid 7l/a in plane of the solid surface, that is 7l/a cos 6. Equating these forces gives 

Equation (1.5) is generally referred to as Young s equation. The angle 6 is termed the contact angle. The condition for complete wetting of a solid surface is that the contact angle should be zero. This condition is fulfilled when the forces 

If the contact angle is larger than 0°, the term (cos 9-1) will he negative, as will the value of S. The condition for complete, spontaneous wetting is thus a zero value for the contact angle. 

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