Surface force selective wetting

Two different liquids may compete with each other, if each of them individually wets a solid surface. In this case the resulting contact angle corresponds to that of a better wetting liquid with nature similar to that of the solid surface. If the surface is better wetted by water than by a hydrocarbon ( 0<9O°), it is referred to as hydrophilic (oleophobic) in the case when the surface is better wetted by non-polar hydrocarbon (0>9O°), it is referred to as hydrophobic (oleophilic). For selective wetting, as opposed to wetting in air, the contact angle, 0, can assume any value between 0° and 180°. When 0 = 0°, the more polar liquid spreads over the solid surface, forcing the less polar liquid away when 0 = 180° the situation is opposite the non-polar liquid phase completely forces the polar liquid away from the solid surface. 

Fig. III-21. Balance between the surface tension forces during selective wetting...

The wettability characteristics of an adhesive/adherend pair are determined by the relative values of surface tension of the adhesive and adherend. Surface tension of a liquid is a direct measurement of intermolecular forces and is half of the free energy of molecular cohesion. Surface tension is commonly represented by -q (gamma), and is measured in dynes/cm. The value of the surface tension of the solid substrate, or adherend, is called the critical surface tension, To ensure that the surface of the adherend will be wetted by an adhesive, an adhesive whose surface tension is less than the critical surface tension should be selected, so that... 

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