interactions and minimize the number of unfavorable interactions between the molecules. Consider what may happen when a drop of liquid is placed on a solid surface (Figure 5.8). If the liquid remained as a lens, there would be three different interfaces: solid-liquid, solidgas, and liquid-gas, each with its own interfacial or surface tension. If the liquid spread over the surface, there would be a decrease in the area of the solid-gas interface but an increase in the areas of both the liquid-gas and solid-liquid interfaces. The tendency for a liquid to spread therefore depends on the magnitude of the solid-gas interactions (ySG) compared to the magnitude of the solid-liquid and liquid-gas interactions that replace it (ySL + yLG). This situation is conveniently described by a spreading coefficient, which is defined as (Hunter 1993):
If the energy associated with the solid-gas interface is greater than the sum of the energies associated with the solid-liquid and liquid-gas interfaces (ysg > YSL + Ylg), then S is positive and the liquid tends to spread over the surface to reduce the energetically unfavorable contact area between the solid and the gas. On the other hand, if the energy associated with the solidgas interface is less than that associated with forming the solid-liquid and liquid-gas interfaces (ysg < YSL + Ylg), then S is negative and the liquid tends to form a lens.
The shape of a droplet can be predicted by carrying out a force balance at the point on the surface where the solid, liquid, and gas meet (Figure 5.9) using the Young equation (Hiemenz 1986):
Oil ow ow sw
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