Complexity of solid surfaces and effects on contact angle

3 Complexity of solid surfaces and effects on contact angle 

Equation 4.1 is valid for ideal smooth solid surfaces. Solid-liquid and solid-solid interfaces are very important for understanding wetting, lubrication, adhesion and other phenomena and for characterizing solid surfaces, as discussed further in Chapter 6. 

However, we underline here that solid surfaces are complex in many ways they are often heterogeneous (chemically or geometrically) and rough, they easily become dirty or contaminated (adsorbed liquids or solids, e.g. dust), and their properties change with time due to chemical or physical adsorption. 

Let us discuss some of the above-mentioned factors in some detail. 

Aging phenomena are also cmcial. Even carefully prepared surfaces under vacuum rapidly adsorb (physically or chemically) material from the environment, e.g. water, hydrocarbons, dirt, etc. For example, Mica prepared under vacuum has a surface energy of 4500 mN m but the surface energy decreases to about 300 mN m under normal laboratory conditions, where water can easily adsorb (Israelaschvih, 1985). Moreover, the way the solid surface is prepared (crystallized in water or air) may have a large effect on its surface properties. 

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