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The simultaneous presence of a gas and a liquid does not necessarily result in the formation of foam. Intense mixing of both materials is required for a fine distribution of the gas in the liquid and this results in the creation of new gas/liquid interfaces. In most pure liquids these interfaces are unstable. Consequently, the gas bubbles rise very quickly and collapse on the surface of the liquid. Only the presence of surface active (amphiphilic) substances in the liquid stabilises the gas as foam. (Figure 3.1) [2] [3]. Stabilisation occurs through orientation of the surface-active substances at the newly formed liquid/gas interfaces. In practice, the presence of surface-active substances cannot be avoided in coating systems, as their use is essential to control properties such as wetting and dispersing, emulsification or substrate wetting [4] [5].