ABSTRACT
Returning to the examples used in the dictionary definition, the submarine under the sea is said to rise to the surface, implying that it moves from the bulk to the liquid/air interface. However, other interfacial aspects are also important, as in fact the submarine has a solid/liquid interface when under water, but when it rises to the surface (of the sea) there will be submarine/water (S/L), submarine/air (S/V) and water/air (L/V) interfaces. The relative magnitude of the interfacial forces will determine the behaviour of multi-phase systems. Such interactions are readily demonstrated by considering the cube (used in the dictionary definition), as the six surfaces can exist in different combinations with other phases as will be described in Section 2.3, and will dominate such processes as the dispersion of powders in liquids.At this stage it is worth considering the general properties of interfaces. In energetic terms, the free energy of formation of an interface must be positive. If it were negative or zero then the phases would be, or would readily become, miscible.
1.2 The liquid/vapour interface If there is such a thing as a simple interface, it may be between a liquid and a vapour. It is well known that gasses are such that their molecules travel freely in random directions, whilst liquids can be described as cooled gasses, in which the velocity of the molecules has decreased. Van der Waals interaction forces mean that liquid molecules are held more closely together than those in gasses, and thus liquids are more dense and occupy a definite volume, with a definite surface (liquid/vapour interface). If we compare the forces acting on a molecule in the bulk of the liquid with one at the interface (see Figure 1.1), it can be seen that in the bulk, the molecules are surrounded on all sides by other liquid molecules and will
