Many-body force definition

An ideal gas has by definition no intermolecular structure. Also, real gases at ordinary pressure conditions have little to do with intermolecular interactions. In the gaseous state, molecules are to a good approximation isolated entities traveling in space at high speed with sparse and near elastic collisions. At the other extreme, a perfect crystal has a periodic and symmetric intermolecular structure, as shown in Section 5.1. The structure is dictated by intermolecular forces, and molecules can only perform small oscillations around their equilibrium positions. As discussed in Chapter 13, in between these two extremes matter has many more ways of aggregation the present chapter deals with proper liquids, defined here as bodies whose molecules are in permanent but dynamic contact, with extensive freedom of conformational rearrangement and of rotational and translational diffusion. This relatively unrestricted molecular motion has a macroscopic counterpart in viscous flow, a typical property of liquids. Molecular diffusion in liquids occurs approximately on the timescale of nanoseconds (10 to 10 s), to be compared with the timescale of molecular or lattice vibrations, to 10 s. 

The Webster definition of adhesion is simply the molecular attraction exerted between the snrfaces of bodies in contact (7). In the more technical world, it is understood that there are many complex components to adhesion in a system, both chemical and physical. Further, the wide variety of complex combinations limits the merit of general predictive mles of thumb. Instead, perhaps, it is useful to simply let a specific situation define an appropriate adhesion test, and thus the practically adherent system. A sufficiently adherent interface is one in which the appropriate applied load or sttess did not exceed the total interfacial adhesive forces. Stresses in coated systems can be either compressive or tensile when experienced normal to the surface, shearing when in a parallel orientation, or some combination of these. Much can be learned from automotive adhesion requirements, because so many durable coating applications in so many situations are found in this industry. Table 7 summarizes some of the tests used to measure adhesion. The following sections offer summaries of important aspects of adhesion in practical coating systems. 

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