The Properties of Liquids and Lubrication

The fluids most widely used for lubrication are the liquid oils. Each liquid has its individual chemical composition, and consequently the way a liquid functions as a lubricant depends on how its chemical structure governs its density and viscosity behavior. Such influences are in part the subject of the discussions in this chapter. 

The general theory of fluids makes no distinction between liquids and gases. The basic relations for the flow of viscous fluids under pressure apply to both liquids and gases. However, there are overt differences between liquids and gases which are obvious from everyday experience. For example, under ordinary circumstances a liquid has a distinct interfacial boundary separating it from the ambient gaseous atmosphere. The deeper, fundamental difference between liquids and gases lies in their internal structure, a subject of considerable complexity. 

The physicochemical differentiation of the liquid state from the gaseous and the solid states requires elaborate and formal treatment. But characterization of the liquid state in a fashion useful for lubrication problems can be made much simpler than is required by exact theory. It will suffice for our purposes to begin with the treatment of liquid viscosity in descriptive terms. Then those constitutive and structural aspects of liquids and the liquid state which influence viscosity will be discussed. Similar treatment will be applied to the density and compressibility of liquids. 

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