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1. 1 Preamble The science to which this work is appended is termed rheology and rheology means the study of flow and deformation of liquids and solids with emphasis on the underlying physical processes. Characteristic for solids is that they respond to a force by deforming, and, on removal of the force, by returning to their original shape. This type of re sponse is termed elastic. Characteristic for liquids is that they deform and continue to deform as long as the force is present. This type of response is termed viscous. The two definitions just given for viscous and elastic response represent ~wo extremes of response to an external force. Materials do not always fall readily into one or the other of the above-mentioned categories. An additional parameter is needed to describe material response more fully. The extra param eter is time. As a general rule the faster the deformation the closer the re sponse is to being elastic, the slower the deformation the closer the response is to being viscous. Slow and fast . are factors relative to some natural time, T, of the material. This natural time may be thought connected with the rates of spontaneous diffusion of its molecular and atomic constituents. For every day fluids as water, the natural time is very short, of the order of 10-10s 1) and hence for most purposes is considered as being viscous.
