ABSTRACT
In normal viscous fluids, only the rate of deformation is of interest. In the absence of external and body forces, no stresses are developed and there is no means of distinguishing between a natural state and deformed state [65]. It is rather disturbing to think of the very large overall deformations obtained in the flow of fluids being associated in any way with substances that have elasticity. The rationalization lies in realizing that for the substances considered here, the behavior is essentially that of a fluid; although much translation and rotation may occur, the “elastic” distortion of the elemental volumes around any point is generally small. This “elastic” distortion or material’s strain is nevertheless present and is a feature that cannot be neglected. It is responsible for the recovery of reverse flow after the removal of applied forces and for all the other non-Newtonian effects [68]. These distortions or strains are determined by the stress history of the fluid and cannot be specified kinematically in terms of the large overall movement of the fluid. Another way of looking at the situation is to say that the natural state of the fluid changes constantly in flow and tries to catch up with the instantaneous state or the deformed state. It never quite succeeds in doing so, and the lag is a measure of the memory or the elasticity. In elastic solids, the natural state does not change and there is perfect memory [65].
