<Li> if the materials behaves as a combination of viscous and elastic components, then the material is viscoelastic . Theoretically such materials can show both instantaneous deformation as elastic material and a delayed time dependent deformation as in fluids . </Li> <Li> Plasticity is the behavior observed after the material is subjected to a yield stress: A material that behaves as a solid under low applied stresses may start to flow above a certain level of stress, called the yield stress of the material . The term plastic solid is often used when this plasticity threshold is rather high, while yield stress fluid is used when the threshold stress is rather low . However, there is no fundamental difference between the two concepts . </Li> <P> Rheology has applications in materials science engineering, geophysics, physiology, human biology and pharmaceutics . Materials science is utilized in the production of many industrially important substances, such as cement, paint, and chocolate, which have complex flow characteristics . In addition, plasticity theory has been similarly important for the design of metal forming processes . The science of rheology and the characterization of viscoelastic properties in the production and use of polymeric materials has been critical for the production of many products for use in both the industrial and military sectors . Study of flow properties of liquids is important for pharmacists working in the manufacture of several dosage forms, such as simple liquids, ointments, creams, pastes etc . The flow behavior of liquids under applied stress is of great relevance in the field of pharmacy . Flow properties are used as important quality control tools to maintain the superiority of the product and reduce batch to batch variations . </P> <P> Examples may be given to illustrate the potential applications of these principles to practical problems in the processing and use of rubbers, plastics, and fibers . Polymers constitute the basic materials of the rubber and plastic industries and are of vital importance to the textile, petroleum, automobile, paper, and pharmaceutical industries . Their viscoelastic properties determine the mechanical performance of the final products of these industries, and also the success of processing methods at intermediate stages of production . </P>

When a contrast medium is described as aqueous this means that