1.1. Rheology:

            Rheology is a branch of science which deals with the flow and deformation of matter. It is all about the response of materials to the applied stress. Knowledge of rheology is necessary since we have attempted to focus our attention on the concepts necessary for purpose of the present monograph, rather than to provide an extensive account of the wide topic which rheology has become nowadays.  Control of rheology is essential for the manufacture and handling of numerous materials and products. Before that we must understand the rheology and ability to measure rheological properties. We all know that flow is irreversible action, when the stress is removed. The material does not revert to its original form. By this action we understood that work is converted to heat. At the same time thermodynamics of fluids under flow comes in to picture. The flow properties of a liquid are defined by its resistance to flow i.e. viscosity and may be measured by determining the rate of flow, the resistance of flow when the fluid is sheared between two surfaces. There has been significant appreciation of the importance of rheology in the industrial field; outstanding researchers have been done in the field of bio-rheology, rheology in polymers and in suspension rheology.

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1.2. History of Rheology:

            The term “Rheology” is coined by professor Bingham of Lafayette College, PA. In 1678, Robert Hooke introduced the theory called “True theory of Elasticity”, which gives the theory behind the classical elasticity. Till the date fluid like materials, are concerned, an interfacial contribution came in 1867 from the article named “On the dynamic theory of Gases”. Author of this article was James Clerk Maxwell; his article proposed the mathematical model for the fluid exhibits some elastic properties.

1.3. Non Linearity in Rheology:

            Elastic and viscous behaviour of fluids obeys the laws of Hooke’s and Newton respectively. Both these laws are linear, which exhibits direct proportionality between stress and strain. Within these linear systems, much rheological behaviour can be accommodated. It is observed that, material behaviour such as modulus of rigidity and viscosity can vary with the applied stress and the respective stress need not be high. This change can be instantaneous or even may take long time. Shear thinning process is the form of non linearity. This is the reduction of viscosity by increasing the shear rate in steady flow. Special term for time dependent shear thinning is followed by recovery “Thixotropy”. Shear thinning is just non linear behaviour. It is not worth to study the rheology without understanding the proper importance of non linearity.