Rheology is the study of flow and deformation of materials, both solids and liquids. Deformation and flow are referred to as strain or strain rate, respectively, and indicate the distance over which a body moves under the influence of an external force or stress. For this reason, rheology is also considered to be the study of stress-strain relationships in materials.
Rheology generally accounts for the behavior of non-Newtonian fluids (i.e. fluids whose viscosity does change with flow conditions and isn't described by a single value) by characterizing the minimum number of functions that are needed to relate stresses with rate of change of strain or strain rates. A rheometer is used during the rheological analyses of materials. Rheometer is a precision instrument that contains the material of interest in a geometric configuration, controls the environment around it, and applies and measures wide ranges of stress, strain, and strain rate.
Rheology has many 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, 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.