The term rheology was coined by Eugene C. Viscosity and rheology pdf, a professor at Lafayette College, in 1920, from a suggestion by a colleague, Markus Reiner. Newtonian fluids can be characterized by a single coefficient of viscosity for a specific temperature. Although this viscosity will change with temperature, it does not change with the strain rate.

Only a small group of fluids exhibit such constant viscosity. Rheology generally accounts for the behavior of non-Newtonian fluids, by characterizing the minimum number of functions that are needed to relate stresses with rate of change of strain or strain rates. Some other non-Newtonian materials show the opposite behavior, rheopecty: viscosity going up with relative deformation, and are called shear thickening or dilatant materials.

Since Sir Isaac Newton originated the concept of viscosity, the study of liquids with strain rate dependent viscosity is also often called Non-Newtonian fluid mechanics. The experimental characterization of a material’s rheological behaviour is known as rheometry, although the term rheology is frequently used synonymously with rheometry, particularly by experimentalists. Materials with the characteristics of a fluid will flow when subjected to a stress which is defined as the force per area.

Much of theoretical rheology is concerned with associating external forces and torques with internal stresses and internal strain gradients and flow velocities. The study of the physics of continuous materials with a defined rest shape.

Describes materials that return to their rest shape after applied stresses are removed. Describes materials that permanently deform after a sufficient applied stress. The study of materials with both solid and fluid characteristics.

The study of the physics of continuous materials which deform when subjected to a force. Non-Newtonian fluids do not undergo strain rates proportional to the applied shear stress. Newtonian fluids undergo strain rates proportional to the applied shear stress. In this sense, a solid undergoing plastic deformation is a fluid, although no viscosity coefficient is associated with this flow.

Granular rheology refers to the continuum mechanical description of granular materials. These experimental techniques are known as rheometry and are concerned with the determination with well-defined rheological material functions. Such relationships are then amenable to mathematical treatment by the established methods of continuum mechanics. The study of extensional flows is called extensional rheology.