Rheology is the branch of physics that deals with deformation, including flow, of matter. Although this definition was proposed in 1929, the recognition of rheological phenomena dates back to antiquity.
Rheological principles stem from two fundamental laws derived in the late seventeenth century: Robert Hooke’s law of elasticity (ca. 1676) and Isaac Newton’s law of flow (1687).
The flow of liquids by parallel layers, moving past each other and dragging adjacent layers along, is called laminar or streamline flow. At higher velocities and/or if the plates have rough surfaces, eddies or swirls develop, whereby mass transfer occurs from one layer or lamina to another. Theoretically, this complex phenomenon—referred to as turbulent flow—may be described by a set of partial differential equations, known as the Navier-Stokes equations, which govern fluids in motion.
Empirical relationships aside, numerical methods for the characterization of non-Newtonian flow were developed in the 1960s, but computational rheology has emerged to address previously intractable problems, such as three-dimensional transient flows of polymeric liquids, non-isothermal non-Newtonian flows, or turbulent flow of generalized Newtonian and viscoelastic materials.
Many of the instruments designed to measure steady shear flow and the corresponding viscosity do not enable the precise delineation of sample dimensions, the forces exerted, or the resultant deformation or flow. These devices are referred to in this text as viscometers. Those instruments that enable a thorough, geometrically accurate analysis of deformation and flow to be made are referred to as rheometers. The term viscometer should be reserved for a subset of rheometers that measure only Newtonian viscosity. When flow or deformation is not well defined or is indeterminate, the resulting measurements are likely to correspond, at best, to approximations of the true rheological behavior of the sample.