A. Introduction
(by R. C. Armstrong)

Many fluids do not obey Newton’s law of viscosity; these materials are commonly grouped together under the broad heading of non-Newtonian fluids. Examples of non-Newtonian fluids include polymer solutions and melts, paints, soaps, biological fluids, greases, pastes, and suspensions. The field of study that is aimed at understanding the deformation and flow behavior of these substances is known as rheology. Because they make up the most commonly encountered subgroup of non-Newtonian materials, polymers will be emphasized in this section.

There are many simple experiments that can serve to illustrate the often dramatic differences in behavior between Newtonian and non-Newtonian fluids. If, for example, the "viscosity" of a polymer solution were determined in a falling-ball viscometer and in a tube flow experiment, different results might be obtained. If a rotating shaft is inserted into a beaker of polymeric fluid, the polymer "climbs" the rod. Polymer extruded through a circular orifice may swell to a diameter several times that of the hole. If a filament of molten polymer is suddenly stretched and then released, it will snap back nearly to its original length (Bird et al., 1977).

Because of the very high viscosities exhibited by most concentrated polymer solutions and melts, the flow of these fluids is laminar in most applications, and laminar flow will be the primary focus of this section. In Section B, experimental methods for characterizing non-Newtonian fluids are described. Section C then presents models for describing these properties, and Section D gives examples to illustrate the calculation of quantities of engineering importance. In Section E, turbulent tube flow of non-Newtonian fluids is discussed.

B. Experimental characterization of non-Newtonian fluids
(by R. C. Armstrong)