Rheology, the study of the flow and deformation of matter, is an old discipline undergoing a renaissance. In its widest sense, it includes classical fluid mechanics and elasticity which treat the flow of Newtonian liquids, such as water, and small deformations of hard solids, such as wood and steel.

The use of the special term "rheology" for these subjects alone would not be justified, since they have been extensively studied for more than 170 years, and are an accepted part of the curriculum in most universities.

In practice, the word "rheology" normally refers to the flow and deformation of "non-classical" materials such as rubber, molten plastics, polymer solutions, slurries and pastes, electrorheological fluids, blood, muscle, composites, soils, and paints. These materials can exhibit varied and striking rheological properties that classical fluid mechanics and elasticity cannot describe. Though the word "rheology" was coined in 1929, the rapid development of the subject began 20 years later.

Today, there are societies of rheology in Argentina, Australia (est. 1983), Austria (est. 1969), Belgium (est. 1974), Britain (est. 1940), Canada (est. 1982), China, Czechoslovakia (est. 1968), France (est. 1955), Germany (est. 1951), Greece (est. 1996), India, Israel, Italy, Japan (est. 1973), Korea, Latvia, Mexico, The Netherlands (est. 1951), the Nordic countries (est. 1956), Poland (est. 1997), Portugal (est. 1997), Romania, Slovenia, the former Soviet Union (est 1964), Spain (est. 1982), Switzerland (est. 1993), and the United States (est. 1929). There is also a European Society of Rheology, an International Society of Biorheology, a Japanese Society of Biorheology and an International Society for Clinical Hemorheology and Microcirculation. Nine journals are devoted to rheology: Rhéologie (formerly Les Cahiers de Rhéologie), Clinical Hemorheology and Microcirculation, Biorheology, Journal of Non-Newtonian Fluid Mechanics, Journal of Rheology, Korea-Australia Rheology Journal, Nihon Reoroji Gakkaishi, Rheologica Acta, Applied Rheology and Rheology Abstracts. The latter lists more than 100 other journals which publish articles on a variety of rheological interests. Polymer Engineering & Science devotes six issues per year to Polymer Processing & Rheology. Every four years, the latest developments are archived in the Proceedings of the International Congress on Rheology.

Rheological research involves activities in and draws on knowledge from biophysics, chemical engineering, chemistry, computer science, electronics, engineering mechanics, materials science, mathematics, mechanical engineering, medicine and physics among others. Rheology's interdisciplinary nature stems from the variety of materials investigated and the many new questions that must be answered.

In classical mechanics, material properties are determined when one or two constants are given (viscosity or elastic modulus); the basic "constitutive equations" governing the behavior of each small material element are known. Further research often involves applying these equations, with the momentum conservation equation, to increasingly complicated flows or deformations.

In rheology, on the other hand, the constitutive equations for most materials are unknown, and frequently involve unknown functions; the form of the equations for visco-elastic materials is so different from the classical forms that one must combine continuum mechanics with molecular theory and test the predictions with measurements from a variety of flows or deformations. The molecular theory requires statistical mechanical modeling with computer analysis. The measurements require inventing new instruments for determining nonlinear viscoelastic properties, stress distributions, elastic recoil and flow birefringence. While intuition suggests that structural characteristics like branching should hinder the flow of molecules, it is hard to incorporate such effects with purely continuum arguments. Recent molecular simulations at the Rheology Research Center have helped explain the effect of molecular structure on rheology from first principles.

The challenge of relating molecular structure or morphology to the macroscopic rheological properties of materials is fundamentally important, whether for molten plastics, polymer solutions, suspensions or composites. Meeting this challenge deepens our understanding of rheological properties, guides improvements in processing technology and helps us produce new materials for specific applications.

Polymer rheology is perhaps the most extensively and successfully studied field in rheology. Considerable progress has been made in developing constitutive equations from molecular models and also in developing special rheometers for testing theoretical predictions in the laboratory. Future rheological research will combine constitutive equations with heat flow equations to solve more complicated flow problems to assist the design of polymer processing equipment.

Rheology thus involves, to an unusual degree, a combination of academic and scholarly activities at an advanced level with the possibility of practical applications in a wide range of industrial situations. These include commercial processing of plastics and rubber, textiles, paper, oil and paints, foodstuffs, adhesives and composites.

Last Modified: Monday, 09-Jun-2008 14:46:37 CDT This website is copyrighted. Address comments to rheology@engr.wisc.edu