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Rheology (/riːˈɒlədʒi/; from Greek ῥέω (rhéō) 'flow' and -λoγία (-logia) 'study of') is the study of the flow of matter, primarily in a fluid (liquid or gas) state but also as "soft solids" or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an applied force.[1] Rheology is the branch of physics that deals with the deformation and flow of materials, both solids and liquids.[1]
The term rheology was coined by Eugene C. Bingham, a professor at Lafayette College, in 1920 from a suggestion by a colleague, Markus Reiner.[2][3] The term was inspired by the aphorism of Heraclitus (often mistakenly attributed to Simplicius), panta rhei (πάντα ῥεῖ, 'everything flows'[4][5]) and was first used to describe the flow of liquids and the deformation of solids. It applies to substances that have a complex microstructure, such as muds, sludges, suspensions, and polymers and other glass formers (e.g., silicates), as well as many foods and additives, bodily fluids (e.g., blood) and other biological materials, and other materials that belong to the class of soft matter such as food.
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. The large class of fluids whose viscosity changes with the strain rate (the relative flow velocity) are called non-Newtonian fluids.
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. For example, ketchup can have its viscosity reduced by shaking (or other forms of mechanical agitation, where the relative movement of different layers in the material actually causes the reduction in viscosity), but water cannot. Ketchup is a shear-thinning material, like yogurt and emulsion paint (US terminology latex paint or acrylic paint), exhibiting thixotropy, where an increase in relative flow velocity will cause a reduction in viscosity, for example, by stirring. Some other non-Newtonian materials show the opposite behavior, rheopecty (viscosity increasing 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.[1]
The experimental characterisation of a material's rheological behaviour is known as rheometry, although the term rheology is frequently used synonymously with rheometry, particularly by experimentalists. Theoretical aspects of rheology are the relation of the flow/deformation behaviour of material and its internal structure (e.g., the orientation and elongation of polymer molecules) and the flow/deformation behaviour of materials that cannot be described by classical fluid mechanics or elasticity.
- ^ a b W. R. Schowalter (1978) Mechanics of Non-Newtonian Fluids Pergamon ISBN 0-08-021778-8
- ^ James Freeman Steffe (1 January 1996). Rheological Methods in Food Process Engineering. Freeman Press. ISBN 978-0-9632036-1-8.
- ^ The Deborah Number Archived 2011-04-13 at the Wayback Machine
- ^ Barnes, Jonathan (1982). The presocratic philosophers. Routledge. ISBN 978-0-415-05079-1.
- ^ Beris, A. N.; Giacomin, A. J. (2014). "πάντα ῥεῖ : Everything Flows". Applied Rheology. 24: 52918. doi:10.3933/ApplRheol-24-52918. S2CID 195789095.