What Is Rheology?
Perhaps you have not heard of the term or if you have you may be a little vague! Anything related to the flow of materials might be one explanation; whilst fluids often predominate in the activities of rheologists, it must not be forgotten that solids are also included, as are gaseous materials. The origin of the term is generally attributed to E.B.Bingham in 1929. Roger Tanner in his “Engineering Rheology” gives Bingham’s classic definition as follows: “The study of the deformation and flow of materials”. However matters go back a lot further. Newton gave his name to a class of fluids that exhibit a particular mode of flow behaviour (Linear or Newtonian Flow) and it was Heroclitus in Ancient Greece who stated: “Everything Flows”. Neil Cogswell in his book “Polymer Melt Rheology” reminded us of the Biblical Song of Deborah and Barak: “The mountains melted from before the Lord” (Judges 5:5), from which we obtained the Deborah Number as the characteristic time of a process. Indeed, as we
Rheology is the study of the flow of matter. It is classified as a physics discipline and focuses on substances that do not maintain a constant viscosity or state of flow. That can involve liquids, soft solids and solids that are under conditions that cause them to flow. This includes substances that tend to be very complex, including bodily fluids, sludge and suspensions. Coined in 1920, the term “rheology” takes its name from the Greek words “rei,” meaning flow, and “ology,” meaning study. The study is also called “Non-Newtonian fluid mechanics” because it was English physicist Isaac Newton that originated the concept of viscosity. Rheology studies those substances that don’t follow the rules of viscosity. Take, for example, a jar of peanut butter. When the jar is opened, the top surface of the peanut butter retains the shape that was left by the last person who used it. It can do so for months or even longer. Peanut butter–like mayonnaise, paint or molten plastics–doesn’t follow N
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 y
