Rheometry

Rheology is the science that studies the flow and deformation of soft, complex matter. Its aim is to understand, model, and predict the behavior of such systems. One of the main outcomes of rheology is the elaboration of constitutive equations, that is, relationships between the stress state in a material and the applied deformation history. They are characterized by the presence of the so-called material functions, which are the physical parameters that quantitatively describe the rheological behavior. An official, very complete nomenclature of rheological material functions is provided by the American Society of Rheology.1 Throughout this document, the symbols adopted in the nomenclature cited in the preceding text are used. Rheometry is the experimental, metrological counterpart of rheology. Its main objective is to measure the material functions studied by rheology. This is accomplished by the use of specific apparatuses, the rheometers, where materials undergo specific stress or deformation histories. Here, the main aspects of rheometry are presented and discussed. In order to better understand the present state of rheometry, some historical perspective must be briefly put forward. Modern rheometry was born with the study of non-Newtonian fluids, whose viscosity is a function of the velocity gradient. During the course of the last century, rheology became involved with increasingly complex materials such as viscoelastic liquids, viscoelastic solids, gels, and many others. Most of such materials, which can be classified under the term soft matter, can display either a liquidlike or solid-like behavior, depending on the deformation history and the type and intensity of the applied external fields (forces, magnetic, and electric fields). Correspondingly, rheometry techniques have become more and more sophisticated. In this document, many relevant issues related, but not central, to rheometry will not be considered. One example is the problem of environmental control, especially temperature control. For more in-depth information on such issues, it is suggested to refer to some of the further readings listed at the end of this article.