Materials Rheology: An Overview
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quid crystals, and many others. What thèse materials hâve in common is the complexity of their rheological r e s p o n s e to either flow or déformation. One might say that rheologists study rheologically interesting and sometimes useful materials.
While reading through thèse articles, look for the interplay of the concepts of molecular or domain structures and the flow and déformation of the bulk materials. Examples of interesting rheological effects can be found with the toy, Silly Putty™, as described by A. Wineman. Placed on a table, Silly Putty will flow like a liquid under the force of gravity but will b o u n c e like a rubber bail if thrown against a hard surface. A useful rheological effect cornes with the addition of small a m o u n t s of a watersoluble polymer, polyethylene oxide, to water. The resulting "rapid water" has much higher rates of flow in fire hoses at the same pressure drop compared to water without the polymer. Rheological properties are also important for optimizing the drilling of muds in oil fields, obtaining consistent texture (or response to chewing) in food, and developing product formulations that allow rapid, stable fabrication and processing. In manufacturing paints, the control of rheology is important in the formulation of a dripless and spatterless p r o d u c t , t h e d e s i r a b i l i t y of which can be attested to by weekend painters. The subséquent articles in this issue also note other important rheological effects.
There are several common approaches to studying the rheology of materials, and they can be divided into studies of averaged behavior (e.g., viscosity), corrélations and computations with molecular and structural parameters, and models. Models can be further classified. Some models a r e d e r i v e d from mathematical constructs alone, and some from a combination of mathematical and physical models. Import a n t rheological p a r a m e t e r s such as viscosity are used in many of thèse models to describe average behavior. Thèse parameters are generally neither constant nor linearly related to independent variables such as shear rate or shear stress. The parameters are extremely useful in themselves and can be obtained from experiments without regard to the molecular structure of the materials. Even for complex rheological measurements such as the transient responses of materials, the underlying molecular structure is sometimes ignored. As we shall see, for the best und e r s t a n d i n g of materials, especially those that show complex nonlinear rheological relationships, it is important to include the molecular s t r u c t u r e . Integrating knowledge of the molecular s t r u c t u r e w i t h t h e p r é d i c t i o n s of appropriate models helps relate rheological m e a s u r e m e n t s to molecular structure. Rheology is one of many ways to examine the structure of materials. Dynamic rheological tests (described in Wineman's article) are often used as a measure of the interactions among structures such as molécules or other associations. Like using
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