My fellow authors and I have assembled a series of articles on using rheology to understand the structure and processing of a wide range of materials. We approach this task by presenting an introduction to the concepts of rheology and some illustrative applications, followed by a description of linear vis-coelasticity and a description of the rheological behavior of elastic fluids. These concepts and related tools are then used to describe molten polymers, colloidal suspensions, latex systems, electrorheological fluids, and gels. In each of these areas, we show how rheology provides insights into the structure of materials and the type of information required to process them. We also show that the rheological approach spans macroscopic and microscopic domains. These aspects of rheology are of value to materials scientists and engineers. We hope these examples will clarify the relevance of rheology to materials.

If we are successful in this presentation, you will obtain a flavor of the process of rheological studies. While reading through these articles, look for the interplay of the concepts of molecular or domain structures and the flow and deformation of the bulk materials. These articles may even inspire you to examine the role that rheology may play in your own studies.

The word “viscoelastic” is used to describe the mechanical response of materials exhibiting both the springiness associated with elastic solids and viscous flow characteristics associated with fluids. A familiar example of a material called viscoelastic is Silly PuttyTM. If a blob of Silly Putty is rolled into a ball and then dropped onto a hard surface, it will bounce like an elastic ball. If the ball is placed on a hard surface, its own weight will cause it to flow into a puddle. This behavior indicates that time is an intrinsic parameter in discussing viscoelastic response of materials. The elastic response is associated with a contact force of very short duration. The flow into a puddle occurs when forces act for a long period of time.

Viscoelastic response occurs in materials such as soils, concrete, cartilage, biological tissue, and polymers. Soils and cartilage can be thought of as porous solids filled with fluid. Viscous response is due to the flow of the fluid in the pores; elastic response is due to the distortion of the porous solid.

The term “polymeric liquids” is used to describe both solutions of polymers and molten polymers. All polymeric liquids exhibit non-Newtonian flow behavior, including a shear stress-dependent viscosity and elasticity. However, concentrated solutions and melts of polymers whose molecular weights exceed a certain critical value (the “critical molecular weight for entanglement,” Mc) exhibit quite remarkable deviations from classical fluid behavior, especially marked elasticity. Among the remarkable rheological phenomena exhibited by these materials are elastic recoil and flow-induced an-isotropy. Indeed, in certain situations, such materials can exhibit elastic effects that are almost indistinguishable from those exhibited by cross-linked rubbers. This behavior is important, because most commercial “thermoplastics,” such as polyethylene and polystyrene, have high molecular weights (M > Mc) and are processed in the molten state.

A given generic polymer, polyethylene for example, can exhibit a wide range of properties depending on the molecular weight distribution. Another important aspect of molecular structure is branching. For many monomers (the molecular building blocks that make a polymer molecule), two types of polymer structure are possible, linear and branched. For example, ethylene can be polymerized in two ways to form either linear polyethylene or branched polyethylene. Branching enhances the non-Newtonian and elastic aspects of the melt flow behavior. Yet another possible aspect of polymer molecular structure is the presence of a comonomer.

The rheology of dispersions often appears to be extraordinarily complex since these materials traverse all states of matter from simple Newtonian fluids to Hookean solids. Highly nonlinear viscoplastic or viscoelastic responses, with persistent time dependence, often frustrate attempts to address the mechanics without attempting to characterize the composition or understand the thermodynamics. The difficulty lies in the sensitivity of the stresses to the microstructure, which in turn depends on the interparticle forces and the deformation history.

However, combining rheological measurements with other probes of the structure can permit one to anticipate the response under different conditions and, sometimes, to elucidate the relevant forces. Even better, one may learn to manipulate the composition to obtain the rheology desired for a specific process or a particular product, such as thickeners for coatings. The key lies in linking the composition, interparticle forces, and rheology. This review illustrates the current knowledge of those relationships and identifies some sources, from which the bulk of the content comes for those who wish to pursue the subject further.

This brief treatment first surveys phase behavior and interparticle forces. These concepts, combined with dimensional analysis and simple ideas from statistical mechanics, motivate correlations of data for well-characterized, stable dispersions and both weakly and strongly flocculated dispersions.

Numerous commercial products either exist as concentrated suspensions of small particles or involve the processing of concentrated suspensions during some stage of their manufacture. Examples include foods, adhesives and glues, ceramic dispersions, paints, and polymer dispersions such as polyvinyl chloride plastisols. As a result, it is important for engineers to understand the flow behavior of these systems and how the flow behavior affects the way these materials can be processed.

For mahy years, progress in understanding the flow behavior of concentrated suspensions was slow compared to progress on dilute systems, partly because of how the study of suspensions evolved. Building on Einstein's classical work for dilute suspensions of rigid spheres, many authors attempted to modify his equations to predict the flow behavior of more concentrated suspensions, but the extension of Einstein's work met with limited success, because nonhydrodynamic interactions cari be just as important as the hydrodynamic interactions considered by Einstein, and multiple particle interactions quickly complicate the problem as the particle concentration increases.

The influence of electric fields on the deformation and flow properties of materials has been a subject of interest for many years. Recently, there has been renewed interest in a particular branch of these electric field effects—the electrorheological (ER) effect. The ER effect is also known as the Winslow effect after its founder Willis Winslow. Winslow observed that applying strong electric fields to nonaqueous silica suspensions activated with a small amount of water caused rapid solidification of the originally fluid material. This type of behavior was seen as instrumental in the development of high-speed valves, reactive damping systems, and a host of other applications.

We are accustomed to thinking of matter as being in one of three states: solid, liquid, or gas. Nature, however, has provided us with a marvelous intermediate state, the gel. Industrial applications are just beginning to explore the advantageous gel properties as adhesive, superabsorber, damper, membrane, toner, catalyst support, etc. Gels are good adhesives since they combine the surface wetting property of liquids with the cohesive strength of solids. Strong adhesion and damping properties suggest gels as a binder in composite materials. For example, the car of the future may operate at a much reduced noise level due to a thin layer of gel in its body. Widespread technical applications have not yet materialized because, until recently, it was difficult to measure gel behavior. This has changed, and as a consequence, gels can now be manufactured with reproducible properties.

What the many possible types of gelation processes have in common is that molecules connect into a three-dimensional network structure. Junctions between molecules form as chemical bonds (cross-links) or physical associations (such as crystalline or glassy domains, hydrogen bonds, etc.). The molecular architecture determines the gel properties in ways which have yet to be explored. Typical parameters are the monomer building block, molecular size, branching, chain stiffness, cross-link functionality, and solvent content.