chapter  2
26 Pages

Introduction to Rheology

WithN. Naé Hemi

Rheology is the study of how materials deform and flow under the influence of external forces. The name rheology, from the Greek word rheo (= flow), was suggested by M. Reiner and E. C. Bingham, who are considered the founders of this scientific discipline. Rheological studies are important in the manufacture and applications of plastic materials, lubricating materials, coatings, inks, adhesives, food, pharmaceuticals, cosmetics, and toiletries. Consider a simple experiment: Brush a spoonful of honey on a paper and a spoonful of mayonnaise on another paper. Hang the papers vertically and watch. Honey starts to flow immediately and drips, while mayonnaise does not flow. The flow in this case occurs under the influence of gravitational force. Why then is it more difficult to stir a jar of honey with a spoon than it is to stir a jar of mayonnaise? What happens under other types of external forces, such as when one is painting a wall or when one is blending a formulation in a mixer? To characterize rheological behavior of materials, we need first to define some terms. Consider a material that is subjected to a shearing force as seen in Fig. 1. At rest, the material has a thickness x, length l 0 and width w 0. Assume that the bottom is stationary and the top is movable. Due to the stress, the upper layer is displaced by dl and the thickness by dx. The pulling action is the shear stress (σ), which is defined as the force F over the area A (= 10 · w 0):10 σ = F A