Molecular rheology

Article By:

Cummings, Peter T. University of Tennessee, Knoxville, Tennessee; Chemical Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee.

McCabe, Clare Department of Chemical Engineering, Colorado School of Mines, Golden, Colorado.

Last reviewed:2002

DOI:https://doi.org/10.1036/1097-8542.YB020595

The friction between two moving surfaces, such as a piston in an automobile engine and the cylinder surrounding it, is routinely reduced by a lubricant between the surfaces. The viscosity of a lubricating fluid is determined by measuring the force (per unit area) of the surface required to maintain the surfaces moving at a constant relative speed: the higher this force, the higher the viscosity (Fig. 1). Another way to determine viscosity is to measure the terminal speed with which a metal sphere falls through a column of the fluid: the lower the terminal speed, the higher the fluid's viscosity. Viscosity thus measures a fluid's ability to resist flow. Clearly, water is much less viscous than motor oil, which in turn is much less viscous than a typical polymer melt (such as polyethylene). Moreover, the viscosity of a typical polymer decreases as the shear rate (the ratio of the relative velocity of the two moving surfaces to the distance between them) increases, which is known as shear thinning. Shear thinning behavior is an example of non-newtonian behavior. Ultimately, the differences in the viscosity of these fluids must reflect differences in the shape of the molecules in the fluids and the forces between the molecules. For example, polymer molecules are long and can become entangled, thus yielding a high viscosity; as they are sheared at high rates, they elongate and become unentangled, resulting in non-newtonian shear thinning behavior. The field of molecular rheology aims to understand and predict these rheological properties, particularly viscosity, based on molecular forces and architecture, using theory and/or molecular simulation.

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