Abstract
Liquid kinetics are generally described in terms of shear viscosity, which is defined as the ratio of the shear stress to the shear strain rate. Viscosity varies by over twelve orders of magnitude as a liquid is cooled from high temperatures. A combination of different experimental characterization techniques is required to measure a viscosity versus temperature curve. Of particular note are the rotating cylinder method at high temperatures, the parallel plate method near the softening point, and the beam-bending method at low temperatures. Angell classified liquids as either strong or fragile depending on whether they exhibit Arrhenius or non-Arrhenius scaling of viscosity with temperature. The degree of non-Arrhenius scaling is characterized by the fragility index. The temperature dependence of viscosity is captured by several models, including the Vogel-Fulcher-Tammann (VFT), Avramov-Milchev (AM), and Mauro-Yue-Ellison-Gupta-Allan (MYEGA) equations. The VFT equation leads to an unphysical divergence of viscosity at finite temperature, and the AM equation leads to an unphysical divergence of configurational entropy in the limit of infinite temperature. The MYEGA equation is derived from the Adam-Gibbs model of viscosity, which assumes that the liquid is composed of cooperatively rearranging regions. Special cases of viscous flow include the fragile-to-strong transition, where a liquid converts from fragile behavior at high temperature to strong behavior at low temperatures. Also, with non-Newtonian fluids, the viscosity itself varies with the applied stress.
Published Version
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