Analysis of Viscous Relaxation in Critical Oxide Mixtures

An analysis of shear viscosity and ultrasonic measurements on a series of critical oxide mixtures shows that the supercritical, excess static viscosity is associated with a broadening in the distribution of structural relaxation times, rather than with an interaction between the long-wavelength components of the fluctuations and the shear flow processes. The broadening saturates near the critical point, and the excess static viscosity approaches a finite limit as a function of reduced temperature. These results are in disagreement with existing excess-viscosity theories. A new model is proposed which relates the distribution of relaxation times to microstructure in the melt resulting from the supercritical fluctuations in composition. It also limits the range of interaction associated with structural relaxation to a finite value. The resulting conclusions are consistent with the observed lack of divergence in the excess viscosity and width of the distribution of relaxation times at Tc.