Supercritical Viscosity Anomaly in Oxide Mixtures

Abstract

The viscosity anomaly, previously detected above the critical point of some fluid binary mixtures, is investigated in the sodium–borosilicate oxide system. At the sodium–borosilicate dome, the critical composition is selected with small CaO and Al₂O₃ doping additions to obtain samples whose critical temperatures range over 190°C and have minimum high‐temperature structural differences. Viscosities between 10² and 10⁹ P (10⁻¹ N·sec/m²) are measured. A fractional excess viscosity is defined as the difference between the viscosity of each melt and a normal viscosity function, divided by the normal viscosity. The normal viscosity function is taken to have the form of the viscosity curve from the sample with the lowest critical temperature. The fractional excess viscosity then shows an anomalous increase near the critical point. The fractional excess viscosity, ${\mathrm{\Delta \eta \hspace{0.17em}/\hspace{0.17em}\eta }}_0$ , attains a value of 2 near the consolute temperature but appears to remain finite at the critical point. This critical‐point effect can be detected 200°C above the critical temperature. Such results indicate that the viscosity is quite sensitive to the presence of composition fluctuations above the critical point of this system, and represents a good tool for investigating supercritical phenomena.