SHEAR VISCOSITY OF HEXANE PLUS TETRADECAFLUOROHEXANE NEAR THE UPPER CRITICAL END-POINT

The shear viscosity of hexane+tetradecafluorohexane has been determine d from five Kelvins below the upper critical solution temperature T-c - in the region of limited miscibility - to 19 Kelvins above T-c - in the region of total miscibility. The results show a critical anomaly d escribed by a critical expression eta = A exp (B/T)epsilon(-y) with a multiplicative amplitude or background term eta(0) = A exp (B/T) and a weakly anomalous term epsilon(-y), where epsilon is the reduced tempe rature response function \T-T-c\/T-c calculated with T-c = 22.38(5) de grees C, and the critical exponent y takes the universal value 0.040 p redicted by the mode-mode and renormalisation group theories. The meas ured viscosities show a definite but small dependence on shear gradien t. The shear gradient dependence is entirely described by the Oxtoby t reatment which yields an estimate of the critical amplitude of the cor relation length xi(0) = (5.5 +/- 1.5) Angstrom in fair agreement with the light-scattering value of Skripov et al. of (3 +/- 1) Angstrom. Th e ratio of the limiting amplitudes along the isochoric path and the co existence path is 0.946+/-0.005, close to that predicted by Calmettes, 0.950.