| Abstrakt: |
Shear viscosities are reported for pure liquid methylcyclohexane (MCH) from 298.610 K to 333.694 K, for perfluoromethylcyclohexane (PFMCH) from 319.196 K to 333.114 K and for a MCH PFMCH mixture of overall PFMCH near-critical mole fraction, xc 0.3640, from (TUCS/K − 7) in the region of biphase liquid coexistence to (TUCS/K 20) in the uniphase region, where TUCS 320.13 K is the air-saturated upper liquid−liquid critical solution temperature. The measurements were made using a capillary rheometer that permits the measurement of the viscosity of thermally equilibrated coexisting-liquid phases. The results confirm that the near-critical viscosity exhibits a weak enhancement that strictly speaking becomes a divergence when account is taken of the finite shear gradients in the capillary during measurements. The viscosity of the uniphase mixture of critical composition is well-described after shear gradient correction by a multiplicative combination of an Arrhenius background and a critical power expression with an index close to the now-accepted universal value y 0.0435. The chief objective of the work, in addition to contributing to knowledge of this aspect of near-critical rheology, is the development for the biphase of a simple expression for the temperature dependence of the viscosities of the coexisting phases, and -, that combines (a) an expression for the viscosity diameter <> 1/2(-) similar to that for the viscosity of the critical mixture in the uniphase region, with a similar best critical index y‘ between 0.041 and 0.0435, and (b) an expression for (− -) that behaves like an order parameter, with an index very close to the normal value 0.325 and as many Wegner correction terms as the data require. The best two-phase fit emerges from a freely fitted exponent y‘ 0.037 with one Wegner-extended scaling term, but we believe that were shear gradient correction to be applied, the best y‘ would be the consensus value y‘ 0.0435. The magnitude of y‘ notwithstanding, we believe that our primary objective has been satisfied, namely, the formulation of an expression that affords a good description of the shear and background viscosities of near-critical mixtures in the one- and two-liquid phases in relation to our estimates of the nano- or molecular-viscosity derived from measurements of fluorescence polarization decay rates. |
