Critical Viscosity and Ising‐to‐Mean‐Field Crossover Near the Upper Consolute Point of an Ionic Solution

We report on shear viscosity data near the upper consolute point of the ionic system tetrabutylammonium picrate+ 1-tridecanol (T c = 341.845 K) which shows a large mean-field region in its static critical properties with crossover to Ising behavior close to the critical temperature. After suitable assessment of the non-singular background viscosity we establish that the asymptotic scaling law ητ -y applies in the region τ<5x10 -3 , where τ = (T-T c )/T c is the reduced temperature. The value of the exponent is determined to y = 0.0434±0.001 which is close to the current best value for the 3d-Ising model, y = 0.041. At higher reduced temperatures pronounced deviations from Ising criticality are observed. In agreement with the mean-field prediction y = 0, the critical contribution is vanishing. This crossover is discussed in the framework of current theory.