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 dose to the critical temperature. After suitable assessment of the non-singular background viscosity we establish that the asymptotic scaling law eta proportional to tau(-gamma) applies in the region tau<5x10(-3), where tau = (T-T-c)/T-c is the reduced temperature. The value of the exponent is determined to gamma = 0.0434+/-0.001 which is close to the current best value for the 3d-Ising model, gamma = 0.041. At higher reduced temperatures pronounced deviations from Ising criticality are observed. In agreement with the mean-field prediction gamma = 0, the critical contribution is vanishing. This crossover is discussed in the framework of current theory.