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 th e ionic system tetrabutylammonium picrate + 1-tridecanol (T-c = 341.84 5 K) which shows a large mean-field region in its static critical prop erties with crossover to Ising behavior dose to the critical temperatu re. 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 g amma = 0.0434+/-0.001 which is close to the current best value for the 3d-Ising model, gamma = 0.041. At higher reduced temperatures pronoun ced deviations from Ising criticality are observed. In agreement with the mean-field prediction gamma = 0, the critical contribution is vani shing. This crossover is discussed in the framework of current theory.