Light scattering near a double critical point: Evidence for crossover behavior

We report measurements of osmotic compressibility in a quasibinary liquid mixture of 3-methylpyridine, water, and heavy water. The measurements probed the lower consolute point $(T_L)$ for samples of varying loop sizes $(\triangle T)$. An exact doubling of the critical exponent $(\gamma)$ was observed over the entire experimental temperature range $(T_L-T)$ for a sample with $\bigtriangleup T$ as small as 250 mK. For intermediate $\bigtriangleup T’s$ $(22\geq \triangle T\geq1 °C)$, a smooth crossover from the double $(2\gamma)$ to the single exponent $(\gamma)$ was noticed, as $t [=|(T_L-T)/T_L|]$ $\rightarrow 0$. The universal value of $\gamma (=1.24)$ was recovered for experimental paths terminating in $T_L$ for any $\bigtriangleup T$, provided the field variable used was $t_{UL}$ $[=|(T_U-T)(T_L-T)/T_UT_L|]$ instead of t. This field variable $(t_{UL})$ follows naturally from the geometrical picture as well as the Landau-Ginzburg theory, as applied to the reentrant phase transitions