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 (${\mathit{T}}_{\mathit{L}}$) for samples of varying loop sizes (ΔT). An exact doubling of the critical exponent (γ) was observed over the entire experimental temperature range (${\mathit{T}}_{\mathit{L}}$-T) for a sample with ΔT as small as 250 mK. For intermediate ΔT ’s (22≥ΔT≥1 °C), a smooth crossover from the double (2γ) to the single exponent (γ) was noticed, as t [=‖(${\mathit{T}}_{\mathit{L}}$-T)/${\mathit{T}}_{\mathit{L}}$‖]→0. The universal value of γ (=1.24) was recovered for experimen- tal paths terminating in ${\mathit{T}}_{\mathit{L}}$ for any ΔT, provided the field variable used was ${\mathit{t}}_{\mathit{U}\mathit{L}}$ [=‖(${\mathit{T}}_{\mathit{U}}$-T)(${\mathit{T}}_{\mathit{L}}$-T)/${\mathit{T}}_{\mathit{U}}$ ${\mathit{T}}_{\mathit{L}}$‖] instead of t. This field variable (${\mathit{t}}_{\mathit{U}\mathit{L}}$) follows naturally from the geometrical picture as well as the Landau-Ginzburg theory, as applied to the reentrant phase transitions.