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 (TL) for samples of varying loop sizes ({Δ}T). An exact doubling of the critical exponent ({γ}) was observed over the entire experimental temperature range (TL-T) for a sample with {Δ}T as small as 250 mK. For intermediate {Δ}T 's (22{≥}{Δ}T{≥}1 ^∘{}C), a smooth crossover from the double (2{γ}) to the single exponent ({γ}) was noticed, as t [={}(TL-T)/TL{}]{→}0. The universal value of {γ} (=1.24) was recovered for experimen- tal paths terminating in TL for any {Δ}T, provided the field variable used was tUL [={}(TU-T)(TL-T)/TU{T}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.