PHASE-SEPARATION OF A CRITICAL BINARY MIXTURE SUBJECTED TO A TEMPERATURE-GRADIENT

We present experimental results on phase separation of a binary mixtur e of isobutyric acid and water in a thin horizontal, extended layer at the critical concentration, x(c), and in the vicinity of the consolut e temperature, T(c), subjected to a vertical temperature gradient span ning the critical temperature. For relatively small temperature gradie nts, spinodal decomposition-like patterns are stabilized. A bubble pat tern appears for slightly larger temperature gradients, surprisingly a lways near the hotter boundary, even when T(hot) > T(c). For still lar ger temperature gradients, polygon morphologies are observed. Their bo undaries are probably formed by some kind of surface tension driven in stability caused by the nonuniform surface tension along the bubble's interface. However, hydrodynamic instabilities alone have not been abl e to explain the novel morphologies. The average area of the cellular patterns varies strongly with T(c) - T(cold) and DELTAT across the flu id layer, whereas the mean area of the bubble like patterns changes ju st slightly.