Phase separation of a critical binary mixture subjected to a temperature gradient

We present experimental results on phase separation of a binary mixture of isobutyric acid and water in a thin horizontal, extended layer at the critical concentration, xc, and in the vicinity of the consolute temperature, Tc, subjected to a vertical temperature gradient spanning the critical temperature. For relatively small temperature gradients, spinodal decomposition-like patterns are stabilized. A bubble pattern appears for slightly larger temperature gradients, suprisingly always near the hotter boundary, even when Thot >Tc. For still larger temperature gradients, polygon morphologies are observed. Their boundaries are probably formed by some kind of surface tension driven instability caused by the nonuniform surface tension along the bubble's interface. However, hydrodynamic instabilities alone have not been able to explain the novel morphologies. The average area of the cellular patterns varies strongly with Tc - Tcold and ΔT across the fluid layer, whereas the mean area of the bubble like patterns changes just slightly.