Phase Separation Experiments Near the Critical Point

Abstract

Those who study phase separation in fluids, alloys, glasses, and liquid mixtures have found it useful to classify the process as being either nucleation1 or spinodal decomposition,2,3 These processes are respectively associated with metastability and instability. When a simple fluid, initially in one phase, is quenched into a metastable state, incipient droplets of the second phase will spontaneously grow in size provided they are large enough. Droplets of subcritical radius collapse and disappear due to surface tension. Homogeneous nucleation occurs when local thermodynamic fluctuations produce a sufficient number density of critical sized droplets or bubbles. The phenomonon is associated therefore with the presence of a thermodynamic barrier. This barrier (or more accurately, a saddle point)4,5 must be crossed if phase separation is to occur. Experimentally one finds that if a system is supercooled by an amount δT, phase separation does not take place until a certain critical value δTN is reached. Then the nucleation rate (in droplets/sec) suddenly changes from an unmeasurably small to an unmeasurably large value, and phase separation takes place.