Spinodal Decomposition: Exploring Crossover Points & Metastable States

[Urmi Roy]

Referring to first figure attached, if we put a system of a certain composition at a T and P corresponding to a point inside the spinodal, will it (with very small fluctuations) just jump to the corresponding binodal compositions at that temperature and pressure? As per this figure, the system would assume compositions that we could tell by drawing a horizontal line and seeing which values of x of each component takes.

However on the phase diagrams for, say, water (ref 2nd figure) if we put a system in the spinodal region, it becomes either liquid or gas... what exactly is the difference between the above two cases?

Another thing I was wondering about, is that a system while moving away from the spinodal region need not necessarily move to the binodal composition..isn't it possible for it to just sit in a metastable state or on the spinodal curve itself?

 

[_coyote_]

The difference between the two cases is that in the first figure, the composition of the system is limited to only two components, whereas in the second figure, the composition of the system includes more than two components. In the first case, the system will only jump to the corresponding binodal compositions at that temperature and pressure, as indicated by the horizontal line on the phase diagram. In the second case, the system can assume either liquid or gas form depending on the temperature and pressure, and not necessarily move to the binodal composition.Yes, it is possible for a system to just sit in a metastable state or on the spinodal curve itself, while moving away from the spinodal region. This is because the system can be trapped in a metastable state due to kinetic effects, or because it cannot overcome the energy barrier that keeps it in the spinodal region.