# Spinodal

Entry by Emily Redston, AP 225, Fall 2011

Please see phase separation for an introduction to the regular solution model phase diagram we will be focusing on.

Figure 1 shows the free energy of mixing as a function of composition for one portion of the two-phase region. Let's look at composition <math>\phi_a</math> with free energy <math>F_a</math>. Small fluctuation in composition create a higher energy state <math>{F_{a}}^'</math>, and this composition is therefore considered a metastable state. On the other hand, composition <math>\phi_b</math> is unstable since small fluctuations in composition create a lower energy state, from <math>F_b</math> to <math>{F_{b}}^'</math>. A quick way to visualize stability can be seen in Figure 2. Point (1) is metastable because small fluctuations lead to higher energy states, but if it gets enough energy to make it over the barrier, it can move to a lower energy state. Point (2) is unstable because any fluctuations will cause it to move to a lower energy state. Point (3) is stable.

For every free energy curve, we can find two compositions in the 2-phase region corresponding to the transition from the metastable compositions to unstable compositions -- that is to say, the inflection points on the free energy versus composition curves. Let's consider what the spinodal looks like for the regular solution model. We can find the spinodal line by looking at

By taking this derivative and solving for <math>T_{s}</math> (below which the homogeneous mixture passes from a metastable to an unstable state), we will define a phase boundary on our temperature versus composition phase diagram (Figure 3).

Figure 3 is the phase diagram of a two-component liquid system as described by the regular solution model. The outer blue/purple curve is the phase boundary; within this curve, phase separation occurs. The inner pink curve is the spinodal, and it is where the homogeneous mixture passes from a metastable to unstable state. In summary: (1) outside both curves, the homogenous mixture is *stable*, (2) the homogenous mixture is *metastable* once we are in-between our two derived curves, and (3) the homogeneous mixture is *unstable* once we are within the spinodal line and phase separation will always occur. As a brief aside, the name spinodal, which means "thorn" in Latin, refers to their shape in one-component systems [1].

See also:

Spinodal in Phases and Phase Diagrams from Lectures for AP225.

## References

[1] Spaepen, Frans. *Applied Physics 282: Solids: Structure and Defects*. Harvard University

[2] Haasen, Peter. *Physical Metallurgy*. Cambridge: Cambridge UP, 1996.