Spinodal decomposition

Spinodal gas-liquid separation in a square well fluid, proceeding clockwise from top-left. The initially homogeneous fluid separates into a dense liquid and a less dense gas.

I recently had reason to do one of the most fun parts of any PhD, which is to make pictures.  This one is of phase separation into gas and liquid regions, in a fluid made of solid particles surrounded by short range attractions. Just by moving around at random (Brownian motion) the particles collectively ‘realise’ that they can lower their free energy by splitting off into two distinct phases, so they do.

Often, gas bubbles form within a fluid by ‘nucleation’ — the formation and subsequent growth of a nucleus of gas of some critical size that gets bigger and bigger until, perhaps, it bubbles away because it’s less dense than the liquid. This process often involves the bubble initially forming on some kind of nucleation seed (e.g. a tiny imperfection on the inside of a champagne glass).

In other cases, the initial fluid might be so supersaturated that phase separation is possible no matter how small the amount of the new phase that is formed. In that case, gas-liquid separation happens everywhere throughout the fluid, you get spinodal decomposition, and observe a complex interlocking pattern of the two phases, which gradually coarsens, as seen clockwise from top-left in the above picture.

My science page has more, a Windows demo of some code for simulating crystal formation, and pretty soon should have quite a bit of new stuff — things are busy at the moment.

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