Category Archives: Science

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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A science picture

Click the photo for a high-res version

Here is a photo I recently took with my friend Tom. It’s a little tube containing a suspension of colloidal particles (described a bit more on my Science page). Colloids are small but not microscopically tiny, which means that when they form crystals, the typical distances between particles are larger than those found in molecular or atomic structures. In fact, the distances correspond roughly with the wavelength of visible light, so the crystals scatter incoming light of all different colours and, as shown in the picture, look great. Natural and synthetic opals also display this ‘opalescence’, because they are formed by the crystallisation of colloidal sand particles.

I got this little sample when I was on a conference in Corsica and I’m very fond of it, because the only colloidal suspensions I usually get to see are pretend ones in computer simulations. The motivation for having spent a morning with Tom pointing a camera at a tube is, hopefully, to provide a nice green-screen background for an interview I was involved in for the Physics Department’s website. In the interview I managed to talk for 15 minutes about why Leeds is a good place to do a PhD, while forgetting to once mention its music scene. Well done me.

Thanks very much to Tom West for his incredibly steady camera-hands, and patience.