The Reinhardt Group

When designing practical applications of materials, it is rather useful to know to what extent they are thermodynamically stable. For example, the use of tin to seal fuel cans has been alleged to have contributed to the failure of Robert Scott's second expedition to the South Pole: metallic β-tin is thermodynamically unstable with respect to the brittle α-tin below 13 °C. The transition does not generally occur at 13 °C however, since β-tin can remain metastable to significantly lower temperatures.

Water is one of the most well-studied substances in the physical sciences. This is not surprising in the light of its importance to our everyday lives, and indeed to life itself. Water is the only common substance that appears in all three basic states of matter – vapour, liquid and solid – under everyday conditions. Solid water has an especially large number of polymorphs, although some only appear at very high pressures that are not easily accessible to experiment.

Inside cells, it has been shown that there are regions where the local concentration of some molecules is very considerably higher than elsewhere. Such membraneless compartments are sometimes thought to result from compositional (‘liquid–liquid’) phase separation of proteins and nucleic acids, which is a physical process similar to how oil and water mixtures separate into oil- and water-rich regions.

In the macroscopic world, we would never expect that if we were to shake a box containing parts of a model aeroplane, a perfectly assembled aeroplane would pop out. At the microscopic scale, however, this sort of behaviour is not quite as uncommon: for example, many viruses assemble their capsids spontaneously, without any external energy inputs.

If a system is placed between a hot and a cold reservoir so that a temperature gradient is established, it is natural to expect that there will also be a density gradient, since systems are less dense at higher temperatures. It is perhaps less obvious that anisotropic molecules will also adopt, on average, a preferred net orientation.