Crystals are enormously important in day-to-day technological applications from semiconductors used for solar energy generation, through paint, to pharmaceutical drugs. Crystals of a particular molecule typically come in a number of variants referred to as “polymorphs” and it is often important which polymorph is formed when a crystal nucleates. For example, in the case of pharmaceutical drugs, different polymorphs will dissolve at different rates, which implies that a particular drug may dissolve in your stomach, your belly, or not at all, which could mean that it kills you, cures you, or does nothing. So, controlling crystal nucleation and controlling which polymorph is formed is a very practically important issue. Unfortunately, crystal nucleation is still poorly understood and even more poorly controlled.
In the late 1990s and early 2000s, it was shown that a pulsed laser can be used to induce nucleation of crystals in a supersaturated solution. Most excitingly, it was reported that the laser polarisation could be used to control which polymorph would nucleate, promising an unprecedented degree of control. In the 2010s, it was shown that non-pulsed and therefore much cheaper lasers could be used to nucleate crystals and also control the polymorph. This suggested that cheap lasers might one day be used in industry to crystallise compounds of commercial interest.
However, it was not at all clear how laser-induced crystal nucleation worked, what the physics behind it is. Without understanding the physics of a process, you cannot predict what it will do, how it will work, or how to improve it. First it was suggested that the laser could simply line up molecules through an effect discovered in 1875 by Glasgow scientist the revered John Kerr. However, this effect is much too weak on the level of individual molecules. In the 2010s, it was suggested that the laser might interact with clusters of molecules. But where would these clusters come from?
In 2018, we figured out that you could exploit separation of phases, a bit like separating oil and water. Oil and water don’t normally mix but would mix at a high enough temperature. At the precise temperature and concentration point where the phases start to mix, is the so-called critical point. Near this critical point, it is really easy to manipulate matter, for example, with a laser. This idea formed the basis of the CONTROL research programme.
