Laser-induced nucleation
Nucleation of crystals by laser light
What is nucleation and why is it important?
Nucleation is the process where a new phase of matter is formed from an existing phase: for example, formation of a crystal from solution. Nucleation is of fundamental scientific interest and has significant economic value, e.g., in production of pharmaceuticals and agrochemicals.
Why is nucleation difficult to study? How can lasers help?
Primary nucleation is difficult to study because it occurs randomly in space and time. Our group use short pulses of laser light to initiate nucleation, in supercooled liquids, supersaturated solutions. In the animated image below you can see crystals being nucleated in real time using a single laser pulse (5 ns). The crystals only form where the beam has passed.
Temporal and spatial control of nucleation
The method we use is called non-photochemical laser-induced nucleation (NPLIN) because no reactions are taking place. Using the method we have demonstrated unprecedented temporal (sub-nanosecond) and spatial (sub-micron) control of nucleation. The image below illustrates the spatial control by nucleation of KCl crystals in an agarose gel. The crystals only form where the laser has passed.
What is the mechanism for non-photochemical laser-induced nucleation?
There are a range of different mechanisms depending on how the laser interacts with the sample. For a laser pulse that is short (nanosecond) and not focussed, experiments suggest that (a) the light causes impurity nanoparticles (NP) to be heated, creating a superheated layer (SHL). (b) The super-heated layer boils spontaneously. The process is known as thermocavitation. (c) It is not known where the crystal nucleates, but some models suggest it happens at the interfacial layer (IL) between liquid and gas.
