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Forming crystals from supercooled liquids with lasers

One of the things that we are working on in the lab is the study of ice nucleation making use of optically trapped droplets. Nucleation is in the starting point for processes such as freezing and crystallization, and so obviously is of great scientific and industrial importance.

Another of the things that my group is interested in in laser induced cavitation, and we work with Paul Prentice’s group at Dundee’s Institute of Medical Science and Technology to study the formation of bubbles in laser and ultrasound fields – work which may help us gain a better understanding of processes such as the use of high intensity ultrasound to ablate tumours.

So a recent paper in PCCP by Andrww Alexander’s group from just down the road from us, in Edinburgh, caught my interest as it combined these two ideas, looking at the laser-induced nucleation of supercooled acid solutions. Supercooling is the process in which we can cool a substance below the conventional freezing point, due to the absence of nucleation sites. (There are lots of examples of this on youtube, using bottled water – so you can try it at home). In the study here, the glacial acetic acid (acetic acid with almost no water content) used has a melting point of 16.6 degrees Celsius.

The technique is fairly simple, in that a loosely focused nanosecond pulse with an energy of 1mJ is passed through a test tube containing the supercooled acid. At an appropriate peak power density (the peak power of the laser used divided by the area of the beam) a sample can be made to nucleate, and the fraction of samples that nucleate increases as a function of the peak power density. The samples are cooled down to -9 degrees Celsius at which temperature the laser experiments are carried out. The freezing process happens very rapidly after the intial nucleation is formed, with the freezing complete within half-a-second:

Image of supercooled glacial acetic acid after illumination by a laser pulse. Freezing is complete within half a second. Taken from: PCCP 14 90 (2012) http://pubs.rsc.org | doi:10.1039/C1CP22774B Reproduced with permission from RSC Journals.

This appeals to me for lots of reasons, especially due to the close relationship to many of the things I am interested in in my research. One of the most intriguing things is that the process by which the nucleation occurs is not really well understood (and indeed there may be different processes going on for different substances and types of nucleation). The idea put forward in this paper is that the the application of the field lowers the size of cluster within the acid that is needed to start the nucleation process. This happens by the laser lowering the free energy of the system. Interesting this is vaguely analogous to optical tweezers, in that the free energy is lowered, and hence the energy barrier to nucleation is lowered, in a system where the refractive index (or polarisability) of the solid substance is higher than that of the liquid, so the relative permittivity is positive. This is the same requirement for a particle to be trapped (relative permittivity of the trapped particles and the surrounding medium must be positive) in a normal Gaussian beam. The paper gives a good argument that this explanation is true, as opposed to alternatives, in this case. The fact that the mystery is not yet solved is very interesting though, and leaves room for many new studies…possibly using optically trapped droplets.

Ward, M., McHugh, S., & Alexander, A. (2012). Non-photochemical laser-induced nucleation of supercooled glacial acetic acid Physical Chemistry Chemical Physics, 14 (1) DOI: 10.1039/c1cp22774b

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