A growing understanding of how materials crystallise has revolutionised modern science and technology; from display technologies such as LCDs to solar panels, and from biological sample analysis to pharmaceutical compounds. A material’s crystallisation kinetics, and the conditions at which they undergo phase changes, such as temperature, have a profound effect on its final properties. Correct balance of amorphous and crystalline regions within a sample can be the difference between an inert thin film and an efficient solar cell, or the difference between a successful and failed prototype drug.
As such, research continues into how to understand and manipulate material crystallisation. Several studies have focused on a unique phenomenon known as ‘laser-induced crystal nucleation’ or ’laser tweezing’, where the electromagnetic energy from a laser beam, in combination with temperature control, can be used to force phase changes or nucleation of new phases in a mixed liquid system.
A team of scientists set out to understand how and why lasers trigger crystallisation and how changes made to the properties of the laser could influence which crystal form is generated. Researchers working with Professor Klaas Wynne of The School of Chemistry at the University of Glasgow used Linkam’s THMS600 temperature-controlled stage to explore phase transitions in mixed liquid systems and laser-induced nucleation.
Dr Finlay Walton working on the system with the Linkam THMS600 at the University of Glasgow.
The central hypothesis under investigation was that there were hidden ‘critical points’, either in a single liquid, or when two immiscible liquids mix or separate in response to temperature, where dramatic fluctuations occur in relative concentration, and that at these ‘critical points’ laser light was able to enhance this effect and lead to laser-induced nucleation.
The team constructed a phase-contrast microscope and laser set-up to conduct some preliminary experiments. These first laser-induced phase separation (LIPS) experiments were carried out on mixtures of nitrobenzene and decane at mole fractions and temperatures where the mixture was stable. When the laser was focused in the sample, a bright spot was visible under the microscope. This indicated that LIPS had taken place, and that the fraction that was separated out had a higher refractive index and must therefore be nitrobenzene rich. This was confirmed with the dye methylene blue and fluorescent detection.
Subsequent temperature-controlled experiments using the THMS600 allowed the group to demonstrate that the LIPS effect increases strongly on cooling as the binodal (the critical point at which two distinct phases can coexist) is approached, and proximity of a liquid-liquid critical point can be used by a laser-tweezing potential to induce concentration gradients.
(Left) LIPS and Nucleation (LIPSaN) experiments in nitrobenzene–decane at T = 23.9°C with a 120-mW 785-nm focused laser, on for 30 s. When the laser is switched on a LIPS droplet forms, but nucleation only occurs once the laser is switched off. (Right) Finlay using the setup with a THMS600. Images used with permission of Finlay Walton and Klaas Wynne.
The group concluded that phase manipulation and nucleation can be reliably induced with a straightforward laser set-up. The results could have profound implications for the understanding of these important phenomena. The LIPS experiments, and related theory, not only explain the physics behind non-photochemical laser-induced nucleation, but also suggest potential new ways of manipulating matter.
Dr Finlay Walton, who was a research assistant in Professor Wynne’s group explains: “The Linkam stage integrated so well with our microscope – we have long-field objectives and the stage fitted perfectly into it. The temperature control was extremely good. We want to get really close to this critical point and so having control to 0.1 Kelvin is essential. The Linkam stage was so fast as well, we saw no discernible lag as we approached the target temperature – and the temperature that the stage indicated was the actual temperature, something that we haven’t always seen with other systems.”
References
[1] Walton F, Wynne K. “Control over phase separation and nucleation using a laser-tweezing potential” 2018 Nature Chemistry 506–510 DOI: 10.1038/s41557-018-0009-8
[2] Walton, Finlay, and Klaas Wynne “Using optical tweezing to control phase separation and nucleation near a liquid–liquid critical point” 2019 Soft Matter 15.41 8279-8289 DOI: 10.1039/C9SM01297D
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