Abstract
About 20 years ago, it was shown that lasers can nucleate crystals in super-saturated solutions and might even be able to select the polymorph that crystallises. However, no theoretical model was found explaining the results and progress was slowed down. Here we show that laser-induced nucleation may be understood in terms of the harnessing of concentration fluctuations near a liquid-liquid critical point using optical tweezing in a process called laser-induced phase separation (LIPS) and LIPS and nucleation (LIPSaN). A theoretical model is presented based on the regular solution model with an added term representing optical tweezing while the dynamics are modelled using a Kramers diffusion equation, and the roles of heat diffusion and thermophoresis are evaluated. LIPS and LIPSaN experiments were carried out on a range of liquid mixtures and the results compared to theory.
Highlights
In the late 1990s and early 2000s, it was shown that a nanosecond laser can be used to induce nucleation of crystals in a supersaturated solution through a non-photochemical process.[1]
We have demonstrated that these fluctuations can be harnessed using a laser, which generates a trapping potential that draws the high refractive index component of the mixture into the focus
In critical and near-critical mixed samples that are within B1 1C of the critical point, this potential forces phase separation
Summary
In the late 1990s and early 2000s, it was shown that a nanosecond laser can be used to induce nucleation of crystals in a supersaturated solution through a non-photochemical process.[1]. Switching on an optical tweezing laser will lower the free energy of the product state (even if the crystal nucleus does not exist yet), will increase the driving force and lower the barrier for the nucleation process, and increase the ‘‘reaction rate’’ This simple but novel idea was first tested by us on liquid mixtures.[40,41]. The free-energy change induced by the laser (eqn (9)) gives rise to an optical trap that will draw in the liquid with the highest refractive index giving rise to LIPS kinetics. For mixtures of small-molecule liquids the Soret coefficient is typically on the order of S B 10À3,48 and thermodiffusion will play no role in the experiments described here
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