“Freezing” of NaClO₃ Metastable Crystalline State by Optical Trapping in Unsaturated Microdroplet

Abstract

We reversibly controlled phase conversion between a microdroplet of a NaClO 3 unsaturated aqueous solution and a metastable single crystal, which is usually a short-lived phase in spontaneous crystallization, simply by irradiating a tightly focused visible continuous-wave (CW) laser to the microdroplet. The laser irradiation allowed the metastable crystal to generate and stably grow without a polymorphic transformation. This successful metastable phase control is attributed to the combination of the advantage of optical trapping-induced nucleation that nucleation takes place from unsaturated mother solution and the advantage of microdroplet method, which suppresses additional nucleation leading to the transformation. In situ observation shows the crystal dissolves when the laser irradiation is stopped, whereas the laser irradiation stabilizes the crystal even if the size of the crystal becomes larger than that of focal spot. These observations indicate that a change in the relative magnitudes of chemical potentials between solution/crystalline phases. This change is possibly promoted via crystal growth by trapping of crystalline clusters in optical potential well formed on a crystal surfaces originating from "light propagation" through the crystal. Our results shed a light not only on polymorph control but also on a method to prepare a longer-lived achiral precursor for analysis on achiral−chiral transition by "freezing" a kinetic pathway of chiral crystallization

Using optical tweezing to control phase separation and nucleation near a liquid–liquid critical point

“…In this work, we argue that LIPS is achieved through an opticaltweezing effect. However, phase separation could potentially also be caused by a temperature gradient (such as that generated by the laser, see eqn (19)) through thermophoresis. The onedimensional thermophoresis equation in equilibrium is…”

“…[12][13][14] A series of publications has shown that optical tweezing can be used to nucleate crystals from super-saturated [15][16][17][18] and strangely even from under-saturated solutions. 19 Optical tweezing is a technique used widely in physics and biology and involves the trapping of a high-refractive-index particle through optical forces. 20 Thus, it was assumed that in these experiments the laser was tweezing pre-existing clusters or pre-nuclei.…”

“…However, very excitingly, these experiments also demonstrated polymorph selection. [15][16][17][18][19] As in the case of non-photochemical laser-induced nucleation, no sensible theory is available to describe the physics of these results. Thus, it is fair to say that a physical understanding of all of these phenomena is still sorely lacking.…”

Control over phase separation and nucleation using a optical-tweezing potential

“…[12][13][14] A series of publications has shown that optical tweezing can be used to nucleate crystals from super-saturated [15][16][17][18] and strangely even from under-saturated solutions. 19 Optical tweezing is a technique used widely in physics and biology and involves the trapping of a high-refractive-index particle through optical forces. 20 Thus, it was assumed that in these experiments the laser was tweezing pre-existing clusters or pre-nuclei.…”

“…These experiments also demonstrated polymorph selection. 15,19 As in the case of non-photochemical laser-induced nucleation, no sensible theory is available to describe the physics of these results. Thus, it is fair to say that a physical understanding of all of these phenomena is still sorely lacking.…”