Evolving Crystal Morphology of Potassium Chloride Controlled by Optical Trapping

Abstract

Dynamic morphology evolution of potassium chloride (KCl) crystals was demonstrated by surface optical trapping with a focused continuous-wave near-infrared laser. Optical trapping at an air/solution interface triggered the crystallization, and then, the dynamic change in crystal morphology was observed in real time. We observed three different crystal morphologies of needle, rectangle, and cubic at the early stage of crystallization. As the laser power increases, the probability of generation of a cubic crystal increases, especially upon the irradiation with linear polarization. We also found laser polarization-dependent morphology evolution by the continuous irradiation to the generated crystals. Upon linearly polarized laser irradiation, the stepwise morphology evolves from needle to rectangle, and eventually to cubic, which is an equilibrium shape of KCl crystals. Meanwhile, circularly polarized laser irradiation only induced morphology evolution from needle to rectangle, without morphology change into cubic, because the rectangle crystal was dissolved while the crystal was rotating. It was made possible to observe such a unique morphological evolution due to the spatiotemporal controllability of our crystallization method. The dynamics and mechanism of these intriguing phenomena are discussed from the perspective of a dense cluster domain formed by optical trapping before nucleation.

Optical Force-Induced Dynamics of Assembling, Rearrangement, and Three-Dimensional Pistol-like Ejection of Microparticles at the Solution Surface

“…This optical scattering and propagation at interface holds also for molecular solution, which explains the molecular crystallization of amino acids , 23,39 and inorganic compounds such KCl 40 . We have demonstrated that the optically generated crystals are much larger than the focused laser beam size (about 1 µm 2 ) and they can even enlarge up to a few tens of µm 2 .…”

Optical Spectroscopy of Crystal Nucleation One Nucleus at a Time

“…1a). While OTIC has been performed using a NIR laser in the past, [23][24][25][26][27][28] we employed a 532 nm laser that served the dual role of inducing a crystal nucleation and Raman excitation (Fig. 1b).…”

“…Optical trapping induced crystallization (OTIC) was first demonstrated by Sugiyama et al in 2007 by focusing a near infrared (NIR) laser in supersaturated glycine/D2O solution. 23 Since the discovery, OTIC has been applied to a variety of systems, [24][25][26] and furthermore high quality single crystals can be prepared while the polymorphs can be controlled by laser polarization. 26,27 Our approach applies OTIC to spatially control a single crystal nucleation event (i.e.…”

Optical Spectroscopy of Crystal Nucleation One Nucleus at a Time

“…1a). While OTIC has been performed using a NIR laser in the past, [23][24][25][26][27][28] we employed a 532 nm CW laser that served the dual role of inducing a crystal nucleation and Raman excitation (Fig. 1b).…”

“…Optical gradient force is, however, not large enough to trap a single molecule, and therefore it is generally assumed that the crystallization occurs as a result of local concentration increase by trapping aggregates in solution. Since the discovery, OTIC has been applied to a variety of systems, [24][25][26] and furthermore high quality single crystals can be prepared while the polymorphs can be chosen by laser polarization. 26,27 Our approach applies OTIC to spatially control a single crystal nucleation event (i.e.…”