Evolving Crystal Morphology of Potassium Chloride Controlled by Optical Trapping

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.