Tightly focusing a continuous-wave, near-infrared laser beam at the air/solution interface of a mm-thick layer of glycine in D₂O forms a crystal through a high-speed nucleation process known as Gradient-Force Laser-Induced Nucleation. However, when this same beam is focused at the glass/solution interface of a film of aqueous glycine a Laser-Induced Phase-Separated (LIPS) solution droplet is formed instead. In addition to providing insight into the light-matter interactions of glycine nanodroplets in solution, the LIPS droplet has peculiar properties: it is visible millimeters beyond the focal region of the attracting beam, it has much higher glycine concentration than the surrounding solution, and it does not nucleate for as long as one focuses the laser at the glass/solution interface. In order to better understand the LIPS droplet, two optical experiments were conducted: (1) Non-Photochemical Laser Induced Nucleation (NPLIN) of the LIPS droplet using a single near-IR pulse of a high powered (0.4 GW/cm²) unfocused nanosecond pulsed beam and (2) dynamic light scattering of the LIPS droplet using a modular, custom built, in-situ apparatus with a low-power cw blue laser beam (5 mW/cm²) . These experiments revealed that NPLIN could nucleate crystals within a LIPS droplet and that the LIPS droplet was observed to be more labile to spontaneous nucleation than a control solution for the first 40 min of relaxation. The resulting crystals were analyzed using powder X-ray diffraction, and 100% of crystals formed within the LIPS droplet induced by NPLIN and by spontaneous nucleation were α-glycine. Furthermore, both experiments suggest that the LIPS droplet and the surrounding solution are not equilibrium phases of aqueous glycine, but phases in which gradient optical forces have induced a partitioning of large and small solute clusters. Additionally, that longer focusing times of the optical tweezers expel smaller nanodroplets and clusters while larger nanodroplets are consolidated near the focal region of the beam. This understanding can allow for a more tunable crystallization mechanism of materials that undergo two step nucleation.

Bruce Garetz

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