A centimeter-sized, laser-induced phase-separated (LIPS) solution droplet, which was formed by tightly focusing a continuous-wave near-infrared laser beam at the glass/solution interface of a millimeter-thick layer of glycine in D2O with a supersaturation ratio, S, of 1.36 was irradiated with a single unfocused nanosecond near-infrared laser pulse in order to study the effect of non-photochemical laser-induced nucleation (NPLIN) on the droplet, as well as to help characterize the behavior of the LIPS droplet. Additionally, a control NPLIN experiment was conducted on an S = 1.50 supersaturated solution of glycine/D2O in the same cell to better understand the differences between NPLIN in a LIPS droplet and an ordinary supersaturated solution. These experiments revealed that NPLIN could nucleate crystals within a LIPS droplet, although the growth of these crystals was inhibited during the first 5 min of the droplet’s relaxation. For the first 40 min of its relaxation, the LIPS droplet was observed to be more labile to spontaneous nucleation than the control S = 1.50 solution, although the growth of spontaneously nucleated crystals was also inhibited during the first 5 min of the droplet’s relaxation. This suggests that although the macroscopic phase boundary between the LIPS droplet and the surrounding solution disappeared after approximately 5 min, the full microscopic relaxation of the LIPS droplet took at least 40 min. The resulting crystals were analyzed using powder X-ray diffraction, and 100% of crystals formed within the LIPS droplet induced by NPLIN with linearly polarized light and by spontaneous nucleation were α-glycine, while crystals formed outside of the LIPS droplet were mixtures of α- and γ-glycine. The results suggest that the LIPS droplet and the surrounding solution are not equilibrium phases of aqueous glycine, but phases in which optical gradient forces have induced a partitioning of large and small solute clusters.

We have observed two new morphologies of crystalline glycine grown from supersaturated aqueous solutions in agarose gels: tree-branch dendrites that nucleate spontaneously from a solution interface or by nonphotochemical laser-induced nucleation (NPLIN) at the air–solution interface, and stellar dendrites that nucleate in the bulk of the solution induced only by laser irradiation. The tree-branch dendrites always consist of parallel, needle-like microcrystals of α-glycine and always grow unidirectionally in the c-direction, forming branches with small branching angles. The four-armed stellar dendrites consist of conglomerates of plate-like microcrystals of either α- or γ-glycine or a mixture of microcrystals of the two polymorphs, with the γ-glycine microcrystals concentrated in the core of the dendrite. The plate-like microcrystals of α-glycine grow primarily in the c- and a-directions. The stellar dendrite arm orientation is uncorrelated with the plane of polarization of the incident light, which does not lend support to the induced-polarization mechanism for NPLIN.