A microfluidic device was designed and fabricated for non-photochemical laser-induced nucleation (NPLIN) in continuous laminar flow, which enabled real-time in situ characterization of crystal size, shape, growth, and polydispersity. On-chip thermoelectric cooling created supersaturation by lowering the solution temperature. The influences of laser power density, laser exposure time, flow rate, and supersaturation were examined for aqueous KCl solutions. The observed threshold peak power densities and solution labilities agreed with those reported by Alexander et al. using static cells. The mean crystal size just downstream from the irradiated region was observed to increase with increasing supersaturation. The number of crystals nucleated was found to increase with increasing supersaturation and laser power density but was independent of the number of laser pulses to which the solution was exposed. These results are consistent with the dielectric polarization model of Alexander et al. Our findings broaden the scope of nucleation in a light field by introducing a way to directly characterize the crystallization.

Nonphotochemical laser-induced nucleation (NPLIN) of supersaturated aqueous glycine solutions was studied at a wavelength of 1064 nm using a microfluidic device. Crystal shape, size, and number were characterized in situ in real time on the chip. The influence of the laser pulse intensity on the nucleation kinetics was reported. Aging of the supersaturated solutions was necessary to observe NPLIN; fresh solutions did not nucleate. Crystal structure was found to switch from the αto the γ-polymorph as the supersaturation increased. The observed number of crystals formed exhibited a threshold intensity but was otherwise proportional to the laser intensity, consistent with the dielectric polarization model, although the "lability" calculated from classical nucleation theory was too large by many orders of magnitude. Dynamic light scattering data revealed nanodroplets, hundreds of nanometers in diameter, formed in aged supersaturated aqueous glycine solutions; these submicron sized nanodroplets were apparently necessary for NPLIN. A new model combining the dielectric polarization model and two-step nucleation theory via submicron nanodroplets was proposed to explain these observations, providing a reasonable match between experiment and theory.

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.

Tightly focusing a continuous-wave, near-infrared laser beam at the air/solution interface of a millimeter-thick layer of glycine in D2O forms a crystal through a polymorphically and spatially controlled nucleation process known as gradient-force laser-induced nucleation or optical-tweezer laser-induced nucleation. However, when this same beam is focused at the glass/solution interface of a film of aqueous glycine, a highly concentrated laser-induced phase-separated (LIPS) solution droplet is formed that does not nucleate while the focusing beam remains on. Two competing theories have emerged about the nature of the LIPS droplet: one proposes that it is a merger of prenucleation metastable nanodroplets and clusters into one large homogeneous “dense liquid droplet”, and the other stipulates that it is the result of the partitioning of larger droplets into the new phase, but not a merging of droplets, around the focal point of the beam. In order to determine the nature of the LIPS droplet, dynamic light scattering was used to detect the presence of nanodroplets undergoing Brownian motion within the droplet and to measure their relative size following a range of laser exposure times. The observation of nanodroplets in motion in the center of the LIPS droplet revealed that the application of optical tweezers at the glass/solution interface forms a relatively monodisperse collection of large nanodroplets (>700 nm) concentrated around the focal point of the beam with smaller particles (<100 nm) depleted within the first 2 min of laser exposure. The LIPS droplet quickly reaches a steady state and is not affected by increasing focusing times. These findings allow for a better understanding of the interactions of optical tweezers with aqueous glycine nanodroplets. This understanding will help in studying the fundamental nature of metastable nanodroplets. More practically, laser-induced phase separation makes possible the nucleation-free separation of large nanodroplets from small clusters, facilitating materials technologies such as high purity, polymorphically selective nucleation of crystals and co-crystals used for pharmaceuticals, dyes, and photovoltaics.