Microfluidic Laser-Induced Nucleation of Supersaturated Aqueous Glycine Solutions

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 α- to 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.