Nonphotochemical Laser-Induced Nucleation in Levitated Supersaturated Aqueous Potassium Chloride Microdroplets Ke Fang, Stephen Arnold, and Bruce A. Garetz* Department of Chemical & Biomolecular Engineering, NYU Polytechnic School of Engineering, New York University, Brooklyn, New York 11201, United States ABSTRACT: We have observed nonphotochemical laser-induced nucleation (NPLIN) in levitated 8 pL microdroplets of supersaturated aqueous potassium chloride irradiated with nanosecond pulses of 532 nm light. A much higher supersaturation ratio S of 1.20 was required to observe NPLIN in a microdroplet, compared to the value of 1.06 for bulk solutions, attributed to a 40-million-fold lower nucleation rate in a microdroplet at S = 1.06 as a result of its much smaller volume. This finding has application to high spatial and temporal resolution pump-probe studies of nucleation in a containerless environment. ■ INTRODUCTION Nucleation from solution, the first step in crystallization, is a rare, stochastic, symmetry-breaking event that is difficult to study experimentally and to model theoretically, and that is still incompletely understood. 1 It is an essential tool for purification and control of crystal structure in the pharmaceutical industry, 2 and it is the first step in biomineralization, a process essential for many living organisms. 3 Not only must solute molecules cluster to form a separate phase, they also must organize to form an ordered crystalline structure, and there is evidence that these two steps can occur sequentially. 4,5 In 1996, our group reported that intense near-infrared nanosecond laser pulses could induce a supersaturated aqueous urea solution to nucleate, which we attributed to the optical Kerr effect, in which the optical electric field induces the alignment of solute molecules in a liquid-like cluster, helping the molecules to organize into a critical nucleus. Although photochemical light-induced nucleation had been known since the 19th century work of Tyndall, 6 we ruled that out because the peak light intensities and the near-infrared photon energies were too low to induce photochemistry, and we called the phenomenon “nonphotochemical laser-induced nucleation” (NPLIN). 7 In 2002, we reported that, by simply switching the polarization state of the light between linear and circular, we could induce aqueous glycine to nucleate into different polymorphs. 8,9 Such control over crystal structure had previously been possible only with chemical additives or modifying the physical conditions. Since these initial reports, NPLIN has been observed in a wide variety of materials, including supersaturated solutions of aqueous L-histidine, 10 hen egg white lysozyme, 11 potassium chloride and bromide 12,13 and carbon dioxide, 14 as well as supercooled melts of sodium chlorate, 15 glacial acetic acid, 16 and the liquid crystal 5CB. 17 Simulations by Peters and co-workers suggest that optical Kerr interaction energies are too small to account for NPLIN. 18 While several alternative mechanisms have been proposed to explain some of these observations, including isotropic electronic polarization 12 and nanocavitation, 14 no single mechanism appears to fit all of the materials in which NPLIN has been observed. Because spontaneous nucleation is a random process, an experimenter has little control over when or where a nucleation event will occur. We noted in ref 7 that NPLIN had the potential to allow an experimenter to control precisely the time and place that nucleation occurs. 7 In 2009, Alexander and co- workers demonstrated macroscopic 3D spatial control of potassium chloride (KCl) nucleation in agarose gels using NPLIN. 19 In this paper, we report a realization of this goal on the microscopic scale: the observation of NPLIN in 8 pL levitated supersaturated solution microdroplets of KCl in water. Alexander and co-workers have characterized NPLIN in bulk aqueous solutions of KCl. 12,13 They proposed that the interaction of the optical electric field with the isotropic electronic polarizability of subcritical solute clusters decreases their free energy, making some of them become supercritical. This system exhibits a relatively low laser intensity threshold (5.6 MW/cm 2 ) at a wavelength of 532 nm and with a minimum supersaturation ratio S of 1.06. (S = c/c sat , where c is the concentration of a given solution and c sat is the concentration of a saturated solution.) We chose to study KCl because of its low intensity threshold. Received: March 27, 2014 Published: April 2, 2014 Article pubs.acs.org/crystal © 2014 American Chemical Society 2685 dx.doi.org/10.1021/cg5004319 | Cryst. Growth Des. 2014, 14, 2685-2688