(224f) Nanoparticle Concentration Dependence for Microfluidic Laser Induced Nucleation of Supersaturated Aqueous KCl Solutions

Laser-induced nucleation offers an alternative method for crystallization of solids. Upon laser irradiation, crystals nucleate from a solution in a shorter induction time than in the absence of light. Three mechanisms have been proposed: Optical Kerr effect (electric field aligns disordered clusters), Dielectric Polarization model (electric field lowers energy of sub-critical clusters) and Impurity-Heating Mechanism (nanobubble formation around heated nanoparticles induce nucleation). However, some experimental observations cannot be explained by the Optical Kerr and Dielectric Polarization mechanisms such as: threshold laser intensity and filtration suppressing nucleation. The Impurity-Heating mechanism describes that a nanoparticle impurity absorbs energy from a laser pulse, instantaneously heats up the surrounding liquid to form a vapor cavity, the nanobubble expands and grows, and solute clusters aggregate at the vapor-solution interface leading to a nascent crystal. We show with Dynamic Light scattering measurements that a large population of about 900nm particles are removed during filtration, nanofiltration with a 20nm pore size removed particles to low enough concentrations not detected by the standard light scattering technique. The observed laser intensity threshold with the nano-filtered aqueous potassium chloride was about 15 MW/cm² and crystal yield drastically reduced with filtration, which highlights the crucial role of the impurity concentration in laser-induced experiments. Experiments with varying nanoparticle concentration by intentionally doping the solutions with 30nm iron oxide nanoparticles reveal a dramatic increase in nucleation efficiency as a function of the laser intensity. The crystal yield is also shown to have a non-linear dependence with laser intensity. Impurity concentrations of 10⁹ nanoparticles/mL are suspected to have been present in reported literature. We discuss the results with reference to the Dielectric Polarization model, a mechanism used to explain previous findings of non-photochemical laser induced nucleation of aqueous potassium chloride solutions.