Laser-Induced Cavitation for Controlling Crystallization from Solution

Primary nucleation control is crucial for obtaining crystals with specific properties, such as purity, size, morphology, and polymorphic form. Non-photochemical laser-induced nucleation (NPLIN) has attracted interest due to its ability to control these properties without chemical reactions, using non-invasive methods, and allowing spatio-temporal precision. However, the exact mechanism underlying NPLIN remains debated in the literature.

This dissertation explores how micron-sized vapor bubbles, formed by laser interaction with supersaturated aqueous solutions, can trigger crystal nucleation. Despite aqueous solutions generally being transparent to laser wavelengths of 532 nm and 1064 nm, transient bubbles can still form due to energy absorption by impurities or by focusing the laser. The research begins by examining the crystallization of KCl in aqueous solutions, initiated by bubbles formed using focused laser light with nanosecond pulse width. Findings show that solute accumulation at the bubble surface exceeds the saturation limit, leading to localized supersaturation. A finite element method model, validated by experimental bubble size data, is used to estimate solute transfer and supersaturation levels. The model demonstrates a concentrated solute boundary layer around the bubble, driven by high solvent evaporation rates associated with bubble growth. The experimental results for crystallization probability and crystal count align with classical nucleation theory predictions based on the numerically estimated supersaturation at the vapor-liquid interface.

The bubble formation mechanism proposed for NPLIN is extended to other solutes like NH4Cl, NaCl, KBr, and CH4N2O. Experiments with NH4Cl and NaCl yield a general analytical relation for supersaturation in the liquid surrounding the bubble, explaining NPLIN activity for these solutes when an unfocused laser is used. The predicted bubble sizes, based on Mie theory, correlate with the minimum nucleation rate necessary for crystal formation, indicating that the bubble-driven mechanism is a key factor in NPLIN.

Since isolated bubbles are rare in irradiated volumes due to the random distribution of impurities, the study also investigates bubble-bubble interactions and their effect on crystallization. The dynamics of single laser-induced bubbles in microchannel geometries are analyzed, revealing a unified theory for bubble size and lifetime as a function of laser energy. This analysis also uncovers a transient flow instability, rare in low Reynolds number flows, which originates from the channel walls and is characterized by the Womersley number and flow timescale. The research further demonstrates crystallization using bubble pairs in microchannels with KMnO4 as a model salt. The interaction between bubbles produces microjets that alter nucleation kinetics through induced shear, enabling crystallization at lower laser energies and solution supersaturation compared to single bubbles. A numerical model based on the boundary integral element method is used to predict microjet velocities and the resulting shear, correlating these factors with crystallization probabilities.

Overall, this work advances understanding of NPLIN, suggesting that bubble formation and interactions can be harnessed to achieve targeted crystallization with lower energy inputs and reduced supersaturation, paving the way for more efficient laser-induced crystallization processes.