Primary nucleation control of crystals is central to having targeted crystal properties such as purity, size, morphology and polymorphic form. Non-photochemical laser-induced nucleation (NPLIN) has gained attention due to its presumed absence of chemical reactions, non-invasive procedures, spatio-temporal control, and ability to influence the polymorphic form of the crystals. Yet, the existing literature has no general agreement on the underlying mechanism.
This dissertation demonstrates that micron-sized vapor bubbles, formed due to the direct interaction of the laser with the supersaturated aqueous solutions, can trigger crystal nucleation. While most aqueous solutions are transparent to laser wavelengths of 532 nm and 1064 nm, transient bubbles can still be formed in these solutions due to the absorption of the laser energy by intrinsic impurities or by intentionally focusing the laser. This work first discusses the crystallization in aqueous solutions of KCl, due to bubbles formed using focused laser light with nanoseconds pulse width. The results reveal enhanced solute accumulation above the saturation limit at the vapor-liquid interface (bubble surface). A numerical model employing the finite element method, validated against the bubble size evolution from experiments, is leveraged to estimate the solute transfer and, therefore, local supersaturation. The model shows the emergence of a more concentrated solute boundary layer in the liquid surrounding the bubble due to high solvent evaporation rates [~100 kg/(m2s)] associated with the bubble growth. The experimentally recorded crystallization probability and crystal count are successfully correlated to the numerically estimated supersaturation at the vapor-liquid interface using classical nucleation theory.
The proposed mechanism to NPLIN via bubble formation is then extended to other aqueous solutes such as NH4Cl, NaCl, KBr and CH4N2O. With the experiments performed for NH4Cl and NaCl, similar to KCl as discussed above, a general analytical relation for supersaturation in the liquid surrounding the bubble is developed. This analytical relation is then used to rationalize the observed NPLIN activity of all abovementioned solutes in experiments performed in literature employing an unfocused laser. An unfocused laser is expected to generate bubbles via energy absorption by impurities. The predicted bubble sizes surrounding impurities using Mie theory successfully correlate to the minimum necessary nucleation rate as a function of supersaturation at the vapor-liquid interface. Thus, the study on the laser-induced bubbles provides understanding of the dominant mechanism underlying NPLIN, necessary to engineer a laser-induced crystallizer.
Because single isolated bubbles are hardly present within an irradiated volume due to the random distribution of impurities, a study considering bubble-bubble interaction and its influence on crystallization is necessary. Moreover, unbounded liquid domains are absent in a continuous functioning system. Therefore, first, the dynamics of a single laser-induced bubble within diverse quasi-1D microchannel geometries is analyzed using both experiments and theory. The proposed analytical and scaling relations show a unified theory for dynamic bubble size and lifetime as a function of laser energy, including insights on a transitory flow instability - uncommon to low Reynolds flows (<1000). This instability originates from the channel walls and exists due to the oscillating nature of the flow, with its origin and growth characterized by the Womersley number and convective timescale of the flow, respectively. Following this, crystallization using laser-induced bubble pairs in a microchannel is demonstrated using supersaturated KMnO4 as a model salt. The microjet emerging due to bubble-bubble hydrodynamic interaction is shown to alter the nucleation kinetics via the induced shear over the liquid surrounding the bubble. Furthermore, the microjets allow crystallization at lower laser energy and solution supersaturation in the bulk compared to single bubbles. A concomitant numerical model is developed to study the bubble interactions employing the boundary integral element method. The inferred scaling relations from the model are used to predict the microjet velocities and therefore the shear generated due to the jet's impingement over the surrounding liquid. The associated shear and the supersaturation of the bulk liquid are correlated to the recorded crystallization probabilities in experiments using classical nucleation theory. This study using confined bubbles and their interaction with each other suggests a novel pathway for crystal nucleation under laser light for solutes that might otherwise require large supersaturation and laser energies. Thus, using bubble pairs avoids the process difficulties associated with handling solutions with large supersaturation and usage of high-powered lasers that might potentially cause photochemistry.
Together, the findings in this report serve to a priori predict the non-photochemical laser-induced nucleation (NPLIN) activity of a solution, based on the laser intensity and physicochemical properties of the solute, solvent and intrinsic impurities.
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