Crystallization is a process known to humankind for centuries. Everyday items can be a result of crystallization, e.g. snowflakes being formed in cold weather and sugar that is obtained from glucose-water solutions. Crystallization follows from the process which is called nucleation. This phenomenon is explained in two different theories: classical nucleation theory and two-step nucleation theory. Also, two different ways of crystallization can be distinguished: heterogeneous nucleation, which is nucleation that evolves from external surfaces and homogeneous nucleation, which obviously evolves without the help of external surfaces. Methods to perform crystallization, based on aforementioned mechanisms, are useful for production scale facilities as it is a very useful method to separate chemicals in manufacturing processes. However, large scale crystallization processes are known to be hard to control and energy intensive, something that does not fit into today’s society. To keep up with the global demand for sustainable production novel crystallization methods need to be established. A promising finding, 20 years ago, consisted of laser-induced nucleation. By means of a laser, Garetz et al.[1] were able to induce crystallization, calling it non-photochemical laser induced nucleation (NPLIN). Up to this date, it is not known what mechanisms underlie the observed behavior. Because the phenomenon was only observed at the milliliter range, it is important to understand the mechanisms before further upscaling can be done. Over the course of the last decade, 4 mechanisms have been proposed: Optical Kerr effect, Isotropic electronic polarization, nano-impurities, and shockwaves. During the discovery of NPLIN, it was found that the wavelengths and power intensity were incapable of creating a photochemical effect on the compound used. For this reason, the LIN was ascribed to nonphotochemical behavior. In this research, more detail is provided on this presumed non-photochemical effect. An experimental build is established to perform multiple experiments for detecting radicals and obtain consensus on the attributed name. Initial tests revealed that there is no particular interaction between the solute and laser electric field. Throughout the research, several factors were found to play a role in observed NPLIN behavior. Over the course of this project, glycine samples started to turn yellow due to the degradation of glycine. pH was found to be an important factor for polymorphic control, however, it was constant for all samples since glycine acted as a buffering agent. Future research with new chemicals should keep track of the pH together with polymorphism. Glass geometries tests showed that container curvatures are affecting the polarization of the laser and thus the outcomes for polymorphic structures. At last, the effect of impurities on NPLIN was elaborated. Doping samples intentionally with nanoimpurities resulted in higher crystallization probabilities. On the other hand, reducing the impurity levels also decreased the nucleation probability. It is concluded that impurities are highly correlated to nucleation performance. Moreover, the laser-interaction volume is also affecting the nucleation probability in significant amounts. Supplementary research is required to obtain a set of operating parameters that have an effect on NPLIN behavior. With such a model, NPLIN can be controlled and a state of the art production scale laser induced nucleation unit becomes the new standard.

Crystallization is employed in a wide range of industries but our ability to control it remains far from perfect. New methods are being continuously developed and improved to provide enhanced kinetics and control. Non-photochemical laser induced nucleation (NPLIN) is one of the avenues being looked into extensively since its accidental discovery about two decades ago. Despite providing improved nucleation kinetics and potential polymorph control, the mechanism behind NPLIN is still unknown. Four different theories have been proposed in the literature with varying amounts of agreement between
different research groups.
The main aim of this thesis is to try to determine the mechanism behind NPLIN. This report can be divided into two parts, each focusing on a possible mechanism. The first is the optical Kerr effect,
which involves investigating the effect of polarization of light on glycine polymorph formed. This is achieved by varying the laser light polarisation and number of pulses for a range of glycine
supersaturation. The second part deals with an experimental setup designed to work with microscale volumes. This will give us the capability to isolate the nuclei and observe the events leading up to their formation.
For studying the optical Kerr effect, the experiment performed by Sun et al. was repeated. A significant temperature increase inside the solution was obtained because of exposure to a high number
of pulses (600) of infrared light. No dependence of laser light polarization on polymorph formation was found. The polymorph formed by laser is different than that obtained by crash cooling. In the
second part of the thesis, the attention is shifted towards the role of impurities present in the solution which can also absorb the laser light leading to formation of a cavitation bubble. This possibility was
examined with the help of the setup mentioned above. It was noted that the crystals were nucleating at multiple points around the laser focus at a distance which is similar to the size of the cavitation bubble previously reported in literature. These observations made can be attributed towards the presence of a bubble.

Nucleation is the initial step for the creation of new crystalline phase. A precise control over nucleation and its kinetics is important for both research and industries. Thus, alternative methods are sought after to extend the toolbox for controlling nucleation. In the 1990's, Non-Photochemical Laser Induced Nucleation (NPLIN) was suggested as a promising method to alter the nucleation kinetics. Since then, several reports have demonstrated that NPLIN dramatically reduces the nucleation induction time and controls polymorphism of various fine chemicals relevant for industrial practice. Although different hypotheses have been proposed in literature to explain the experimental observations, the mechanism behind NPLIN is still unknown.

The objective of this work is to extend the mechanistic understanding of NPLIN. This has been approached by qualitatively studying the effect of different factors on the nucleation efficiency of the non-photochemical process using unfocused pulsed laser in aqueous supersaturated solution of KCl. The factors investigated include wavelength, peak intensity, supersaturation, mixing, and impurity level of the solution. Each of these parameters are studied using high number of samples (80-100) to generate a robust set of results and to avoid the stochastic nature of nucleation.

In a separate series of experiments, an acoustic wave was detected in the solution due to the non-linear interaction of the unfocused laser with the system by measuring the pressure signal with a piezo-electric transducer placed just below the air-liquid interface. Further experiments were executed to understand the nature of the acoustic wave and its influence on NPLIN. The results show that laser could induce nucleation at significantly low peak intensities, much below the previously reported intensity threshold in literature. It is also observed that NPLIN shows a strong dependence on peak intensity, supersaturation, impurity level, and mixing of the solution while the dependence on wavelength was found to be weak. Furthermore, the acoustic wave experiments show that the laser induced pressure fluctuations do not affect the nucleation efficiency of the process. Overall, the results suggest that several mechanisms play a role during laser induced nucleation. To summarize, the research provides a robust analysis of different factors that can influence NPLIN. The results can be further utilized to enhance the understanding and applicability of the process.