The principles behind Non-Photochemical Laser Induced Nucleation

Abstract

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.