Understanding Non-Photochemical Laser Induced Nucleation

Abstract

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.