Investigating possible mechanisms of Non-Photochemical Laser Induced Nucleation

Abstract

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.