Laser shadowgraphy and Mach-Zehnder interferometric imaging of convection, concentration and growth kinetics during unidirectional solution growth of benzophenone crystal

Abstract

Fluid convection and concentration during unidirectional growth of benzophenone crystal were imaged in an online and in-situ manner using optical techniques of laser shadowgraphy and Mach-Zehnder (MZ) interferometry. The fluid dynamic behavior of the plumes, the frequency of their appearance and the number of plumes provide information about the crystal growth mechanism and also growth rates as a function of supersaturation. Comparing the images captured using above techniques, it was found that shadowgraphy was most appropriate to visualize the convective field. It was used to image the life cycle of convection plume starting from its birth at the crystal surface to its diffusion in to the bulk of the solution. Comparative analysis of the images grabbed using different type of imaging techniques reveal that MZ infinite fringe technique is best suitable for visualizing the concentration gradient. This is followed by MZ horizontal wedge setting and then MZ vertical wedge setting fringes. The shadowgraph technique is not able to discern the concentration gradient as clearly as seen in the MZ images. The solutal boundary layer (SBL) was better visualized using shadowgraphy as compared to Mach-Zehnder interferometry. The thickness of SBL was an indicator of the relative growth kinetics. It is demonstrated that by choosing appropriate optical technique, one can monitor as well as control the growth process to achieve the desired results.