Color schlieren deflectometry for characterization of crystal growth processes: KDP and lysozyme

Abstract

Color schlieren deflectometry is a technique that has great potential in terms of both quality and ease of implementation for the study of convection phenomena during crystal growth. The advantage stems from its non-intrusive and inertia-free nature. Application of this technique for crystal growth is not available in the literature and is the focus of the present work. The present study discusses the implementation of color schlieren deflectometry for characterization of KDP and protein crystal growth processes. The technique is evaluated both for qualitative visualization and as a tool for quantitative analysis. Crystal growth processes, in general, reveal complex transport phenomena. Solutal convection is present during KDP crystal growth while the protein crystal growth process includes both evaporation and condensation. The present study demonstrates the effectiveness of color schlieren technique for understanding these physical phenomena for both crystal growth processes. The color schlieren images are qualitatively compared with those from other optical techniques such as Mach–Zehnder interferometry, monochrome schlieren and shadowgraph techniques. The quantitative analysis procedure of color schlieren images is validated by using the Rayleigh–Benard convection experiment in a rectangular cavity. It is seen that the temperature profiles and the heat flux distribution in the Rayleigh–Benard experiment and concentration distribution during KDP crystal growth derived from color schlieren deflectometry match well with data reported in the literature. In the protein crystal growth experiment using lysozyme, evaporation of water from the droplet and its condensation on the reservoir surface followed by subsequent diffusion and complete mixing with the reservoir solution are clearly revealed from color schlieren deflectometry experiments. Overall, the present study establishes the color schlieren deflectometry as an attractive tool for characterization of crystal growth processes.