Comparison of interferometry, schlieren and shadowgraph for visualizing convection around a KDP crystal

摘要

Buoyancy-driven convection patterns around a KDP crystal growing from its aqueous solution have been visualized using optical techniques. The growth process is initiated in the experiments by inserting the KDP seed into its supersaturated solution and is followed by slow cooling of the solution. The deposition of the excess salt from the solution changes the concentration field around the crystal and initiates convection in the growth cell. Convection currents have been recorded around the growing crystal as a function of time. Optical techniques employed for visualization of the convective field are laser interferometry, schlieren, and shadowgraph. Images recorded by these techniques have been compared with respect to the ease of instrumentation, quality of images and the possibility of quantitative analysis. The optical techniques show quite clearly the spatial distribution of concentration differences in the vicinity of the growing crystal. With the passage of time, the solution in the growth cell was found to develop stable density stratification, leading to a practically stagnant solution. This phenomenon was consistently revealed by the three techniques. The overall assessment to emerge from the study is that schlieren is more suitable for monitoring buoyancy-driven convection in crystal growth when compared to interferometry and shadowgraph.