A Schlieren study of the effect of ramp rate and rotation on convection around a crystal growing from an aqueous solution

The present work examines the influence of process parameters on convection around a growing KDP crystal from its aqueous solution. Parameters that are adjusted to control the crystal growth process are the cooling rate of the solution and crystal rotation. The size of the growing crystal indirectly influences the convective field through buoyancy and centrifugal forces. Experiments have been conducted in the buoyancy-controlled and rotation-controlled regimes of convection in the solution. The growth process of the crystal is initiated in the experiments by inserting the KDP seed into its supersaturated solution, followed by slow cooling of the solution. Transport of excess salt from the bulk of the solution to the growing crystal gives rise to concentration gradients. In the absence of rotation, these gradients initiate density-driven convection in the growth cell. Experiments are continued as long as the crystal remains clear and transparent. A laser schlieren technique has been employed as a diagnostic tool to map the concentration field of KDP in the solution. Results show that the growth rates are well-correlated with regions of large concentration gradients. The growth rate is seen to be highest in the buoyancy-driven convection regime, while the quality of the grown crystal is superior in the presence of rotation.