The present lecture reports measurement of the solute concentration distribution around a KDP crystal growing from its aqueous solution under slow cooling conditions. The choice of KDP (molecular mass 136.09) was based on the availability of refractive index and supersaturation data in the literature for image analysis. The solution adjacent to the crystal is depleted of the salt and is close to the saturated state, while it is supersaturated in the far-field. The difference in solute concentrations drives a diffusive mass flux in the initial stages of growth, while the flux is controlled by fluid convection at later times (Figure 5.11). In the context of crystal growth, the present module contains discussions on refractive-index methods, their validation in a buoyancy-driven flow experiment, comparison of interferograms, schlieren and shadowgraph for crystals growing under nominally identical experimental conditions, schlieren imaging and analysis of convection patterns, possibility of control by rotation, and tomographic reconstruction of the concentration field around the growing crystal. The physical problem taken up for validation and comparison of the three optical methods is buoyancy-driven convection in a fluid medium confined in a rectangular cavity. The fluid is heated from below, cooled from the top and the cavity has insulating side walls. The flow pattern associated with this configuration shows a sequence of transitions from steady laminar to unsteady turbulent flow.

Figure 5.11: Schematic drawing of the concentration distribution around a growing crystal. (a) Diffusion dominated growth shown by dots, (b) Growth in stable convection regime in the presence of a buoyant plume. Arrows indicates fluid motion in the bulk of the solution.