Figure 5.27 shows normalized concentration maps above the growing crystal as derived from the schlieren images of Figure 5.26. The value of represents a saturated solution, while the limit refers to the supersaturated solution at its instantaneous temperature in the growth chamber. The maximum crystal size at the end of each experiment has been used to non-dimensionalize the x and y coordinates. For hour, the concentration contours of Figure 5.27 in the bulk of the solution reveal an almost symmetric solute distribution. Symmetry can also be gauged by the identical nature of contour distributions for each view angle. The contours are not densely spaced, indicating low concentration gradients. The contours are localized in the vicinity of the crystal, with the bulk of the solution being at the supersaturated state . Hence this phase of the experiments corresponds to the growth process with transport across a diffusion boundary layer attached to the crystal. With the passage of time, the crystal size increases and the concentration gradients increase in strength. At this stage, the concentration field is accompanied by a partial loss of symmetry. Experiments of the present study revealed a marginal unsteadiness as well. This phase defines a transition process from diffusion-dominated transport of solute to the onset of convection. With the passage of time, a weak convection current is set up around the crystal. The images recorded at t=70 hours also show a greater loss of symmetry in the concentration contours and a denser spacing closer to the crystal are to be seen.

Figure 5.28 shows the variation of concentration with respect to the (vertical) y-coordinate in the crystal growth chamber. The concentration referred here is an average obtained in the viewing direction of the laser beam and across the image as well. For averaging the concentration values in the x-direction, namely normal to the light beam, 11 columns along the width were selected. The concentration profiles for t=1 hour (Figure 5.28(a)) overlap in the bulk of the solution above the crystal for all the four view angles, thus revealing a uniform distribution of solute concentration in this region. At t=35 hours (Figure 5.28(b)), the differences in the profiles are quite noticeable. These differences can be attributed to the temporary unsteadiness that sets in when a transition from diffusion-dominated growth takes place towards convection. With the passage of time, the profiles become increasingly symmetric and overlap with each other. The growth now takes place in the presence of an upward rising convective plume (Figure 5.28(c)). The symmetry in concentration distribution indicates an almost uniform and symmetric deposition of solute onto the crystal surfaces. It is to be noticed that for the three time instants, the profiles show differences in the vicinity of the crystal . The morphology of the KDP crystal is pyramidal, as shown in Figure 5.29 in a photograph of a grown KDP crystal in the diffusion-dominated regime, at the end of 70 hours. The differences in the average concentration near the crystal can be traced to the non-symmetric shape of the crystal as well as large gradients at the crystal edges.