The intensity contrast in the images of Figure 5.19, as well as their average intensity progressively increases with time, indicating a depletion of the salt in the aqueous solution. For long times, a stable stratification in density (and hence salt concentration) was obtained in the growth chamber. The rate of increase of the crystal size was negligible at this stage.
Figure 5.20 shows the transient evolution of the convective field around a rotating crystal when the rpm is 15. The first image (Figure 5.20(i)) shows the existence of a diffusion boundary layer around the surfaces of the seed crystal, as seen by the almost uniform distribution of intensity all over the growing faces. After a growth period of about 10 hours, the gradients near the crystal increase in strength. There is a tendency for the plumes to rise vertically, demonstrating that buoyancy forces are larger than centrifugal, on an average. Unlike the schlieren images in Figure 5.19 where the buoyant plume moved almost symmetrically along the seed holder over a considerable part of the growth phase, the images shown in Figure 5.20(ii-iii) reflected temporary unsteadiness in the experiments and asymmetric behavior of the convection currents on either side of the crystal. The unsteadiness can be attributed to two factors: (a) temporary unsteadiness in convection due to the development of prismatic faces from the seed crystal, and (b) the dominance of centrifugal force over the buoyancy force due to crystal rotation, causing the convection currents to be pushed sideways (Figure 5.20(iii)). At all subsequent time periods, the combined effect of buoyancy and rotational forces govern the overall orientation and movement of the convection currents. As the crystal size increases, relatively stronger convection currents rising upwards due to buoyancy lead to stronger concentration gradients near its surfaces. The effective movement of the fluid particles is along a helical path, seen in Figures 5.20(iv-vii), but most clearly in Figure 5.20(vii). The width of the helical structure of the rising plumes scales well with the horizontal dimension of the growing crystal. The increasing strength of buoyant convection is evident from the vertical extent upto which the well-defined helical shape is preserved above the crystal. For example, the respective images for 45 hours (Figure 5.20(iv)) and 55 hours (Figure 5.20(v)) show a breakdown of helical structure in the central region between the upper face of the crystal and the free surface of the solution. Helical symmetry is preserved for the greatest vertical extent (i.e., the distance between the crystal and the free surface of the solution) during the time interval of 90-120 hours (Figures 5.20(vii-viii)). Hence, at a certain level of supersaturation and crystal size, a delicate balance exists between the rotational and buoyancy forces. The balance provides a geometric pattern to the plume, and hence favorable conditions required for the growth of good quality crystals from their aqueous solution is obtained. This particular phase of the growth process can be termed as the stable growth regime in which the crystals of highest transparency and symmetry can be grown. Unlike the growth process in purely buoyancy-driven mode, rotation enforces homogenization of concentration gradients around the crystal over the longer duration of experimental run time. Therefore, the possibility of solute stratification as observed in Figure 5.19(viii) (and at later times) is delayed.