Figure 5.23 shows the transient evolution of the convective field in a purely buoyancy-dominated growth regime (0 rpm, 5.23(a)) and under the combined effects of buoyancy and rotation of the crystal (15 rpm, 5.23(b)). In comparison to the sequence shown in Figure 5.19 (ramp rate=, 0 rpm) the region in the vicinity of the growing crystal reveals a bright intensity distribution, indicating larger concentration gradients in the buoyancy-driven regime. The spread of the bright patch at t=1 hour shows that fluid motion has set in quite early. Thus, even the initial growth is not diffusion-controlled. The buoyancy-driven fluid motion is more vigorous at the higher ramp rate, leading to a greater vertical spread of the depleted solution, and a narrower plume. A clear movement of the buoyant plumes on either sides of the seed holder can be seen at t=35 hours in Figure 5.23(a), whereas at a comparable time instant, weak convection currents were observed at a lower ramp rate of Figure 5.19. Schlieren images in Figure 5.23(a) show well-defined, strong buoyant plumes that transport solute from the bulk of the solution to the crystal surfaces at a faster rate. Consequently, the crystal size at 65 hours in Figure 23(a) is larger than that at 90 hours in Figure 5.19. Rotation of the growing crystal tends to diminish the high concentration gradients in the growth chamber. It is clearly seen from the schlieren images shown in Figure 5.23(b) for hours. In contrast to pure buoyancy, the convection currents do not rise vertically upwards; instead they are dispersed in the solution as they are determined by the combined effects of buoyancy and rotational forces (Figure 5.23(b), t=25 hours). The sizes of the crystal and the convection pattern in the mixed regime (when rotation and buoyancy are of comparable magnitude) are similar in Figures 5.20 and 5.23(b). Furthermore, lower concentration gradients due to the homogenization of the solution are reflected by the weak intensity contrast in Figure 5.23(b) in comparison to that in Figure 5.23(a).

As the crystal size increases, vigorous convection currents in the solution due to buoyancy result in steep concentration gradients around the crystal (Figure 5.23(a), t=65 hours). In the presence of rotation (Figure 5.23(b), t=50 and 80 hours), the plumes orient themselves in the vertical direction, showing that buoyancy predominates over rotation. However, the flow field is clearly disorganized, leading to the appearance of light streaks that are smeared in the horizontal direction by the circulating flow. There is a considerable loss of symmetry in the concentration field at all time instants. At t=80 hours, the higher ramp rate removes a considerable amount of solute from the solution, leading to density stratification in the growth chamber (Figure 5.23(b)). Rotation has no impact on the concentration field at this stage.