Figure 5.20 shows that the convection regime is purely forced for short time, and is governed by crystal rotation (< 8 hours). The stirring effect is to be seen by the streaks of light in the image that spread out deep into the solution. Between 8 and 20 hours, the spread becomes narrower, as buoyancy forces re-direct the plume in the vertical direction. For times greater than 45 hours, the plumes show a swirl component, but are vertically directed. The relative importance of rotation and gravity is governed by the ratio of buoyancy and centrifugal forces. The force ratio can be shown to be proportional to the crystal size; hence buoyancy is the guiding force at later times, when the crystal has become large. However, rotation provides a kinematic condition for fluid motion (in the form of a boundary condition), causing the buoyant plumes to become helical, and hence structured.    

Figure 5.21 shows the concentration contour maps (normalized between 0 and unity) around the growing crystal for 0 and 15 rpm.

Figure 5.21: Concentration contours around a growing crystal with the passage of time with and without crystal rotation. The central vertical filled band in each plot represents the seed holder. Ramp rate=