GROWTH PATTERNS IN THE DIFFUSION REGIME

Figure 5.26: Schlieren images of the convective field around the crystal growing from its aqueous solution in a platform techniques at three different time instants recorded from the four view angles are presented.

Figure 5.26 shows schlieren images of a growing crystal in the diffusion-dominated regime at three different time instants. All four view angles are included in the figure. Adjacent to the crystal the deposition of solute from the solution to the crystal surfaces results in a change of concentration. This introduces a variation in density, refractive index and hence results in intensity contrast. For small time (less than one hour), intensity changes are restricted to the vicinity of the crystal. Here, transport of the solute to the crystal is purely by diffusion. At later times, a weak convection current is to be seen in the spread of light intensity above the crystal, though diffusion is still the dominant mechanism for solutal transport. For times greater than 70 hours, steady convection was seen to be clearly established in the form of a buoyant plume. In view of the small degree of supersaturation, the origin of convection here is related to an increase in the crystal size, rather than density differences.