P - Technische Universiteit Eindhoven

3. Crystallization under wetting/non-wetting conditions 39
Fig. 3.6: Normalized sodium content, normalized water content, and average concentration during
drying at 6 ◦ C of a 300 µl droplet having an initial Na 2 SO 4 concentration of 1 M
for a wetting surface, i.e., a quartz-glass sample holder. The dashed lines result from
a free cubic spline interpolation and are shown as guides to the eye.
t [s]
also shown in recent work of Hua Hu et al. [65]. A local enhancement of the concentration
is expected at the droplet sides, leading to formation of thenardite near the contact lines,
as was also observed by Bonn at al. [62,63]. In case of mirabilite the crystallization should
appear instantly, having a growth rate of about 2 cm/s at the experimental temperature
of 6 ◦ C [59].
After about 20000 s the average concentration reaches the heptahydrate supersolubility
line. From that moment on formation of heptahydrate starts in the middle of the
droplet, as indicated in figure 3.7-C. The bipyramidal shape of these crystals also confirms
them to be heptahydrate [22, 59]. As the drying continues, i.e., from 25000 s onwards,
crystals are growing both at the center of the droplet and near the edge of the droplet,
indicating the growth of two types of crystalline phases. Because we have a mix of two
crystal phases, the average concentration in the droplet will be in between the heptahydrate
and thenardite solubility, which is confirmed by the NMR measurements shown in
figure 3.6.
When all water has evaporated from the solution (t > 32000 s), dehydration of
heptahydrate takes place. It is again accompanied by the formation of a thenardite layer
covering the heptahydrate crystals (see Fig. 3.7-D), as was also observed under nonwetting
conditions.