P - Technische Universiteit Eindhoven

3. Crystallization under wetting/non-wetting conditions 32
Actually, all these studies have shown that by cooling of a sodium sulfate solution the
heptahydrate phase is formed.
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Fig. 3.1: Phase diagram of sodium sulfate. The equilibrium solubility is given for mirabilite
(Na 2 SO 4·10H 2 O), thenardite (Na 2 SO 4 ), and the metastable form, i.e., heptahydrate
(Na 2 SO 4·7H 2 O). The dashed line indicates the supersolubility curve for heptahydrate,
i.e., an upper bound for the supersaturation. The regions in-between the solubility
curves allow coexistence of the solution with certain crystalline phases. The combination
of horizontal and vertical arrows indicates the various stages of the experiments.
In the present study we investigate the crystallization from sodium sulphate solutions
induced by drying of the solution. This was studied by time-lapse microscopy of sodium
sulfate solution droplets by Shahidzadeh-Bonn [62,63], but in that study the concentration
of the solution was not measured. Here we combine time-lapse microscopy of the sodium
sulphate droplet with the simultaneous measurement of the concentration of the solution
by NMR. We have studied the crystallization on both wetting and non-wetting surfaces.
We will first briefly discuss the main consequences of the (non) wetting surface on the
drying behavior of a droplet of a salt solution. Next the experimental setup and the
results will be presented.
3.2 The influence of the surface on the drying
When a droplet on a solid surface is drying, a non-wetting or a wetting condition can be
present, depending on the surface material. These conditions have large implications for
the contact angle between fluid and solid and therefore for the drying process. In the case