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From: Nicolas Tsapis, Laboratoire de Pharmacie Galénique, UMR CNRS 8612, Faculté de Pharmacie, Châtenay-Malabry, France.
http://www.umr-cnrs8612.u-psud.fr/tsapis/perso/droplets.html
To reproduce the non-wetting situation of the droplet in the lab, we have used the Leidenfrost effect: when placed on a hot plate (temperature above 150 degrees celsius), water droplets are sustained on their own vapor, thus they are not wetting the substrate and are spherical.
When we dry a 10 microliters drop containing 2mg/ml 170nm colloidal polystyrene, we first observe that the drop shrinks staying spherical and then it buckles as shown here.
We define the buckling radius as the radius measured just before the shell starts to buckle (i.e. when the shell start becoming non spherical). We observe that the buckling radius scales linearly with the initial drop and with colloid volume fraction phi to the 1/3.
Our explanation of the buckling instability is the following: the peclet number of the system (ratio of drying time over diffusion time) is large so diffusion is too slow to redistribute particles within the drop. Therefore colloids accumulate near the air-water interface forming a dense region. At the beginning because of the electrostatic repulsions between colloids they slip past each other without sticking. As the compact region grows the capillary pressure due to evaporation increases until it overcomes the electrostatic repulsions. At this moment the colloids start to stick to each other and form a solid shell that is buckling because it experiences compression due to the volume decrease coming from evaporation. (See the next slide.)
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