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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
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.
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