Drying of a colloidal suspension in confined geometry.

We describe experiments on drying of a hard-sphere colloidal suspension in confined geometry where a drop of the suspension is squeezed in between two circular transparent plates and allowed to dry. In this situation, the geometry controls the vapor removal rate and leads to a facilitated observation directly inside the drop. We monitor the drying kinetics of colloids of two sizes and several volume fractions; in most cases, the drying kinetics leads to the formation of a crust at the level of the meniscus which can be either crystalline or glassy, the transition between the two cases being triggered by the local deposition velocity, itself slaved to the evaporation rate. It yields a final dry state which is either polycrystalline or amorphous. The crust is also responsible for a shape instability of the quasi-two-dimensional drop shrinking upon evaporation but with a crust opposing mechanical and flow resistance, and possibly a partial adhesion on the substrate.