Direct observation of concentration profiles induced by drying of a 2D colloidal dispersion drop.

We report experimental results on the drying of a colloidal dispersion drop in a circular thin cell. This confined geometry is well adapted to quantify concentration profiles inside the drop using fluorescence microscopy. Two stages have been identified in the drop evolution. In the first one the drop is shrinking such as if a pure drop, keeping axisymmetry. In the second one strong distortions occur and result in the appearance of a local depression at the drop surface. This process results in the spontaneous formation of a complex drop shape with both concave and convex interfaces. The influence of the interface concavity on the concentration profiles inside the drop and the drying kinetics are investigated. Particularly, concentration profiles are related to the nonuniform evaporation rate at the distorted drop surface.