  We performed a thorough investigation of the drying dynamics of a charged
colloidal dispersion drop in a confined geometry. We developed an original
methodology based on Raman micro-spectroscopy to measure spatially-resolved
colloids concentration profiles during the drying of the drop. These
measurements lead, for the first time, to estimates of the collective diffusion
coefficient of the dispersion over a wide range of concentration. The
collective diffusion coefficient is one order of magnitude higher than the
Stokes-Einstein estimate showing the importance of the electrostatic
interactions for the relaxation of concentration gradients. At the same time,
we also performed fluorescence imaging of tracers embedded within the
dispersion during the drying of the drop, which reveals two distinct regimes.
At early stages, concentration gradients along the drop lead to
buoyancy-induced flows. Strikingly, these flows do not influence the colloidal
concentration gradients that generate them, as the mass transport remains
dominated by diffusion. At longer time scales, the tracers trajectories reveal
the formation of a gel which dries quasi homogeneously. For such a gel, we show
using linear poro-elastic modeling, that the drying dynamics is still described
by the same transport equations as for the liquid dispersion. However, the
collective diffusion coefficient follows a modified generalized Stokes-Einstein
relation, as also demonstrated in the context of unidirectional consolidation
by Style et al. [Crust formation in drying colloidal suspensions, Style et al.,
Proc. R. Soc. A 467, 174 (2011)].
