Mass transport in a drying drop of a charged colloidal dispersion: new insights using Mach-Zehnder interferometry

In the present work, we use Mach-Zehnder interferometry to thoroughly investigate the drying dynamics of a 2D confined drop of a charged colloidal dispersion. This technique makes it possible to measure the colloid concentration field during the drying of the drop at a high accuracy (about 0.5.{\%}) and with a high temporal and spatial resolution (about 1 frame/s and 5 $\mu $m/pixel). These features allow us to probe mass transport of the charged dispersion in this out-of-equilibrium situation. In particular, our experiments provide the evidence that mass transport within the drop can be described by a purely diffusive process for some range of parameters for which the buoyancy-driven convection is negligible. We are then able to extract from these experiments the collective diffusion coefficient of the dispersion $D(\varphi )$ over a wide concentration range $\varphi =$ 0.24.$-$0.5, i.e. from the liquid dispersed state to the solid glass regime, with a high accuracy. The measured values of $D(\varphi )\simeq $5.$-$12$D_{\mathrm{0}}$ are significantly larger than the simple estimate $D_{\mathrm{0}}$ given by the Stokes-Einstein relation, thus highlighting the important role played by the colloidal interactions in such dispersions.