Drying dynamics of a charged colloidal dispersion in a confined drop

TL;DR

This study investigates the drying behavior of charged colloidal drops in confined spaces, introducing a novel Raman spectroscopy method to measure concentration profiles and revealing the role of electrostatic interactions and gel formation during drying.

Contribution

It presents a new methodology for spatially-resolved concentration measurements and estimates the collective diffusion coefficient, highlighting electrostatic effects and gel formation in drying colloidal drops.

Findings

Collective diffusion coefficient is an order of magnitude higher than Stokes-Einstein estimate.

Buoyancy-induced flows occur early but do not affect concentration gradients.

Drying dynamics of gels follow similar transport equations as liquid dispersions.

Abstract

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)].