Imaging the drying of a colloidal suspension

We present an experimental investigation of the drying kinetics seen from inside a sessile droplet laden with a colloidal sol of silica nanoparticles. We use fast, two-color confocal microscopy imaging to quantitatively extract on the one hand the concentration field of the rhodamine-tagged nanosol and on the other hand the velocity field and the mobility field of large, fluorescein-tagged tracers. By changing the initial concentration at which the drop dries up, we propose a method that yields a self-consistent way to obtain the rheology of the sol. Based on these results, we analyse the drying kinetics in terms (i) of flow patterns that include evaporating and Marangoni flows which compete to determine the final concentration profile and (ii) of truncated dynamics that we quantitatively relate to the rheology of the sol.

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