Migration and precipitation of soluble species during drying of colloidal films

The evaporation-induced convection resulted in a transport of dissolved species, a water-soluble polymer (carboxymethylcellulose) and dissolved CaCO ₃, to the drying front of silica and CaCO ₃ dispersions where the material eventually precipitates. Scanning electron microscopy and chemical analysis showed that the concentration of carboxymethylcellulose, CMC, is highest in the centre of the dried silica film and decreases towards the perifery. The colloidal films of the monodisperse silica particles displayed a high degree of structural order even at high concentrations of the non-adsorbed polymer CMC, which suggests that any depletion induced interparticle attraction is insufficient to affect the assembly of the colloidal crystal. The CaCO ₃ particles are slightly soluble and we found that rod-like crystals reprecipitated in the centre of the particle films on top of the polyacrylate-coated particles. Addition of CMC disturbs the formation of distinct crystal shapes which was attributed to a complexation of Ca ²⁺ in solution.