Drying dip-coated colloidal films

Joaquim Li1, Bernard Cabane1, Michael Sztucki2, Jérémie Gummel2, and Lucas Goehring3

    1PMMH, ESPCI, Paris, France

    2ESRF, Grenoble, France

    3MPI Dynamics and Self-Organization, Göttingen, Germany

Langmuir, 28, 200-208 (2012).

Download preprint pdf: Dip_coat_rev.pdf

Supplementary information: Li2011_SI_revised.pdf

We present the results from a Small Angle X-ray Scattering (SAXS) study of lateral drying in thin films. The films, initially 10 μm thick, are cast by dip-coating a mica sheet in an aqueous silica dispersion (particle radius 8 nm, volume fraction φs = 0.14). During evaporation, a drying front sweeps across the film. An X-ray beam is focused on a selected spot of the film, and SAXS patterns are recorded at regular time intervals. As the film evaporates, SAXS spectra measure the ordering of particles, their volume fraction, the film thickness and the water content, and a video camera images the solid regions of the film, recognised through their scattering of light. We find that the colloidal dispersion is first concentrated to φs = 0.3, where the silica particles begin to jam under the effect of their repulsive interactions. Then the particles aggregate until they form a cohesive wet solid at φs = 0.68 ± 0.02. Further evaporation from the wet solid leads to evacuation of water from pores of the film, but leaves a residual water fraction φw = 0.16. The whole drying process is completed within three minutes. An important finding is that, in any spot (away from boundaries), the number of particles is conserved throughout this drying process, leading to the formation of a homogeneous deposit. This implies that no flow of particles occurs in our films during drying, a behavior distinct to that encountered in the iconic coffee-stain drying. It is argued that this type of evolution is associated with the formation of a transition region that propagates ahead of the drying front. In this region the gradient of osmotic pressure balances the drag force exerted on the particles by capillary flow toward the liquid-solid front.