Vitrification in restricted geometry: dry films of colloidal particles
We prepared dry films of colloidal particles from surfactant co-stabilized polystyrene and poly(methyl methacrylate) latex spheres of moderate polydispersity. The film thickness was varied from monolayer to some 20 μm. The average structure and the influence of polydispersity was analyzed by small-angle X-ray scattering. An analysis of local microscopic structures was performed by force microscopy on the least polydisperse sample. Irrespective of the thickness, we observe the formation of only short-ranged order which is rather pronounced but changes its qualitative character in the vicinity of the substrate. The observations suggest a bulk structure of small, defect-rich and stacking faulted clusters of local face-centred cubic symmetry and the co-presence of small compact voids. Closer to the substrate we find clear evidence of a tendency to form layers parallel to the substrate. This is interpreted as induction of orientational order by the presence of the restricting wall. While the local environs are often well ordered, the range of crystalline or in-layer order is restricted to only a few dozen particle diameters. This behaviour differs significantly from that observed in other systems and may be due to particularly small adsorbate-adsorbate, adsorbate-substrate and capillary forces.
Key wordsHard spheres Restricted geometry Vitrification Layering Colloids
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