Expansion of charged colloids after centrifugation: formation and crystallisation of long-range repulsive glasses

We studied long-range repulsive glasses formed in suspensions of sterically stabilised charged colloidal poly(methyl methacrylate) particles (sigma = 2.23 mu m) with low polydispersity (4%) in the low-polar solvent cyclohexyl bromide (epsilon(r) = 7.92). Particle interactions were described by a long-range repulsive Yukawa potential. Glasses were obtained upon compression of the suspensions by centrifugation from a body-centred-cubic crystalline structure at low initial volume fraction (eta approximate to 0.02) to a close-packed amorphous structure (eta approximate to 0.64). Subsequent expansion of the sediment in gravity resulted in long-range repulsive glassy structures with volume fractions eta = 0.16-0.64. The presence of small clusters (mostly dumbbells; clustered fraction f(cl) >= 0.12) formed by centrifugation prevented the glasses from crystallising for several weeks, while the sediment was still expanding. We used confocal microscopy to obtain three-dimensional datasets of the system and quantitatively analysed the structure of the glasses. The structure of the glasses was found to be remarkably similar to that of hard-sphere glasses, for which experimental data were obtained by centrifugal compression of silica spheres with a hard potential, despite the much longer-range interaction potential. After more than ten weeks the clustered fraction decreased due to spontaneous dissociation of the clusters, and finally bulk crystallisation of the glasses was observed into face-centred-cubic crystals with a volume fraction of around 0.22.