8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

Thu 811:10-14:00P8.59Compressing charged colloids by centrifugation:formation of soft glassesMarjolein van der Linden, 1 Marjolein Dijkstra, 1 and Alfons van Blaaderen 11 Soft Condensed Matter, Debye Institute, Princetonplein 1, 3584 CC, Utrecht,NetherlandsOur model system consists of fluorescently labelled charged colloidal particles suspended in a lowpolarsolvent, for which the pair interaction can be modelled by the Yukawa (screened Coulomb)potential. The particles are surrounded by an electrical double layer formed by ions present in thesolvent, with a double layer thickness (or Debye screening length) larger than the particle diameter,resulting in very long-ranged repulsive interactions. We compressed this system by centrifugationfrom a low initial density. In this way, we were able to obtain glassy structures (amorphous, butlacking significant diffusive motion) at packing fractions well below the packing fraction of thehard-sphere glass transition (φ ≈ 0.58). We studied these structures using laser-scanning confocalmicroscopy. We obtained the particle coordinates from 3D datastacks of confocal images andanalysed quantitatively using bond-orientational order parameters and their correlations. Due tothe long-range interactions between the particles, the way in which the system is compressed israther different from hard-sphere systems. In a typical compression experiment we do not reachrandom close packing (φ ≈ 0.64). While it is easily obtained for hard spheres, in our case eventhe densest (and amorphous) part of the sediment stays well below this density. In addition, weobserve expansion of the layer of sedimented particles against gravity, from the moment we stopcentrifugation until the system stabilises when the equilibrium sedimentation profile is reached.We study the stability of the glassy structures, ordering and crystallisation in the system over time.59