Sedimentation and multi-phase equilibria in mixtures of platelets and ideal polymer

Utrecht University, Utrecht, Utrecht, Netherlands
EPL (Europhysics Letters) (Impact Factor: 2.1). 01/2007; 66(1):125. DOI: 10.1209/epl/i2003-10140-1
Source: OAI


The role of gravity in the phase behaviour of mixtures of hard colloidal plates without and with non-adsorbing ideal polymer is explored theoretically. By analyzing the (macroscopic) osmotic equilibrium conditions, we show that sedimentation of the colloidal platelets is significant on a height range of even a centimeter. Gravity enables the system to explore a large density range within the height of a test tube which may give rise to the simultaneous presence of multiple phases. As to plate-polymer mixtures, it is shown that sedimentation may lead to a four-phase equilibrium involving an isotropic gas and liquid phase, nematic and columnar phase. The phenomenon has been observed experimentally in systems of colloidal gibbsite platelets mixed with PDMS polymer.

  • Source
    • "We have checked that the results from our approach, using the free-volume theory of Ref. [25] to describe the bulk phase diagram of the mixture, match those of the effective one-component approach by Wensink et. al [26] and the experiments by van der Kooij et. al [27]. "
    [Show abstract] [Hide abstract]
    ABSTRACT: The observation of stacks of distinct layers in a colloidal or liquid mixture in sedimentation-diffusion equilibrium is a striking consequence of bulk phase separation. Drawing quantitative conclusions about the phase diagram is, however, very delicate. Here we introduce the Legendre transform of the chemical potential representation of the bulk phase diagram to obtain a unique stacking diagram of all possible stacks under gravity. Simple bulk phase diagrams generically lead to complex stacking diagrams. We apply the theory to a binary hard core platelet mixture with only two-phase bulk coexistence, and find that the stacking diagram contains six types of stacks with up to four distinct layers. These results can be tested experimentally in colloidal platelet mixtures. In general, an extended Gibbs phase rule determines the maximum number of sedimented layers to be $3+2(n_b-1)+n_i$, where $n_b$ is the number of binodals and $n_i$ is the number of their inflection points.
    Full-text · Article · May 2013 · Soft Matter
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: A geometry-based density-functional theory is presented for mixtures of hard spheres, hard needles, and hard platelets; both the needles and platelets are taken to be of vanishing thickness. Geometrical weight functions that are characteristic for each species are given, and it is shown how convolutions of pairs of weight functions recover each Mayer bond of the ternary mixture and hence ensure the correct second virial expansion of the excess free-energy functional. The case of sphere-platelet overlap relies on the same approximation as does Rosenfeld's functional for strictly two-dimensional hard disks. We explicitly control contributions to the excess free energy that are of third order in density. Analytic expressions relevant for the application of the theory to states with planar translational and cylindrical rotational symmetry--e.g., to describe behavior at planar smooth walls--are given. For binary sphere-platelet mixtures, in the appropriate limit of small platelet densities, the theory differs from that used in a recent treatment [L. Harnau and S. Dietrich, Phys. Rev. E 71, 011504 (2004)]. As a test case of our approach we consider the isotropic-nematic bulk transition of pure hard platelets, which we find to be weakly first order, with values for the coexistence densities and the nematic order parameter that compare well with simulation results.
    Full-text · Article · Feb 2006 · Physical Review E
  • [Show abstract] [Hide abstract]
    ABSTRACT: The results of an experimental study focused on the effect of added silica nanospheres on the structure of an aqueous suspension of disc-shaped kaolinite particles are presented. In the absence of any additives, kaolinite particles rapidly aggregate and settle. When only nanoparticles were added to a 14% vol. kaolinite suspension, some stabilization was observed, although a thick, fluid-like sediment still formed. Adding both nanoparticles and salt (NaCl or KCl), however, caused the entire suspension to transition into a solid material that was strong enough to actually be sliced. A phase diagram was constructed showing the concentration of salt and nanoparticles needed to produce this transition. With smaller nanoparticles, the transition occurred at much lower nanoparticle volume fractions. Scanning electron micrographs of both the sediment and solid-like material, obtained by cryogenic drying, showed that the latter consisted of a porous, 'sponge-like' structure. The characteristic size of the pores decreased as the number density of the added nanoparticles increased. Although the nanoparticles were not visible in the SEM images, it is believed that they had separated into the pores of the solid-like material. While a similar type of transition could be produced in suspensions containing only the silica nanospheres, the structure and flow behavior of this material were markedly different from that obtained with the added clay.
    No preview · Article · Jun 2006 · Journal of Colloid and Interface Science
Show more