[Show abstract][Hide abstract] ABSTRACT: We consider colloidal platelets under the influence of gravity and an external aligning (magnetic) field. The system is studied using a fundamental measures density functional theory for model platelets of circular shape and vanishing thickness. In the gravity-free case, the bulk phase diagram exhibits paranematic-nematic phase coexistence that vanishes at an upper critical point upon increasing the strength of the aligning field. Equilibrium sedimentation profiles display a paranematic-nematic interface, which moves to smaller (larger) height upon increasing the strength of gravity (the aligning field). The density near the bottom of the system decreases upon increasing the strength of the aligning field at fixed strength of gravity. Using a simple model for the birefringence properties of equilibrium states, we simulate the color variation with height, as can be observed in samples between crossed polarizers.
The Journal of Chemical Physics 04/2010; 132(14):144509. DOI:10.1063/1.3378264 · 2.95 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We study bulk and interfacial properties of a model suspension of hard colloidal platelets with continuous orientations and vanishing thickness using both density functional theory, based on either a second virial approach or fundamental measure theory (FMT), and Monte Carlo (MC) simulations. We calculate the bulk equation of state, bulk isotropic-nematic (IN) coexistence, and properties of the (planar) free IN interface and of adsorption at a planar hard wall, where we find complete wetting of the nematic phase at the isotropic-wall interface upon approaching bulk IN coexistence. We investigate in detail the asymptotic decay of correlations at large distances. In all cases, the results from FMT and MC agree quantitatively. Our findings are of direct relevance to understanding interfacial properties of dispersions of colloidal platelets.
The Journal of Physical Chemistry B 08/2007; 111(27):7825-35. DOI:10.1021/jp068870b · 3.30 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We use density functional theory to study the capillary phase behaviour of a discotic system of colloidal platelets that are confined in a planar slit pore. The model plates have circular shape, continuous orientations and vanishing thickness; they interact via hard-core repulsion with each other and with the walls which induces homeotropic wall anchoring of the nematic director. We find that the isotropic–nematic capillary binodal is shifted to lower values of the chemical potential as compared to bulk isotropic–nematic coexistence. Capillary isotropic–nematic coexistence vanishes below a critical wall separation distance which is significantly larger than it is in a reference system of thin hard (Onsager) rods confined between two parallel hard walls that act on the particle centres.
[Show abstract][Hide abstract] ABSTRACT: We study interfacial phenomena in a colloidal dispersion of sterically stabilized gibbsite platelets, exhibiting coexisting isotropic and nematic phases separated by a sharp horizontal interface. The nematic phase wets a vertical glass wall and polarized light micrographs reveal homeotropic surface anchoring both at the free isotropic-nematic interface and at the wall. On the basis of complete wetting of the wall by the nematic phase, as found in our density functional calculations and computer simulations, we analyze the balance between Frank elasticity and surface anchoring near the contact line. Because of weak surface anchoring, the director field in the capillary rise region is uniform. From the measured rise (6 m) of the meniscus at the wall we determine the isotropic-nematic surface tension to be 3 nN=m, in quantitative agreement with our theoretical and simulation results.
[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.
[Show abstract][Hide abstract] ABSTRACT: A binary quenched-annealed hard core mixture is considered in one dimension in order to model fluid adsorbates in narrow channels filled with a random matrix. Two different density functional approaches are employed to calculate adsorbate bulk properties and interface structure at matrix surfaces. The first approach uses Percus' functional for the annealed component and an explicit averaging over matrix configurations; this provides numerically exact results for the bulk partition coefficient and for inhomogeneous density profiles. The second approach is based on a quenched-annealed density functional whose results we find to approximate very well those of the former over the full range of possible densities. Furthermore we give a derivation of the underlying replica density functional theory. Comment: 22 pages, 11 figures, to be published in JSP