A geometry-based density functional theory is presented for mixtures of hard spheres, hard needles and hard platelets; both the needles and the 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.

Using a geometry-based density functional theory we investigate the free interface between demixed bulk fluid phases of a colloidal mixture of hard spheres and vanishingly thin needles. Results are presented for the spatial and orientational density distributions of the particles, as well as for the interface tension. Density profiles display oscillations on the sphere-rich side of the interface provided the sphere liquid phase is on the oscillatory side of the Fisher-Widom line in the bulk phase diagram. Needles tend to align parallel (perpendicular) to the interface on the needle-rich (sphere-rich) side displaying biaxial (uniaxial) order. Furthermore, we generalize the theory to the Onsager limit for interacting rods, and give explicit expressions for the functional in simple geometries.

Equilibrium sedimentation density profiles of charged binary colloidal suspensions are calculated by computer simulations and density functional theory. For deionized samples, we predict a colloidal "brazil nut" effect: heavy colloidal particles sediment on top of the lighter ones provided that their mass per charge is smaller than that of the lighter ones. This effect is verifiable in settling experiments. PACS numbers: 82.70.Dd, 61.20.Ja, 05.20.Jj Binary systems of granular matter separate upon shaking in gravity, so that the larger particles lie on top of the smaller ones even if they are heavier and denser than the latter. This is due to a sifting mechanism in which tiny grains filter through the interstices between the large particles which is well-known as "brazil nut" effect: in a jar of mixed nuts or in a package of cereal, the largest species rises to the top [1,2]. This clearly distinguishes granular matter from ordinary fluids where the rising species is controlled by Archimedes' law. Understanding the full details of the brazil nut effect is still a problem; recently even a reverse brazil nut effect of large light grains sinking in a granular bed has been predicted [3,4,5] and verified in experiments [6].In this letter, we report on equilibrium density profiles of binary charged colloidal fluids ("macroions") under gravity. Using extensive Monte-Carlo computer simulations of the "primitive" model [7] of strongly asymmetric electrolytes and a density functional theory, we predict that the heavier particles sediment on top of the lighter ones provided the charge per mass of the heavier particles is higher. In analogy to granular matter, we call this counter-intuitive phenomenon a colloidal brazil nut effect. It is generated by the entropy of the microscopic counterions in the solution, which are coupled to the macroions by strong Coulomb binding. Clearly, though this effect is qualitatively similar to the granular brazil nut effect insofar as heavy particles are on top of lighter ones, its physical origin is different: first, the particle charge (and not the size) is crucial. Second, the colloidal brazil nut effect is a pure equilibrium phenomenon while the granular brazil nut effect happens intrinsically in non-equilibrium. The colloidal brazil nut effect can be verified, e.g., in depolarized-light scattering or real-space experiments on sediments of strongly deionized binary charged suspensions [8,9]. Similar techniques have been used to measure one-component colloidal density profiles [9,10] where deviations from the ideal barometric law [11] are still an ongoing debate [12,13,14,15,16].We simulate the asymmetric "primitive" model of binary charged suspensions in which the solvent only enters via a continuous dielectric background with permittivity ǫ but all charged particles (two species of negatively charged macroions and microscopic counter-and coions) are treated explicitly at constant temperature T . With Z 1 e, Z 2 e, −qe and σ 1 , σ 2 , σ c denoting the charges and the diameters of the two ...

The influence of substrate roughness on the wetting scenario of adsorbed van der Waals films is investigated by theory and experiment. Calculating the bending free energy penalty of a solid sheet picking up the substrate roughness, we show that a finite roughness always leads to triple-point wetting reducing the widths of the adsorbed solid films considerably as compared to that of smooth substrates. Testing the theory against our experimental data for molecular hydrogen adsorbed on gold, we find quantitative agreement. DOI: 10.1103/PhysRevLett.88.055702 PACS numbers: 64.70.Hz, 67.70. +n, 68.08.Bc, 68.35.Rh Wetting of a solid substrate, exposed to a gas in thermodynamic equilibrium, is an ubiquitous phenomenon, with both fundamental aspects [1,2] and important applications [3 -5]. Microscopically, substrate wetting by a liquid film is caused by a strong substrate-particle attraction mediated by van der Waals forces. At present, an almost complete microscopic understanding of wetting on flat solid substrates is available [1,2,6] predicting the thickness of the liquid film as a function of the substrate-particle and interparticle interactions for given thermodynamic parameters such as temperature and pressure. The following basic theoretical predictions were confirmed by experiments using, e.g., noble gases [1] on different substrates: (i) For fixed thermodynamic conditions, the thickness of the wetting layer grows for increasing substrate-particle attraction. (ii) Complete wetting (i.e., a diverging thickness of the liquid layer) occurs if the substrate-particle attraction is stronger than the interparticle interaction and the thermodynamic conditions approach liquid-gas coexistence. The latter condition can be achieved only if the system temperature T is above the triple temperature T 3 . For T , T 3 , on the other hand, a solid film shows up near the sublimation line. Various experiments have shown [7][8][9][10] that the width of the solid layer always remains finite when approaching gas-solid coexistence. It is only near the triple point that a liquid layer on top of the solid sheet is formed, with a diverging width as the triple point is approached. This universal behavior is called "triple-point wetting."One major difference between a liquid and solid wetting layer is that a solid cannot relax the elastic compression caused by the substrate attraction as embodied in the (reduced) wall-particle Hamaker constant R. This fact is the basic ingredient in the traditional Gittes-Schick theory [11] of solid adsorption on flat substrates. It predicts that, for a particular value R R 0 of the substrate attraction, complete wetting is possible, while for R . R 0 , in contrast to liquid wetting, the thickness of the solid film ᐉ s decreases with increasing R. In this Letter we show that the key parameter governing adsorption of solid films is the substrate roughness rather than the elastic deformation caused by the substrate attraction. As a result of our theoretical analysis, a finite substrate roughness leads inev...

The physics of wetting phenomena at structured surfaces by crystalline layers as investigated by theory, computer simulation and experiments is reviewed. Both realizations on the molecular scale and more mesoscopic realizations in colloidal models systems are included. We explore how a crystalline wall pattern affects the wetting by a crystalline phase in the context of a simple hard sphere model relevant for sterically stabilized colloids. We further discuss decoration lattices generated by adsorption of colloidal particles on stripepatterned substrates. For molecular systems, the influence of a rough and preplated surface on triple-point wetting of hydrogen is calculated. Finally, we present data for fluid layering in primitive model simulations of charged colloids near neutral walls.