We report experimental evidence for an enhancement, due to the hydrodynamic interactions (HI), of the self-diffusion function D s ͑t͒ of colloidal particles at intermediate and long times. Monolayers of paramagnetic polystyrene spheres confined to an air/water interface are studied using digital videomicroscopy. The interparticle potential tuned by an external magnetic field is accurately calibrated by comparing measured radial distribution functions with computer simulation results. This allows one to separate the effects of HI on D s ͑t͒ from those of the direct interactions.[S0031-9007(97)

The optical and structural properties of dense colloidal suspensions in the presence of long-range electrostatic repulsion are determined from both light and small-angle neutron scattering experiments. Short-range structural order induces an enhancement of the scattering strength while at the same time the total transmission shows strong wavelength dependence, reminiscent of a photonic crystal. Interestingly, the interplay between diffusive scattering and local order leads to negative values of the scattering anisotropy parameter. The tunable optical properties of these liquids furthermore suggest potential applications such as transparency switches or filters. DOI: 10.1103/PhysRevLett.93.073903 PACS numbers: 42.25.Bs, 42.70.Qs, 82.70.Dd When light is incident on a nonabsorbing material, its further propagation is strongly influenced by the microscopic structure of the material itself. For a bulk homogeneous medium, light is refracted according to Snell's law. Local variations in the dielectric properties lead to isolated scattering events that disperse the light beam. The scatterer density and cross section define the scattering mean free path l. As the number of scattering events increases, the transport of light becomes diffusive and the material appears turbid or ''white'' [1,2]. The relevant scattering length for diffusive light transport is the transport mean free path l . Both quantities are connected by the scattering anisotropy parameter g defined as the average of the cosine of the scattering angle g hcosi, l=l 1 ÿ g. Our current understanding of the diffusive transport is based on the knowledge of these key quantities. In the absence of positional correlations, l is usually equal to or larger than l [2 -5]. For instance, Mie particles (or human tissue [1]) scatter strongly in the forward direction (small scattering angles ) and hence g ' 1 while for Rayleigh scatterers g ' 0. Here we show that these common properties of diffusive transport can be manipulated by tuning the interaction between scatterers. By the appropriate control of the Coulomb repulsion between highly charged particles in suspension, we are now able to access the whole possible interval of g values (from forward scattering g ! 1 to the unusual case of strong backscattering g ! ÿ1).When mesoscopic variations of the dielectric constant can be neatly controlled over macroscopic distances, totally new, so-called photonic properties may appear [6]. At the core of the design of new photonic materials lies the intelligent way structures are assembled on length scales comparable to the wavelength of light. There are two main promising concepts to achieve lossless guidance and manipulation of light based on seemingly opposite principles: order or disorder. Photonic band gap materials are based on periodic structures predicted to inhibit light propagation completely [6]. In the case of disorder, light cannot propagate in the material due to recurrent interference called strong Anderson localization [7].Tailoring microstructures with an app...

Depletion forces are accounted for by a contraction of the description of colloidal mixtures based on the integral equations theory of simple liquids. The applicability of this treatment is illustrated for binary mixtures of hard spheres, in the bulk and near a hard wall. The Asakura and Oosawa potential is obtained as the dilute limit of our equations. At higher concentrations the depletion potential has an oscillatory behavior and becomes more long ranged. If charge is put on the small particles there are energy-driven depletion forces in addition to those of entropic origin, which result in repulsive interaction at contact.

The many-particle Langevin equation, written in local coordinates, is used to derive a Brownian dynamics simulation algorithm to study the dynamics of colloids moving on curved manifolds. The predictions of the resulting algorithm for the particular case of free particles diffusing along a circle and on a sphere are tested against analytical results, as well as with simulation data obtained by means of the standard Brownian dynamics algorithm developed by Ermak and McCammon [J. Chem. Phys. 69, 1352 (1978)] using explicitly a confining external field. The latter method allows constraining the particles to move in regions very tightly, emulating the diffusion on the manifold. Additionally, the proposed algorithm is applied to strong correlated systems, namely, paramagnetic colloids along a circle and soft colloids on a sphere, to illustrate its applicability to systems made up of interacting particles.

Depletion forces can be accounted for by a contraction of the description in the framework of the integral equations theory of simple liquids. This approach includes, in a natural way, the effects of the concentration on the depletion forces, as well as energetic contributions. In this paper we systematically study this approach in a large variety of dilute colloidal systems composed of spherical and nonspherical hard particles, in two and in three dimensions, in the bulk and in front of a hard wall with a relief pattern. We show by this way the form in which concentration and geometry determine the entropic interaction between colloidal particles. The accuracy of our results is corroborated by comparison with computer simulations.