Digital video microscopy is used to study the Brownian motion in quasibidimensional colloidal systems, consisting of spherical polystyrene particles suspended in water and confined between two glass plates. This technique allows the direct measurement of the lateral ͑two-dimensional͒ probability distribution function, P(⌬r,t), of the random variable ⌬r ͑the particle displacement͒ at time t, and the mean squared displacement W(t). We studied the effect of confinement in highly diluted samples, where W(t) is found to be a linear function of time. The hydrodynamic interactions between the colloidal particles and the glass walls are found to be more important than predicted by approximate hydrodynamic theories. Keeping fixed the separation between the plates, we studied the effect of direct and hydrodynamic interactions between the particles by increasing the particle concentration. In this case, the short time dynamics is characterized by means of a theoretical approach that describes self-diffusion in terms of the static structure of the suspension. In all the samples studied, we found negligible deviations of P(⌬r,t) from Gaussian behavior.

We report measurements of the effective pair potential between charged colloidal particles in a bidimensional matrix of fixed obstacles. A binary mixture of polystyrene spheres in water is confined between two glass plates. The larger particles are trapped by the plates in a disordered configuration with respect to which the smaller species of particles equilibrates. The structures of both the mobile and the fixed species are measured by videomicroscopy. The pair potential, obtained by deconvoluting the structural information via the Ornstein-Zernike equation, exhibits two attractive components.[S0031-9007(98)06744-1] PACS numbers: 82.70.Dd, 05.40. + j, 47.55.Mh, 61.20.Gy Recent measurements of the direct interaction between charged colloidal particles in suspension, such as polystyrene spheres in water, have provided evidence of an attractive effective pair potential between the particles under conditions of confinement [1-3]. The physical properties of such systems have been extensively studied in the homogeneous three-dimensional (3D) space. In this case, the assumption of a repulsive screened Coulomb pair potential, as the one derived by Derjaguin, Landau, Verwey, and Oveerbek [4], provides a functional form for the interparticle potential in terms of which the experimental observations have been reasonably well described [5,6]. However, for the same kind of systems but now confined between two parallel glass plates, direct and indirect determinations of the effective interparticle potential show an attractive component, rather than repulsive, at intermediate distances [1][2][3]. This observation is quite interesting and raises the question about the physical origin of such effective attractive interaction, and also whether this interaction is further modified under different conditions of confinement, for instance, in an arbitrary geometry such as a disordered porous medium. The answer to these questions is quite important because the accurate determination of colloidal interactions is essential in order to understand on a fundamental basis colloidal properties such as their stability, structure, dynamics, thermodynamics, and so on. Here we address the latter question, namely, the question about the sensitivity of the effective colloidal interactions on the local environment. In this Letter, we report indirect measurements of the effective interaction between charged colloidal particles in suspension when it is permeating a bidimensional porous matrix. We measure both the structure of the colloidal suspension and the porous matrix. Then, the effective pair potential is determined by deconvoluting the information contained in the structural properties of the system, by using the multicomponent Ornstein-Zernike (O-Z) integral equation and a closure relation. This deconvoluting method was employed to determine the effective interaction potential between polystyrene spheres in water, confined between two parallel glass plates in such a way that the system becomes an effective two-dimensional colloidal suspens...

Partially quenched quasibidimensional colloidal suspensions are obtained by confining a bidisperse aqueous suspension of polystyrene spheres between two glass plates. The larger particles are fixed between the plates, forming a disordered bidimensional matrix of obstacles, with respect to which the colloidal suspension of the smaller species of particles equilibrates. Digital video microscopy is used to measure the static structure of both the colloidal suspension and the matrix for various fixed particles concentrations. The effective pair potentials between the mobile particles u 11 (r) and between mobile and fixed particles u 12 (r) are obtained by deconvoluting the structural information of the system via the Ornstein-Zernike equation. The measured pair potential u 11 (r) exhibits two attractive components. One of them, at intermediate distances, is present at all fixed particle concentrations, while the second one, of longer range, develops as the number of obstacles increases. The pair potential u 12 (r) also has an attractive component at intermediate distances.

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