Martín Chávez-Páez

Coalescence in double emulsions.

Coalescence processes in double emulsions, water-in-oil-in-water, are studied by optical microscopy. The time evolution of such systems is determined by the interplay of two coalescence processes, namely, between inner water droplets and between the inner water droplets and the continuous external water phase. The predominance of one of those processes over the other, regulated by the relative amount of hydrophilic and lipophilic surfactants, leads to different evolutions of the system. We present here results for a class of systems whose evolution follows a master behavior. We also implemented a computer simulation where the system is modeled as a spherical cavity filled with smaller Brownian spheres. Collisions between spheres allow coalescence between them with probability P(i), whereas collisions between a sphere and the wall of the cavity allow coalescence with the external phase with probability P(e). The phenomenology observed in the experimental systems is well reproduced by the computer simulation for suitable values of the probability parameters.

Dynamic equivalence between atomic and colloidal liquids

We show that the kinetic-theoretical self-diffusion coefficient of an atomic fluid plays the same role as the short-time self-diffusion coefficient DS in a colloidal liquid, in the sense that the dynamic properties of the former, at times much longer than the mean free time, and properly scaled with DS, will be indistinguishable from those of a colloidal liquid with the same interaction potential. One important consequence of such dynamic equivalence is that the ratio DL/DS of the long-time to the short-time self-diffusion coefficients must then be the same for both an atomic and a colloidal system characterized by the same inter-particle interactions. This naturally extends to atomic fluids a well-known dynamic criterion for freezing of colloidal liquids (L?wen H. et al., Phys. Rev. Lett., 70 (1993) 1557). We corroborate these predictions by comparing molecular and Brownian dynamics simulations on the hard-sphere system and on other soft-sphere model systems, representative of the ?hard-sphere?dynamic universality class.

Structural properties of colloidal suspensions

Structural properties of three- and two-dimensional colloids composed by hard spheres and/or by Yukawa particles, which can have different diameters and charges, are studied by solving the Ornstein-Zernike equation, together with Percus-Yevick, hypernetted chain and Rogers-Young approximations. From the partial radial distribution functions gij(r) the partial structure factors Sij(k) are determined, and with them the compressibility structure factor Sx(k), the measured structure factor SM(k) and the Bhatia-Thornton structure factors SNN(k), SNQ(k) and SQQ(k). As an effect of diameter and/or charge polydispersity on the structure of binary mixtures, the position and height of the main peak of SM(k), and its value at k = 0, change non-monotonously with the composition. In the case of binary mixtures of hard and Yukawa spheres the structure is given by two different scales. A liquid-solid phase transition induced by a change in the dimensionality was found for monodisperse systems.

Structure and thermodynamics of discrete potential fluids in the OZ?HMSA formalism

We study the structural and thermodynamic properties of three discrete potential fluids: the square well (SW), the square well–barrier (SWB), and the square well–barrier–well (SWBW) fluids by means of the Ornstein–Zernike (OZ) integral equation and the HMSA (hybrid mean spherical approximation) closure relation. The radial distribution functions, structure factors, and pressure of the systems are calculated as a function of the strength of the attractive and repulsive parts of the potential in an extended range of densities, mainly covering the range 0.1 ≤ ρ* ≤ 0.9. We find that far away from the liquid–vapour coexistence region the HMSA theory is an accurate approach that compares well with Monte Carlo simulations. We also find that when the attractive parts of the potential dominate over the repulsive part the structure factor at low q values shows a considerable increase, which suggests the formation of large-scale domains that locally exhibit fluid-like structure.

Correlations among colloidal particles confined to a spherical monolayer

The internal structure of spherical colloidal monolayers of charged particles is studied here, both by means of Monte Carlo computer simulations and of an integral equation approach based on the application of the Ornstein–Zernike equation for spherical surfaces. The latter is complemented with a relatively fast and accurate numerical method for its solution, obtained by expanding the corresponding correlation functions in series of Legendre polynomials. It is found that the density correlations among the particles within the monolayer have some special features that differentiate them from the corresponding bulk corrections in open spaces. In particular, for a sufficiently small radius of the spherical monolayer, the distribution of particles around a particle fixed at one of the poles exhibits a peak at the opposite pole which is noticeably larger than the peaks immediately before it. It is also shown here that the introduction of a simple functional form with one adjustable parameter for the bridge fun...

The overdamped van Hove function of atomic liquids

Using the generalized Langevin equation formalism and the process of contraction of the description we derive a general memory function equation for the thermal fluctuations of the local density of a simple atomic liquid. From the analysis of the long-time limit of this equation, a striking equivalence is suggested between the long-time dynamics of the atomic liquid and the dynamics of the corresponding Brownian liquid. This dynamic equivalence is confirmed here by comparing molecular and Brownian dynamics simulations of the self-intermediate scattering function and the long-time self-diffusion coefficient for the hard-sphere liquid.

Brownian motion in asymmetric two-dimensional colloidal mixtures

Abstract In this paper we report some aspects of our work on the description of tracer-diffusion phenomena in model two-dimensional binary colloidal mixtures. Here we study the effects of asymmetries between the two brownian species. In particular, we consider the effects of the dynamic asymmetry produced when the free-diffusion mobility of one of the species is reduced, thus resembling a sluggish porous matrix permeated by a colloidal liquid (the species with a “normal” free-diffusion mobility). This study consists mostly of Brownian dynamics simulations, but a theoretical interpretation of the results is also attempted, with the assistance of an approximate theory of tracer-diffusion in equilibrium colloidal mixtures (the single exponential approximation).

A Monte Carlo study of the electrical double layer of a shape-asymmetric electrolyte around a spherical colloid

In this paper, we present a Monte Carlo simulation study on the structure of the electrical double layer around a spherical colloid surrounded by a binary electrolyte composed of spherical and non-spherical ions. Results are provided for the radial distribution functions between the colloid and ions, the orientation correlations between the colloid and non-spherical particles, and the integrated charge. Work is reported mainly for non-spherical particles modeled as spherocylinders, although a particular comparison is made between spherocylindrical particles and dimers. For the conditions investigated here, spherocylinders and dimers produce essentially the same structural information. Additionally, it is shown that spherocylinders mostly orient tangentially to the colloid at its surface; this preferred orientation disappears for larger distances. We also evidence that, near the colloid, the integrated charge attenuates monotonically when the macroparticle is highly charged, whereas for intermediate and low charged states of the colloid, the integrated charge can display charge reversal, overcharging, or both, with magnitudes that are sensitive to the salt concentration and to the localization of charge inside the spherocylinders.