José Luis Arauz-Lara

Time-dependent self-diffusion in model suspensions of highly charged Brownian particles

A quantitative comparison is reported between the predictions of two theories of the time-dependent self-diffusion properties of suspensions of highly charged Brownian particles. The first theory, based on the overdamped N-body Fokker-Planck dynamics, involves a mode-mode coupling approximation for the time-dependent self-friction function, whereas the second one makes use of short-time conditions derived from the N-body Smoluchowski equation. In both cases, the relevant dynamic properties can be expressed in terms solely of the radial distribution function g(r). This quantity is first calculated using the rescaled mean spherical approximation (RMSA). By comparing with computer simulation results for g(r), it is found that the RMSA becomes increasingly inaccurate as the freezing transition is approached. We observe, however, that the RMSA itself provides a device to fit the simulation results for g(r). Using this procedure, the time-dependent self-diffusion coefficient, calculated from both theories, is in good agreement with Brownian dynamics simulations.

Like-charge attraction in confinement: myth or truth?

It is general wisdom that like-charged colloidal particles repel each other when suspended in liquids. This is in perfect agreement with mean field theories being developed more than 60 years ago. Accordingly, it was a big surprise when several groups independently reported long-ranged attractive components in the pair potential () of equally charged colloids. This so-called (LCA) was only observed in thin sample cells while the pair-interaction in unconfined suspensions has been experimentally confirmed to be entirely repulsive. Despite considerable experimental and theoretical efforts, LCA remains one of the most challenging mysteries in colloidal science. We experimentally reinvestigate the pair-potential () of charged colloidal particles with digital video microscopy and demonstrate that optical distortions in the particles images lead to slightly erroneous particle positions. If not properly taken into account, this artefact pretends a minimum in () which was in the past misleadingly interpreted as LCA. After correcting optical distortions we obtain entirely repulsive pair interactions which show good agreement with linearized mean field theories.

Statics and tracer-diffusion in binary suspensions of polystyrene spheres: experiment vs. theory

Extensive measurements of the static and dynamic field autocorrelation function of dilute bidisperse suspensions of charged polystyrene spheres, obtained by static and dynamic light scattering experiments, are discussed in terms of the two-component macroion fluid model. The suspensions under study consist of a small amount of large spheres immersed in a system of small spheres. For these suspensions the self-scattering function at short and intermediate times can be determined by dynamic light scattering (DLS), performed at large wavenumbers. The self-scattering function contains information on the tracer-diffusion of both small and large spheres. The experimentally observed self-scattering functions of the binary mixtures are compared with theoretical calculations based on the so-called single exponential approximation (SEXP). The SEXP is based on the exact short-time behavior of the single particle dynamics and within it the dynamic properties are entirely expressed in terms of the partial static structure functions. The latter are calculated using the extended rescaled mean spherical approximation (RMSA) for colloidal mixtures. Good agreement between experimental data and the SEXP theory has been found.

STRUCTURE AND SELF-DIFFUSION IN A MODEL TWO-DIMENSIONAL BROWNIAN LIQUID

The static structure and the time‐dependent self‐diffusion motion of interacting Brownian particles in a model two‐dimensional suspension are discussed. For the static structure we report Brownian dynamics results assuming a hard disk plus Yukawa pair potential. The self‐diffusion properties of this model system are calculated from two independent theoretical approaches. In order to assess the accuracy of the predictions of these two theories, we also performed Brownian dynamics calculations of the time‐dependent self‐diffusion coefficient for a wide range of values of both the particle concentration and the pair potential coupling constant. We find that both theories reproduce very well the main features exhibited by the Brownian dynamics data. Quantitatively, there are some discrepancies between both theoretical predictions and the Brownian dynamics results, which are negligible at moderate couplings, but become larger for strongly coupled systems and long times.

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.

Theory of self-diffusion of highly charged spherical Brownian particles

Starting from the N-particle Smoluchowski equation without hydrodynamic interactions the authors derived an expression for the mean square displacement by modelling the memory function entered in the equation for the incoherent scattering function. The numerical evaluation of that expression is done for the case of spherical particles interacting via a screened Coulombic potential. The results of this theory compare favourably with computer simulation results.

Dynamic light scattering by optically anisotropic colloidal particles in polyacrylamide gels.

The translational and rotational motions of optically anisotropic spherical particles embedded in cross-linked polyacrylamide gels is studied by dynamic light scattering. The particles are liquid crystal droplets solidified in the nematic phase. The amount of cross linkers is varied to cross the sol-gel transition where the system becomes nonergodic for both translational and rotational diffusion modes of the probes. The translational and rotational dynamic correlation functions are obtained by measuring the intensity correlation function between crossed polarizers in the parallel and perpendicular geometries. Data from nonergodic systems is analyzed using an extension, to include rotations, of the method of Pusey and van Megen [Physica A 157, 705 (1989)]. Both diffusion modes are observed to be arrested as the rigidity of the gel increases.

Brownian motion in binary colloidal suspensions

Abstract The statics and dynamics of dilute binary suspensions of charged polystyrene spheres have been investigated by static and dynamic light scattering experiments (SLS and DLS). The samples under study consist of a small amount of large spheres immersed in a system of small spheres. The SLS data of the measured static structure factor are compared with a theoretical approach, the so-called extended resealed mean spherical approximation, for colloidal mixtures. Dynamic light scattering at large wavenumbers was used to measure the self-scattering function which, in general, contains information on the Brownian motion of both species. The experimentally observed self-scattering functions are compared with theoretical calculations based on the single exponential approximation and good agreement has been found within the experimentally accessible time window. It is demonstrated that for appropriately chosen system parameters, the Brownian motion of only the large spheres is observed.

Confinement-induced fluid-gel transition in polymeric solutions

The viscoelastic properties of confined polymer solutions are probed by particle-tracking microrheology. The mean squared displacement of spherical probe particles embedded in the solution and the storage and loss moduli of the system are measured as the level of confinement is increased. It is found that those quantities change continuously as the confinement increases, and, at severe conditions, when the constrain reaches the size of the polymer molecule, the system undergoes a transition from a viscoelastic fluid to a gel.

Static structure of the two‐dimensional hard‐disk plus Yukawa fluid

The static structure of the two‐dimensional hard‐disk plus Yukawa fluid is studied on the basis of the hypernetted chain approximation. We find that the most relevant features exhibited by the three‐dimensional hard‐sphere plus Yukawa system are also exhibited by its two‐dimensional counterpart.