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We analyze the dynamics of a tracer particle embedded in a bath of hard spheres confined in a channel of varying section. By means of Brownian dynamics simulations, we apply a constant force on the tracer particle and discuss the dependence of its mobility on the relative magnitude of the external force with respect to the entropic force induced by the confinement. A simple theoretical one-dimensional model is also derived, where the contribution from particle-particle and particle-wall interactions is taken from simulations with no external force. Our results show that the mobility of the tracer is strongly affected by the confinement. The tracer velocity in the force direction has a maximum close to the neck of the channel, in agreement with the theory for small forces. Upon increasing the external force, the tracer is effectively confined to the central part of the channel and the velocity modulation decreases, which cannot be reproduced by the theory. This deviation marks the regime of validity of linear response. Surprisingly, when the channel section is not constant, the effective friction coefficient is reduced as compared to the case of a plane channel. The transversal velocity, which cannot be studied with our model, follows qualitatively the derivative of the channel section, in agreement with previous theoretical calculations for the tracer diffusivity in equilibrium. Published by AIP Publishing.

The equilibrium phase diagram of a colloidal system composed of 1:1 mixture of positive and negative particles with equal charge is studied by means of Monte Carlo simulations. The system is the colloidal analog of the restricted primitive model (RPM) for ionic fluids. A liquid-gas transition is found in the low-temperature-low-density region, similar to the liquid-gas transition in the RPM. The fluid-crystal transition is also studied, and the liquid phase is shown to be stable in a narrow range of temperatures. In the liquid, the pair distribution function shows alternating layers of particles with opposite sign of charge surrounding every particle. In the vapor phase, clusters of particles are observed, again in agreement with the RPM. However, a decreasing distribution of clusters is obtained, instead of the discrimination between charged and neutral clusters found in the RPM.

Colloidal aggregation induced by attractive interactions is tackled experimentally and by Brownian dynamics simulations using a mixture of positive and negative particles. The structure of the aggregates and the aggregation kinetics are used to characterize the aggregation behavior. The clusters show uniform internal structures, with a fractal dimension lower than that of clusters formed in diffusion, indicating a more branched architecture. The aggregation kinetics also differs from the diffusive one, slowing down as time proceeds. Both results are totally confirmed by simulation. The transition from the attractive driven to the diffusion controlled regimes is studied varying the range of interaction. Continuous transitions are observed both for the aggregation kinetics and cluster structure.

Dynamical heterogeneities in a colloidal fluid close to gelation are studied by means of computer simulations. A clear distinction between some fast particles and the rest, slow ones, is observed, yielding a picture of the gel composed by two populations with different mobilities. Analyzing the statics and dynamics of both sets of particles, it is shown that the slow particles form a network of stuck particles, whereas the fast ones are able to move over long distances. Correlation functions show that the environment of the fast particles relaxes much faster than that of the slow ones, but at short times the bonds between fast particles are longer lived due to the flexibility of their structure. No string-like motion is observed for the fast particles, but they occupy preferential sites in the surface of the structure formed by the slow ones.

A model for describing heteroaggregation kinetics has been developed and tested with experiments in a colloidal system composed of positively and negatively charged particles. At first order, the model reduces to the classical Hogg, Healy, and Fuerstenau approximation. Static light scattering was used to monitor the aggregations. The dependence of the initial aggregation rate on the relative particle fraction and on the surface charges was studied and compared with the theoretical predictions. The experimental results indicated that first-order approximation cannot be used to describe the kinetics. However, good agreement was found when second order was introduced in the theoretical model.