Transient dynamics during stress overshoots in binary colloidal glasses.

We investigate, using simultaneous rheology and confocal microscopy, the time-dependent stress response and transient single-particle dynamics following a step change in shear rate in binary colloidal glasses with large dynamical asymmetry and different mixing ratios. The transition from solid-like response to flow is characterised by a stress overshoot, whose magnitude is linked to transient superdiffusive dynamics as well as cage compression effects. These and the yield strain at which the overshoot occurs vary with the mixing ratio, and hence the prevailing caging mechanism. The yielding and stress storage are dominated by dynamics on different time and length scales, the short-time in-cage dynamics and the long-time structural relaxation respectively. These time scales and their relation to the characteristic time associated with the applied shear, namely the inverse shear rate, result in two different and distinct regimes of the shear rate dependencies of the yield strain and the magnitude of the stress overshoot.

Single-file diffusion in periodic energy landscapes: the role of hydrodynamic interactions.

We report on the dynamical properties of interacting colloids confined to one dimension and subjected to external periodic energy landscapes. We particularly focus on the influence of hydrodynamic interactions on the mean-square displacement. Using Brownian dynamics simulations, we study colloidal systems with two types of repulsive interparticle interactions, namely, Yukawa and superparamagnetic potentials. We find that in the homogeneous case, hydrodynamic interactions lead to an enhancement of the particle mobility and the mean-square displacement at long times scales as t(α), with α=1/2+ε and ε being a small correction. This correction, however, becomes much more important in the presence of an external field, which breaks the homogeneity of the particle distribution along the line and, therefore, promotes a richer dynamical scenario due to the hydrodynamical coupling among particles. We provide here the complete dynamical scenario in terms of the external potential parameters: amplitude and commensurability.

On the calculation of the structure of charge-stabilized colloidal dispersions using density-dependent potentials.

The structure of charge-stabilized colloidal dispersions has been studied through a one-component model using a Yukawa potential with density-dependent parameters examined with integral equation theory and Monte Carlo simulations. Partial thermodynamic consistency was guaranteed by considering the osmotic pressure of the dispersion from the approximate mean-field renormalized jellium and Poisson-Boltzmann cell models. The colloidal structures could be accurately described by the Ornstein-Zernike equation with the Rogers-Young closure by using the osmotic pressure from the renormalized jellium model. Although we explicitly show that the correct effective pair-potential obtained from the inverse Monte Carlo method deviates from the Yukawa shape, the osmotic pressure constraint allows us to have a good description of the colloidal structure without losing information on the system thermodynamics. Our findings are corroborated by primitive model simulations of salt-free colloidal dispersions.

Structure and effective interactions in parallel monolayers of charged spherical colloids.

We study the microstructure and the effective interactions of model suspensions consisting of Yukawa-like colloidal particles homogeneously distributed in equally spaced parallel planar monolayers. All the particles interact with each other, but particle transfer between monolayers is not allowed. The spacing between the layers defines the effective system dimensionality. When the layer spacing is comparable to the particle size, the system shows quasi-three-dimensional behavior, whereas for large distances the layers behave as effective two-dimensional systems. We find that effective attractions between like-charged particles can be triggered by adjusting the interlayer spacing, showing that the distance between adjacent layers is an excellent control parameter for the effective interparticle interactions. Our study is based on Brownian dynamics simulations and the integral equations theory of liquids. The effective potentials are accounted for by exploiting the invariance of the Ornstein-Zernike matrix equation under contractions of the description, and on assuming that the difference between bare and effective bridge functions can be neglected. We find that the hypernetted chain approximation does not account properly for the effective interactions in layered systems.

Dynamic arrest in charged colloidal systems exhibiting large-scale structural heterogeneities.

Suspensions of charged liposomes are found to exhibit typical features of strongly repulsive fluid systems at short length scales, while exhibiting structural heterogeneities at larger length scales that are characteristic of attractive systems. We model the static structure factor of these systems using effective pair interaction potentials composed of a long-range attraction and a shorter range repulsion. Our modeling of the static structure yields conditions for dynamically arrested states at larger volume fractions, which we find to agree with the experimentally observed dynamics.

Superparamagnetic colloids confined in narrow corrugated substrates.

We report a Brownian dynamics simulation study of the structure and dynamics of superparamagnetic colloids subject to external substrate potentials and confined in narrow channels. Our study is motivated by the importance of phenomena like commensurable-incommensurable phase transitions, anomalous diffusion, and stochastic activation processes that are closely related to the system under investigation. We focus mainly on the role of the substrate in the order-disorder mechanisms that lead to a rich variety of commensurate and incommensurate phases, as well as its effect on the single-file diffusion in interacting systems and the depinning transition in one dimension.

Density dependent interactions and structure of charged colloidal dispersions in the weak screening regime.

We determine the structure of charge-stabilized colloidal suspensions at low ionic strength over an extended range of particle volume fractions using a combination of light and small angle neutron scattering experiments. The variation of the structure factor with concentration is analyzed within a one-component model of a colloidal suspension. We show that the observed structural behavior corresponds to a nonmonotonic density dependence of the colloid effective charge and the mean interparticle interaction energy. Our findings are corroborated by similar observations from primitive model computer simulations of salt-free colloidal suspensions.

Magnetic properties of synthetic eumelanin--preliminary results.

We report an experimental and theoretical study of magnetic properties of synthetic eumelanin. The magnetization curves are determined by using both a vibrating sample magnetometer and a superconducting quantum interferometer device in an extended range of magnetic fields ranging from -10 kOe to 10 kOe at different temperatures. We find that the eumelanin magnetization can be qualitatively explained in terms of a simple model of dipolar spheres with an intrinsic magnetic moment. The latter one is experimentally measured by using X-band electron paramagnetic resonance. Our findings indicate that synthetic melanins are superparamagnetic.

Macroion virial contribution to the osmotic pressure in charge-stabilized colloidal suspensions.

Our interest goes to the different virial contributions to the equation of state of charged colloidal suspensions. Neglect of surface effects in the computation of the colloidal virial term leads to spurious and paradoxical results. This pitfall is one of the several facets of the danger of a naive implementation of the so called one component model, where the microionic degrees of freedom are integrated out to only keep in the description the mesoscopic (colloidal) degrees of freedom. On the other hand, due incorporation of wall induced forces dissolves the paradox brought forth in the naive approach, provides a consistent description, and confirms that for salt-free systems, the colloidal contribution to the pressure is dominated by the microionic one. Much emphasis is put on the no salt case but the situation with added electrolyte is also discussed.

Entropic forces in dilute colloidal systems.

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.

Macroion correlation effects in electrostatic screening and thermodynamics of highly charged colloids.

We study macroion correlation effects on the thermodynamics of highly charged colloidal suspensions using a mean-field theory and primitive model computer simulations. We suggest a simple way to include the macroion correlations into the mean-field theory as an extension of the renormalized jellium model of Trizac and Levin [Phys. Rev. E 69, 031403 (2004)]. The effective screening parameters extracted from our mean-field approach are then used in a one-component model with macroions interacting via a Yukawa-like potential to predict macroion distributions. We find that inclusion of macroion correlations leads to a weaker screening and hence smaller effective macroion charge and lower osmotic pressure of the colloidal dispersion as compared to other mean-field models. This result is supported by comparison to primitive model simulations and experiments for charged macroions in the low-salt regime, where the macroion correlations are expected to be significant.

A reference interaction site model approach to depletion forces induced by hard rodlike particles.

We study the effective interactions among large hard spherical colloidal particles induced by small hard rodlike particles and compare them with those induced by small hard spherical particles to highlight the specific effects due to the anisotropic shape of the former. This is done by determining the effective pair potentials within the framework of the reference interaction site model approach. The rodlike particles are modeled as N nonoverlapping spherical units arranged in a straight line, so that their total length is N times their transversal diameter. These results are compared against those obtained in the Asakura-Oosawa limit.