Biomedical subjects

A Moncho-Jordá

Publications and source records attributed to A Moncho-Jordá.

8 recordsLinked to original sources

PubMed2005

Formation and structure of stable aggregates in binary diffusion-limited cluster-cluster aggregation processes.

Binary diffusion-limited cluster-cluster aggregation processes are studied as a function of the relative concentration of the two species. Both, short and long time behaviors are investigated by means of three-dimensional off-lattice Brownian Dynamics simulations. At short aggregation times, the validity of the Hogg-Healy-Fuerstenau approximation is shown. At long times, a single large cluster containing all initial particles is found to be formed when the relative concentration of the minority particles lies above a critical value. Below that value, stable aggregates remain in the system. These stable aggregates are composed by a few minority particles that are highly covered by majority ones. Our off-lattice simulations reveal a value of approximately 0.15 for the critical relative concentration. A qualitative explanation scheme for the formation and growth of the stable aggregates is developed. The simulations also explain the phenomenon of monomer discrimination that was observed recently in single cluster light scattering experiments.

Journal Article↗

PubMed2005

Density-functional study of interfacial properties of colloid-polymer mixtures.

Interfacial properties of colloid-polymer mixtures are examined within an effective one-component representation, where the polymer degrees of freedom are traced out, leaving a fluid of colloidal particles interacting via polymer-induced depletion forces. Restriction is made to zero-, one-, and two-body effective potentials, and a free energy functional is used that treats colloid excluded volume correlations within Rosenfeld's fundamental measure theory, and depletion-induced attraction within first-order perturbation theory. This functional allows a consistent treatment of both ideal and interacting polymers. The theory is applied to surface properties near a hard wall, to the depletion interaction between two walls, and to the fluid-fluid interface of demixed colloid-polymer mixtures. The results of the present theory compare well with predictions of a fully two-component representation of mixtures of colloids and ideal polymers (the Asakura-Oosawa model) and allow a systematic investigation of the effects of polymer-polymer interactions on interfacial properties. In particular, the wall surface tension is found to be significantly larger for interacting than for ideal polymers, whereas the opposite trend is predicted for the fluid-fluid interfacial tension.

Journal Article↗

PubMed2005

Short- and long-range topological correlations in two-dimensional aggregation of dense colloidal suspensions.

We have studied the average properties and the topological correlations of computer-simulated two-dimensional (2D) aggregating systems at different initial surface packing fractions. For this purpose, the centers of mass of the growing clusters have been used to build the Voronoi diagram, where each cell represents a single cluster. The number of sides (n) and the area (A) of the cells are related to the size of the clusters and the number of nearest neighbors, respectively. We have focused our paper in the study of the topological quantities derived from number of sides, n , and we leave for a future work the study of the dependence of these magnitudes on the area of the cells, A . In this work, we go beyond the adjacent cluster correlations and explore the organization of the whole system of clusters by dividing the space in concentric layers around each cluster: the shell structure. This method allows us to analyze the time behavior of the long-range intercluster correlations induced by the aggregation process. We observed that kinetic and topological properties are intimately connected. Particularly, we found a continuous ordering of the shell structure from the earlier stages of the aggregation process, where clusters positions approach a hexagonal distribution in the plane. For long aggregation times, when the dynamic scaling regime is achieved, the short- and long-range topological properties reached a final stationary state. This ordering is stronger for high particle densities. Comparison between simulation and theoretical data points out the fact that 2D colloidal aggregation in the absence of interactions (diffusion-limited cluster aggregation regimen) is only able to produce short-range cluster-cluster correlations. Moreover, we showed that the correlation between adjacent clusters verifies the Aboav-Weaire law, while all the topological properties for nonadjacent clusters are mainly determined by only two parameters: the second central moment of number-of-sides distribution mu(2) = sumP (n) (n-6)(2) and the screening factor a (defined through the Aboav-Weaire equation). We also found that the values of mu(2) and a calculated for two-dimensional aggregating system are related through a single universal common form a proportional to mu2(-0.89), which is independent of the particle concentration.

Journal Article↗

PubMed2004

Spontaneous formation of mesostructures in colloidal monolayers trapped at the air-water interface: a simple explanation.

The spontaneous formation of loosely bound ordered aggregates, foam, voids, chains, striations, and loops (see Figure 1a), called mesostructures hereafter, has been observed in colloidal monolayers trapped at the air-water interface. The distance between particles in these mesostructures is of the order of the particle radius (micrometers), implying that the colloidal interaction potential has a minimum at such distances, which could induce the phase separation of colloidal monolayers in dense and dilute regions. This is at odds with the accepted theory (Derjaguin-Landau-Verwey-Overbeek (DLVO)) of colloidal interactions, which predicts a secondary minimum at distances of nanometers between pairs of interacting particles. Moreover, the introduction of capillary, hydrophobic, and dipolar interactions between particles in an extended DLVO theory is not able to explain the spontaneous formation of mesostructures either. Recently, a great deal of effort has focused on understanding the mechanism behind the phenomenon of long-range attraction between colloidal particles confined in interfaces. In particular, this attraction has been employed to explain the spontaneous formation of mesostructures. Here, we show that the appearance of our mesostructures is due to the contamination of colloidal monolayers by silicone oil (poly(dimethylsiloxane)), which arises from the coating of the needles and syringes used to deposit and spread the particle solution at the air-water interface. The difference in the interfacial tension of water and silicone oil accounts for the formation of the experimentally observed mesostructures.

Journal Article↗

PubMed2003

Modeling the aggregation of partially covered particles: theory and simulation.

A theoretical model for describing the initial stages of the aggregation of partially covered colloidal particles is presented. It is based on the assumption of short-range interactions that may be modeled by a sticking probability on contact. Three types of sticking probabilities are distinguished depending on the collision type, i.e., for bare-bare, bare-covered, and covered-covered collisions. Hence, the model allows an analytical expression for the dimer-formation rate constant k(11), to be deduced as a function of the degree of surface coverage and the three sticking probabilities. The theoretical predictions are contrasted with simulated data. The observed agreement between theory and simulations shows the usefulness of the model for predicting the initial stages of this kind of aggregation processes.

Journal Article↗

PubMed2003

Coupled aggregation and sedimentation processes: the sticking probability effect.

The influence of the sticking probability P and the drift velocity on kinetics and structure formation arising in coupled aggregation and sedimentation processes was studied by means of simulations. For this purpose, a large prism with no periodical conditions for the sedimentation direction was considered allowing for sediment formation at the prism base. The time evolution of the cluster size distribution (CSD) and weight-average cluster size (n(w)) were determined in three different regions of the prism. The cluster morphology and the sediment structure were also analyzed. We found that the coupled aggregation and sedimentation processes in the bulk are governed by P for short times, and controlled by the Péclet number Pe for long times. In the lower part of the reaction volume, where the sediment grows, the local n(w) grows at sufficiently large times analytically with an exponent of four. This behavior seems to be independent of Pe and P. The obtained results are in good agreement with the experimental data reported by C. Allain, M. Cloitre, and M. Wafra [Phys. Rev. Lett. 74, 1478 (1995)] and support the idea of a possible internal cluster rearrangement for the experiments. Finally, we discuss how the scale dependent fractal character of the sediment is related to the different stages of the aggregation process.

Journal Article↗

PubMed2002

The effect of the salt concentration and counterion valence on the aggregation of latex particles at the air/water interface.

An experimental study on colloidal aggregation in two dimensions is presented. This study shows that a high amount of electrolyte concentration is necessary to screen the particle interactions and to induce the aggregation process. Our results indicate that the stability of the colloidal particles, with a diameter of 735 nm, increases when they are trapped at the air-water interface. The reason for this stability is the existence of long-range repulsive interactions between the external parts of the particles that are propagated at the air phase. The subphase electrolyte concentration that separates the slow aggregation rate region from the fast aggregation rate region, the critical coagulation concentration (C.C.C.), has been determined for counterions with a different valence. Two regimes can be distinguished: at low salt concentration the aggregation process becomes slower and the aggregation is reaction limited. At high ionic strength the repulsive interactions between the immersed part of the particles are very weak and the aggregation rate tends to grow. However, because of the aerial repulsive interactions, pure diffusion-limited cluster aggregation is never found.

Journal Article↗

PubMed2002

Constant bond breakup probability model for reversible aggregation processes.

Reversible aggregation processes were simulated for systems of freely diffusing sticky particles. Reversibility was introduced by allowing that all bonds in the system may break with a given probability per time interval. In order to describe the kinetics of such aggregation-fragmentation processes, a fragmentation kernel was developed and then used together with the Brownian aggregation kernel for solving the corresponding kinetic master equation. The deduced fragmentation kernel considers a single characteristic lifetime for all bonds and accounts for the cluster morphology by averaging over all possible configurations for clusters of a given size. It became evident that the simulated cluster-size distributions could be described only when an additional fragmentation effectiveness was considered. Doing so, the stochastic solutions were in good agreement with the simulated data.

Journal Article↗