Reversible Diffusion-limited Aggregation Research Articles

A dendrite model of current instability in RbAg 4I 5

A theoretical model of the electronic current instability observed experimentally by Bredikhin et al. in the cell Ag (−)/RbAg 4I 5/ C (+) at various constant voltages is proposed. The model is based on the concept of fractal growth of silver dendritic clusters from the Ag (−)/RbAg 4I 5 interface. It is shown that some of the clusters make connections between opposite electrodes at sufficiently large voltages, which leads to stochastic current oscillations. We show that the number of connecting clusters increases as the voltage is raised and therefore current oscillations become more frequent. All experimental dependences on current instability are well described within the framework of a mean-field approach to the theory of reversible diffusion-limited aggregation without branching.

Kinetics and Modes of Destabilization of Antibody-Coated Polystyrene Latices in the Presence of Antigen: Reactivity of the System IgG-IgM

We thoroughly investigated the aggregation kinetics of a model colloid as a function of the colloid concentration of the suspension and the density of the colloid surface charge. This was done in order to establish the scaling laws of the reversible diffusion-limited and the reaction-limited aggregation processes and to show the complexity of the aggregation modes of real systems. These laws of dynamics were then used to describe the aggregation modes of antibody-coated latices in the presence of IgM antigen. Fully IgG-coated polystyrene latices were used to show the presence of the antigen IgM in the solution. Experiments were performed at hydrogen ion, electrolyte, buffer, and latex concentrations used in the agglutination tests and perikinetic conditions in isopycnic medium were imposed. The Coulter Counter method was used to obtain the histogram of the colloids. Our own method of analysis provided the aggregate size distribution cn in the colloid suspension as a function of the number n of elementary constituents. This enabled us to calculate the number and weight-average sizes of the colloids and thus comprehensively compare our experimental results with those obtained by computer simulation. We were then able to classify the different aggregation modes of the sensitized latex. We determined that the laws of reversible diffusion-limited aggregation governed the rate of the sensitized latex destabilization in the presence of high antigen dosage. At relatively low antigen concentration, the mechanism of site-ligand interaction was clearly revealed and the aggregate reactivity could be determined. We conjectured that elaboration of agglutination tests in which the interaction and binding between sites and ligands precede latex destabilization could be essential for carrying out specific tests.

Computer simulation of surface-induced aggregation of ferritin

Models are presented describing the transient mass-transport limited adsorption and cluster growth of ferritin at a solid surface. Computer simulations are carried out on a hexagonal lattice using a computer model that can be characterized as a two-dimensional stochastic cellular automaton allowing different rules regarding association, lateral interaction and dissociation to be incorporated in the model. The fractal dimensions of individual clusters were extracted from simulated aggregates and for similar rules found to be consistent with literature values on reversible diffusion-limited aggregation in two dimensions. The distribution of clusters versus free surface were shown to be affected by neighbor-dependent association probability. Low fractal dimension clusters were generated by a combination of strong lateral cohesion and neighbor-dependent dissociation to the bulk. By comparing computer simulated aggregation to experimental electron micrographs of adsorbed ferritin layers it is suggested that neighbor-dependent association, neighbor-dependent dissociation and lateral interactions are important factors in the complex dynamics of adsorbed protein layers.

Aggregation modes of colloids in the presence of block copolymer micelles

We investigated the aggregation of colloidal latex induced by the interparticle bridging phenomenon as a function of the surface density of link-forming sites. To perform aggregation experiments on a model system, of reproducible geometry, an unstable colloid was obtained by adsorbing stable spherical diblock copolymer micelles onto latex particles. The conformation of this site, active in the bridging mechanism, did not change either on adsorption or with time, and thus the modified colloid constituted a model to investigate colloid aggregation induced by interparticle polymer bridging. Different aggregation modes were determined in experimental studies of the kinetics of aggregation processes. Dynamic scaling laws were established, which were compared to those determined by computer simulation and theory. By increasing the number of micelles adsorbed onto one latex particle, the following flocculation modes were observed. At very low surface coverage, irreversible reaction-limited processes were found to generate agglomerates having a size frequency function c n ( t) scaling like t − τ with τ equal to 1.50. At a given polymer coating, reversible diffusion-limited aggregation was found to describe the process and a bell-shaped size distribution curve was obtained. At greater surface occupancy, the irreversible reaction-limited aggregation mode was recovered. Due to the presence of adsorbed copolymers, interactions between the colloid/micelle complexes were modified, leading to different reactivity in particle-particle agglomeration. As a result, at each time of the flocculation period, the cluster size distribution c n ( t) scaled like t − τ with a modified exponent τ equal to 1.65. The existence of the unexpected diffusion-limited aggregation process, not evidenced in the presence of the parent flexible polymer, was interpreted in terms of topological constraints governing the collision efficiency in the sticking process. In fact, a minimal surface coverage was found to be necessary to induce this aggregation mode. Above and below this critical domain, the aggregation developed with features characteristic of the classical bridging mechanism induced by water-soluble flexible homopolymers. Diblock copolymers seemed to constitute a new class of very efficient polymeric flocculants.

Computer simulation for structure formation from self-assembling polymers

We have developed a reversible diffusion-limited aggregation model for self-assembling polymers, which contain an associating site at each end of the chain. The simulations show that the associating sites form finite-sized clusters that are interconnected by intervening chains in a three-dimensional network. The average cluster size is seen to decrease with an increase in $\ensuremath{\tau}$ (the residence time of the chain in the cluster).

A Computer Model for the Average “Cluster” Size in Polymer Aggregates

ABSTRACTA reversible diffusion-limited aggregation model for self-assembling polymers has been developed with the aid of computer simulations. The calculations have been performed in three dimensions and apply to linear chains bearing an associating site at each terminus. The results reveal that the associating sites form finite-sized clusters that, in turn, lead to the assemblage of a supermolecular network. The average cluster size is found to vary with the degree of reversibility of site binding.