Kinetics of self-induced aggregation in Brownian particles

We study a model of interacting random walkers that proposes a simple mechanism for the emergence of cooperation in group of individuals. Each individual, represented by a Brownian particle, experiences an interaction produced by the local unbalance in the spatial distribution of the other individuals. This interaction results in a nonlinear velocity driving the particle trajectories in the direction of the nearest more crowded regions; the competition among different aggregating centers generates nontrivial dynamical regimes. Our simulations show that for sufficiently low randomness, the system evolves through a coalescence behavior characterized by clusters of particles growing with a power law in time. In addition, the typical scaling properties of the general theory of stochastic aggregation processes are verified.Comment: RevTeX, 9 pages, 9 eps-figure

Evidence of Kosterlitz-Thouless phase transitions in the Ising model with dipolar interactions

The ferromagnetic bidimensional Ising model with dipolar interactions has been proposed to model ultrathin films with strong out-of-plane anisotropy. The phase diagram presents a rich phenomenology that includes lowtemperature phases characterized by stripes of width n (hn) and a high-temperature phase with domains of stripes with mutually perpendicular orientations, named tetragonal liquid (TL). The latter phase can be reached by two possible ways. One of them is the direct transition hn to TL, and the other one is through an intermediate phase with orientational order but short-range positional disorder, named nematic phase (NM). The regions of the phase diagram where these transitions occur, as well as their character, remain an open question and are the object of the present work. In order to clarify this topic, intensive Monte Carlo simulations were performed by employing short-time dynamics as the main tool for studying the phase transition behavior. The dynamic evolution of the orientational order parameter and its moments are measured for selected values of the ratio between the ferromagnetic exchange and dipolar constants, called δ. The obtained results indicate that the intermediate NM phase is present for δ 2 in narrow ranges of temperatures. Also, the results suggest that both transitions, i.e., hn-NM and NM-TL, have a Kosterlitz-Thouless character. This type of topological transition is observed in continuous bidimensional models and have been proposed for discrete ones, as in the case of the present work.Fil: Bab, Marisa Alejandra. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; ArgentinaFil: Saracco, Gustavo Pablo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentin

Damage spreading in a driven lattice gas model

We studied damage spreading in a Driven Lattice Gas (DLG) model as a function of the temperature T , the magnitude of the external driving field E, and the lattice size. The DLG model undergoes an order-disorder second-order phase transition at the critical temperature Tc (E), such that the ordered phase is characterized by high-density strips running along the direction of the applied field; while in the disordered phase one has a lattice-gas-like behavior. It is found that the damage always spreads for all the investigated temperatures and reaches a saturation value Dsat that depends only on T . Dsat increases for T Tc (E = ∞) and is free of finite-size effects. This behavior can be explained as due to the existence of interfaces between the high-density strips and the lattice-gas-like phase whose roughness depends on T . Also, we investigated damage spreading for a range of finite fields as a function of T , finding a behavior similar to that of the case with E = ∞.Instituto de Investigaciones Fisicoquímicas Teóricas y AplicadasInstituto de Física de Líquidos y Sistemas Biológico

Damage spreading in a driven lattice gas model

We studied damage spreading in a Driven Lattice Gas (DLG) model as a function of the temperature T , the magnitude of the external driving field E, and the lattice size. The DLG model undergoes an order-disorder second-order phase transition at the critical temperature Tc (E), such that the ordered phase is characterized by high-density strips running along the direction of the applied field; while in the disordered phase one has a lattice-gas-like behavior. It is found that the damage always spreads for all the investigated temperatures and reaches a saturation value Dsat that depends only on T . Dsat increases for T Tc (E = ∞) and is free of finite-size effects. This behavior can be explained as due to the existence of interfaces between the high-density strips and the lattice-gas-like phase whose roughness depends on T . Also, we investigated damage spreading for a range of finite fields as a function of T , finding a behavior similar to that of the case with E = ∞.Instituto de Investigaciones Fisicoquímicas Teóricas y AplicadasInstituto de Física de Líquidos y Sistemas Biológico

Influence of Pinning Effects on the Electrochemical Formation of Silver Patterns in Agarose-Containing Sols and Gels

The formation of silver patterns via electrolysis from aqueous silver sulfate + x% w/v agarose sol and gel media, with and without supporting electrolyte, in a quasi-two-dimensional (2D) cylindrical cell at room temperature, is utilized as a reference system to investigate the complexity of pinning effects. From pattern morphology and electrochemical data, both delocalized and localized pinning in the bulk dominate the drift of the growth front, depending on the concentration of agarose in the heterogeneous media. Delocalized pinning results from mobile, small agarose aggregates at the growth front and from their accumulation by the front drift. For gels, localized pinning comes from their own percolated structure. A depinning/pinning transition is observed in going from sols to gels. The relative contribution of diffusion and advection in mass-transport-controlled silver electrodeposition depends on the plating bath composition. On the other hand, silver ion attachment to the cathode appears to be interfered with by some screening caused by weakly adsorbed, mobile agarose aggregates at the metal surface without slowing down the rate of the electron-transfer step at the cathode. Their relative contribution of a delocalized, localized pinning and screening effect to a great extent determines the morphology and transition in the growth mode of silver patterns in both media. The analysis of charge and current transients and the corresponding silver pattern morphologies for open and dense radial patterns is made. Results are qualitatively simulated with a novel, rather simple cellular automaton algorithm.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Damage Spreading in a Driven Lattice Gas Model

We studied damage spreading in a Driven Lattice Gas (DLG) model as a function of the temperature $T$, the magnitude of the external driving field $E$, and the lattice size. The DLG model undergoes an order-disorder second-order phase transition at the critical temperature $T_c(E)$, such that the ordered phase is characterized by high-density strips running along the direction of the applied field; while in the disordered phase one has a lattice-gas-like behaviour. It is found that the damage always spreads for all the investigated temperatures and reaches a saturation value $D_{sat}$ that depends only on $T$. $D_{sat}$ increases for $TT_c(E=\infty)$ and is free of finite-size effects. This behaviour can be explained as due to the existence of interfaces between the high-density strips and the lattice-gas-like phase whose roughness depends on $T$. Also, we investigated damage spreading for a range of finite fields as a function of $T$, finding a behaviour similar to that of the case with $E=\infty$.Comment: 13 pages, 7 figures. Submitted to "Journal of Statistical Mechanics: Theory and Experiment

Irreversible phase transitions induced by an oscillatory input

A novel kind of irreversible phase transitions (IPTs) driven by an oscillatory input parameter is studied by means of computer simulations. Second-order IPTs showing scale invariance in relevant dynamic critical properties are found to belong to the universality class of directed percolation. In contrast, the absence of scale invariance is observed for first-order IPTs.Facultad de Ciencias ExactasInstituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Equilibrium and dynamical behavior in the Vicsek model for self-propelled particles under shear

The effects of an externally imposed linear shear profile in the Vicsek model of self-propelled particles is investigated via computer simulations. We find that the applied field changes in a relevant way both the equilibrium and dynamical properties of the original model. Indeed, short time dynamics analysis shows that the order-disordered phase transition disappears under shear, because the flow acts as a symmetry breaking field. Moreover, the coarsening of particle domains is arrested at a characteristic length-scale inversely proportional to shear rate. A generalization of the original Vicsek model where the velocity of particles depends on the local value of the density is also introduced and shows to affect the domain formation.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Influence of Pinning Effects on the Electrochemical Formation of Silver Patterns in Agarose-Containing Sols and Gels

The formation of silver patterns via electrolysis from aqueous silver sulfate + x% w/v agarose sol and gel media, with and without supporting electrolyte, in a quasi-two-dimensional (2D) cylindrical cell at room temperature, is utilized as a reference system to investigate the complexity of pinning effects. From pattern morphology and electrochemical data, both delocalized and localized pinning in the bulk dominate the drift of the growth front, depending on the concentration of agarose in the heterogeneous media. Delocalized pinning results from mobile, small agarose aggregates at the growth front and from their accumulation by the front drift. For gels, localized pinning comes from their own percolated structure. A depinning/pinning transition is observed in going from sols to gels. The relative contribution of diffusion and advection in mass-transport-controlled silver electrodeposition depends on the plating bath composition. On the other hand, silver ion attachment to the cathode appears to be interfered with by some screening caused by weakly adsorbed, mobile agarose aggregates at the metal surface without slowing down the rate of the electron-transfer step at the cathode. Their relative contribution of a delocalized, localized pinning and screening effect to a great extent determines the morphology and transition in the growth mode of silver patterns in both media. The analysis of charge and current transients and the corresponding silver pattern morphologies for open and dense radial patterns is made. Results are qualitatively simulated with a novel, rather simple cellular automaton algorithm.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada