Narrow Results

We investigate the role played by conservative forces in dissipative particle dynamics (DPD) simulation of single-component and binary fluids. We employ equations from kinetic theory for matching the coefficients of DPD interparticle force to the macroscopic properties of fluid such as: density, temperature, diffusion coefficient, kinematic viscosity and sound velocity. The sound velocity c is coupled with scaling factor π₁ of conservative component of the DPD collision operator. Its value sets up an upper limit on the mass S of a single particle in DPD fluid. The Kirkwood–Alder fluid–solid transition is observed for a sufficiently large S. We emphasize the role of the scaling factor π₁₂ for particles of different types in simulating phase separation in binary fluids. The temporal growth of average domain size R(t) in the phase separation process depends on the value of immiscibility coefficient Δ = π₁₂ - π₁. For small immiscibility, R (t) ∝ t^β, where β ≈ 1/2 for R (t) < R_H and β ≈ 2/3 for R (t) > R_H, R_H is the hydrodynamic length. Finally, both phases separate out completely. For larger immiscibility, R(t) increases exponentially at the beginning of simulation, while finally the domain growth process becomes marginal. We also observe the creation of emulsion-like structures. This effect results from an increase of the surface tension on the two-phase interface along with increasing immiscibility.

We present results on the modeling of on- and off-ramps in cellular automata for traffic flow, especially the Nagel–Schreckenberg model. We study two different types of on-ramps that cause qualitatively the same effects. In a certain density regime ρ_low < ρ < ρ_high one observes plateau formation in the fundamental diagram. The plateau value depends on the input-rate of cars at the on-ramp. The on-ramp acts as a local perturbation that separates the system into two regimes: A regime of free flow and another one where only jammed states exist. This phase separation is the reason for the plateau formation and implies a behavior analogous to that of stationary defects. This analogy allows to perform very fast simulations of complex traffic networks with a large number of on- and off-ramps because one can parametrise on-ramps in an exceedingly easy way.

We present simple arguments and numerical evidence in order to show that "efficient" Kawasaki exchange does not lead to Boltzmann equilibrium distributions.

In order to see the phase conversion taking place in a weak first-order phase transition, we have simulated the q = 3 state Potts model in three dimensions and studied the time evolutions of oriented clusters forming after a rapid temperature quench. Our results indicate that the phase conversion mechanism following a deep temperature quench is spinodal decomposition while a rather shallow quench to temperatures near the phase transition point proceeds through usual nucleation.

Self-organized patterns in an immiscible fluid mixture of dissimilar particles driven from a source at the bottom are examined as a function of hydrostatic pressure bias by a Monte Carlo computer simulation. As the upward pressure bias competes with sedimentation due to gravity, a multi-phase system emerges: a dissociating solid phase from the source is separated from a migrating gas phase towards the top by an interface of mixed (bi-continuous) phase. Scaling of solid-to-gas phase with the altitude is nonuniversal and depends on both the range of the height/depth and the magnitude of the pressure bias. Onset of phase separation and layering is pronounced at low bias range.

In a Potts-like model of Q ethnic groups, we follow Schelling¹ and Meyer-Ortmanns² and simulate the formation of ethnic ghettos as well as their prevention by an increasing social temperature.

This paper extends a cellular automaton model, named modified comfortable driving (MCD) model, to a two-lane roadway. A symmetric lane-changing rule set has been proposed (Set I). The fundamental diagram, the lane-changing frequency and space-time plots are presented. It is found that for the two-lane model, there exists a density range in which phase separation between synchronized flow and wide-moving jams on one lane, and between light synchronized flow and heavy synchronized flow on the other lane, can be maintained for quite long periods of time. In this density range, (i) the outflow from jams is synchronized flow; (ii) wide moving jams are sparse. These are consistent with the empirical observation. We also investigate a slightly different lane-changing rule Set II in which stopped vehicles are not allowed to change lane. It is shown that in this case, the phase separation, between free flow and wide moving jams on one lane and between free flow and heavy synchronized flow on the other lane, can be maintained for sufficiently long periods of time. Consequently, the flux is enhanced comparing to that of rule Set I.

High precision Monte Carlo simulations are used to study domain growth in a compressible two-dimensional spin-exchange Ising model with continuous particle positions and zero total magnetization. For mismatched systems, we measure significant deviations from the theoretically expected late-time domain growth, R(t) = A + Btⁿ with n = 1/3. For a compressible model with no mismatch, we measure only a slight deviation from n = 1/3. Our results indicate that our current understanding of domain growth is incomplete.

The phase-separating system coupled with a simple reversible reaction A ⇌ B in a binary immiscible mixture due to critical quench is investigated with Lowe-Andersen temperature controlling method in two dimensions. The system viscosity strongly influences the asymptotic relationship between the excess energy (characterizing the domain growth) and the reaction rate. The competition between different dynamic factors results in the steady states with characteristic domain sizes. For low viscosities, the domain growth exponent approximates to 0.4 in the cases of low reaction rates and to 0.25 in the cases of high reaction rates, which shows the suppressing effects of high reversible reaction rates on the phase separation. However, in the cases of high viscosities, we find a 0.25 scaling with low reaction rates but a 0.5 scaling with high reaction rates. In these cases, high viscosities prevent mass transport in the binary mixture, consequently result in much smaller steady state domain sizes. Therefore the domain sizes with high viscosities and low reaction rates are very similar to those with low viscosities and high reaction rates, and the dependence of domain sizes on the reaction rates are similar. For the high-viscosity systems with high reaction rates, the domain sizes are predominantly controlled by the reaction rates, therefore we can observe stronger dependence of domain size on the reaction rate.

A systems of self-propelled dumbbells interacting by a Weeks–Chandler–Anderson potential is considered. At sufficiently low temperatures the system phase separates into a dense phase and a gas-like phase. The kinetics of the cluster formation and the growth law for the average cluster size are analyzed.

A discrete model for the late stages of order parameter conserving coarsening is studied in one and in two dimensions. Starting from stationary, unstable solutions, the growth dynamics is related to the evolution of relevant perturbations near these solutions.

We developed a new LGA model which has the applicability for simulation of immiscible two phases with wide difference in density. We introduced long-range interparticle forces into the Rothman and Keller's ILG model to represent density difference between phases. We attempted some simulations of phase separation using our new model. Two-phase interfaces are stably made with density distribution coinciding with particle color distribution. Furthermore, the two-phase interface is clearer than that obtained by the Appert and Zaleski's LG model.

Studies of dynamical properties of first-order phase transition for a scalar field theory indicate that the phase conversion mechanism itself depends on the strength of the first-order transition: if the transition is strongly (weakly) first-order, bubble nucleation (spinodal decomposition) are favored conversion mechanisms, respectively. These distinct scenarios are of phenomenological impact. In order to see which phase conversion mechanism takes place depending on the strength of transition, we have simulated the q=5 state Potts model in two dimensions with an external magnetic field. The transition gets weakened in its first-order as the external field increases. Our results indicate that the phase conversion mechanism changes from nucleation to spinodal decomposition.

We report the application of the Monte Carlo simulation to phase separation dynamics. First, we deal with the phase separation under shear flow. The thermal effect on the phase separation is discussed, and the anisotropic growth exponents in the late stage are estimated. Next, we study the effect of surfactants on the three-component solvents. We obtain the mixture of macrophase separation and microphase separation, and investigate the dynamics of both phase separations.

Slow dynamics of complex domain structures in phase separating polymer systems is investigated with the use of the self-consistent field (SCF) dynamic density functional (DDF) technique where the free energy of the system is calculated using the path integral formalism of the polymer chain conformation. We apply this technique to micellization of block copolymers and to phase separation of polymer blends containing block copolymers as a compatibilizer. In order to study the late stage of the phase separation processes more efficiently, we adopt the so-called Ginzburg–Landau approach, where a phenomenological model free energy functional is used. Numerical results of this approach is quantitatively compared with the results of the SCF approach.

In this paper, we study phase separation in the two-dimensional single-band Hubbard model with the unrestricted Hartree-Fock(UHF) method and the restricted Hartree-Fock (RHF) method. We perform the calculation for square lattices and rectangle lattices. It is observed that the stripe phase exists and it depends on three aspects: geometry of the lattice, Coulomb interaction U and band filling n. To gain more physical insights, we consider the Hubbard model with spin dependent hoppings: t_↑ and t_↓, and study the effect of varying on the phase separation.

In this paper, we have investigated the interplay of the asymmetric exclusion process (ASEP) of parallel updated rules with "Langmuir kinetics" (LK) by equipping the ASEP system with the additional feature of local particle creation at empty sites with rate ω_a and annihilation with rate ω_d. Two slightly different models are proposed. It is shown that the phase separation may occur in both models. However, in one model, the maximum flow phase may occur but in the other model, there only exists the constant bulk density phase.

The results of precision heat capacity measurements near the lower consolute point in aqueous solutions of non-ionic surfactants n-dodecyl hexaethylene glycol monoether (C₁₂E₆), C₁₀E₆, and C₁₂E₈, are reported. These surfactants form homogeneous single phase micellar solutions below the lower consolute point, T_c. The pretransitional specific heat in the homogeneous phase below T_c is found to grow with temperature and exhibits a power-law divergence. Analysis of the data obtained for the three systems reveals that the specific heat exponent, α=-α_R/(1-α_R)≅0.5, gives credence to mean field behavior.

La_0.5Sr_0.5MnO₃ films on (001) LaAlO₃ were prepared by RF magnetron sputtering using powder target and were studied by SQUID and XPS. The XPS results show the formation of Mn³⁺ and Mn⁴⁺, indicating the coexistence of ferromagnetic (FM) and antiferromagnetic (AFM) clusters. SQUID measurement of LSMO on LAO and Si also support an interaction between them at low temperatures, resulting in the shift of hysteresis loops. Large coercivities at low temperature can be attributed to the pinning of both FM and AFM clusters and small coercivities at high temperature to the depinning of both FM and AFM phases, resulting from the thermal motion at higher temperatures which help to overcome the energy barrier and change the magnetic alignments.

To understand several peculiar features of d-wave superconductors such as the cuprates, we have studied a two-dimensional tight-binding (extended Hubbard) model in which the electrons interact with a potential which is attractive at nearest neighbor distance and repulsive elsewhere. Such a model is designed to produce d-wave superconductivity. It also yields antiferromagnetism and phase separation. In the two-phase region, due to vanishing compressibility the system can be easily rendered inhomogeneous by weak perturbations. Thus inhomogeneous superconductivity is a likely consequence of d-wave pairing. In addition to inhomogeneity the model also reproduces a phase diagram strikingly similar to the one observed in the cuprates. As such it throws some light on the nature of the pseudogap state.