arXiv subjects

Erik Luijten

Publications and source records attributed to Erik Luijten.

At least 37 records · Page 2Linked to original sources

arXiv2005

Colloidal stabilization via nanoparticle haloing

We present a detailed numerical study of effective interactions between micron-sized silica spheres, induced by highly charged zirconia nanoparticles. It is demonstrated that the effective interactions are consistent with a recently discovered mechanism for colloidal stabilization. In accordance with the experimental observations, small nanoparticle concentrations induce an effective repulsion that counteracts the intrinsic van der Waals attraction between the colloids and thus stabilizes the suspension. At higher nanoparticle concentrations an attractive potential is recovered, resulting in reentrant gelation. Monte Carlo simulations of this highly size-asymmetric mixture are made possible by means of a geometric cluster Monte Carlo algorithm. A comparison is made to results obtained from the Ornstein-Zernike equations with the hypernetted-chain closure.

cond-mat.soft↗

arXiv2005

Critical polymer-polymer phase separation in ternary solutions

We study polymer-polymer phase separation in a common good solvent by means of Monte Carlo simulations of the bond-fluctuation model. Below a critical, chain-length dependent concentration, no phase separation occurs. For higher concentrations, the critical demixing temperature scales nonlinearly with the total monomer concentration, with a power law relatively close to a renormalization-group prediction based on "blob" scaling arguments. We point out that earlier simulations and experiments have tested this power-law dependence at concentrations outside the validity regime of the scaling arguments. The critical amplitudes of the order parameter and the zero-angle scattering intensity also exhibit chain-length dependences that differ from the conventional predictions but are in excellent agreement with the renormalization-group results. In addition, we characterize the variation of the average coil shape upon phase separation.

cond-mat.soft↗

arXiv2005

Structure and stability of self-assembled actin-lysozyme complexes in salty water

Interactions between actin, an anionic polyelectrolyte, and lysozyme, a cationic globular protein, have been examined using a combination of synchrotron small-angle x-ray scattering and molecular dynamics simulations. Lysozyme initially bridges pairs of actin filaments, which relax into hexagonally-coordinated columnar complexes comprised of actin held together by incommensurate one-dimensional close-packed arrays of lysozyme macroions. These complexes are found to be stable even in the presence of significant concentrations of monovalent salt, which is quantitatively explained from a redistribution of salt between the condensed and the aqueous phases.

cond-mat.soft↗

arXiv2005

Screening in Ionic Systems: Simulations for the Lebowitz Length

Simulations of the Lebowitz length, $ξ_{\text{L}}(T,ρ)$, are reported for t he restricted primitive model hard-core (diameter $a$) 1:1 electrolyte for densi ties $ρ\lesssim 4ρ_c$ and $T_c \lesssim T \lesssim 40T_c$. Finite-size eff ects are elucidated for the charge fluctuations in various subdomains that serve to evaluate $ξ_{\text{L}}$. On extrapolation to the bulk limit for $T\gtrsim 10T_c$ the low-density expansions (Bekiranov and Fisher, 1998) are seen to fail badly when $ρ> {1/10}ρ_c$ (with $ρ_c a^3 \simeq 0.08$). At highe r densities $ξ_{\text{L}}$ rises above the Debye length, $ξ_{\text{D}} \prop to \sqrt{T/ρ}$, by 10-30% (upto $ρ\simeq 1.3ρ_c$); the variation is portrayed fairly well by generalized Debye-Hückel theory (Lee and Fisher, 19 96). On approaching criticality at fixed $ρ$ or fixed $T$, $ξ_{\text{L}}(T, ρ)$ remains finite with $ξ_{\text{L}}^c \simeq 0.30 a \simeq 1.3 ξ_{\text {D}}^c$ but displays a weak entropy-like singularity.

cond-mat.soft↗

arXiv2005

Geometric Cluster Algorithm for Interacting Fluids

We discuss a new Monte Carlo algorithm for the simulation of complex fluids. This algorithm employs geometric operations to identify clusters of particles that can be moved in a rejection-free way. It is demonstrated that this geometric cluster algorithm (GCA) constitutes the continuum generalization of the Swendsen-Wang and Wolff cluster algorithms for spin systems. Because of its nonlocal nature, it is particularly well suited for the simulation of fluid systems containing particles of widely varying sizes. The efficiency improvement with respect to conventional simulation algorithms is a rapidly growing function of the size asymmetry between the constituents of the system. We study the cluster-size distribution for a Lennard-Jones fluid as a function of density and temperature and provide a comparison between the generalized GCA and the hard-core GCA for a size-asymmetric mixture with Yukawa-type couplings.

cond-mat.stat-mech↗

arXiv2005

Generalized Geometric Cluster Algorithm for Fluid Simulation

We present a detailed description of the generalized geometric cluster algorithm for the efficient simulation of continuum fluids. The connection with well-known cluster algorithms for lattice spin models is discussed, and an explicit full cluster decomposition is derived for a particle configuration in a fluid. We investigate a number of basic properties of the geometric cluster algorithm, including the dependence of the cluster-size distribution on density and temperature. Practical aspects of its implementation and possible extensions are discussed. The capabilities and efficiency of our approach are illustrated by means of two example studies.

cond-mat.stat-mech↗

arXiv2004

Rejection-free Geometric Cluster Algorithm for Complex Fluids

We present a novel, generally applicable Monte Carlo algorithm for the simulation of fluid systems. Geometric transformations are used to identify clusters of particles in such a manner that every cluster move is accepted, irrespective of the nature of the pair interactions. The rejection-free and non-local nature of the algorithm make it particularly suitable for the efficient simulation of complex fluids with components of widely varying size, such as colloidal mixtures. Compared to conventional simulation algorithms, typical efficiency improvements amount to several orders of magnitude.

cond-mat.stat-mech↗

arXiv2004

Stabilization of colloidal suspensions by means of highly-charged nanoparticles

We employ a novel Monte Carlo simulation scheme to elucidate the stabilization of neutral colloidal microspheres by means of highly-charged nanoparticles [V. Tohver et al., Proc. Natl. Acad. Sci. U.S.A. 98, 8950 (2001)]. In accordance with the experimental observations, we find that small nanoparticle concentrations induce an effective repulsion that prevents gelation caused by the intrinsic van der Waals attraction between colloids. Higher nanoparticle concentrations induce an attractive potential which is, however, qualitatively different from the regular depletion attraction. We also show how colloid-nanoparticle size asymmetry and nanoparticle charge can be used to manipulate the effective interactions.

cond-mat.soft↗

arXiv2004

Reversible Gel Formation of Triblock Copolymers Studied by Molecular Dynamics Simulation

Molecular dynamics simulations have been employed to study the formation of a physical (thermoreversible) gel by amphiphilic A-B-A triblock copolymers in aqueous solution. In order to mimic the structure of hydrogel-forming polypeptides employed in experiments [W.A. Petka et al., Science 281, 389 (1998)], the endblocks of the polymer chains are modeled as hydrophobic rods representing the alpha-helical part of the polypeptides whereas the central B-block is hydrophilic and semi-flexible. We have determined structural properties, such as the hydrophobic cluster-size distribution function, the geometric percolation point and pair correlation functions, and related these to the dynamical properties of the system. Upon decrease of the temperature, a network structure is formed in which bundles of endblocks act as network junctions. Both at short and medium distances an increased ordering is observed, as characterized by the pair correlation function. Micelle formation and the corresponding onset of geometric percolation induce a strong change in dynamical quantities, e.g., in the diffusion constant and the viscosity, and causes the system to deviate from the Stokes-Einstein relation. The dynamical properties show a temperature dependence that is strongly reminiscent of the behavior of glass-forming liquids. The appearance of a plateau in the stress autocorrelation function suggests that the system starts to exhibit a solid-like response to applied stress once the network structure has been formed, although the actual sol-gel transition occurs only at a considerably lower temperature.

cond-mat.soft↗

arXiv2003

Precise Simulation of Near-critical Fluid Coexistence

We present a novel method to derive liquid-gas coexisting densities, $ρ^{\pm}(T)$, from grand canonical simulations (without knowledge of $\Tc$ or criticality class). The minima of $ Q_{L}\equiv< m^{2} >_{L}^{2}/< m^{4}>_{L}$ in an $L$$\times$$L \times$$L$ box with $m = ρ- <ρ>_{L}$ are used to generate recursively an unbiased universal finite-size scaling function. Monte Carlo data for a hard-core square-well fluid and for the restricted primitive model electrolyte yield $ρ^{\pm}$ to $\pm 1$-2% of $\rhoc$ down to 1 part in $10^4$-$10^3$ of $\Tc$ (and confirm well Ising character). Pressure mixing in the scaling fields is unequivocally revealed and indicates Yang-Yang ratios $R_μ = -0.04_{4}$ and $0.2_{6}$ for the two models, respectively.

cond-mat.stat-mech↗

arXiv2002

Crossover critical behavior in the three-dimensional Ising model

The character of critical behavior in physical systems depends on the range of interactions. In the limit of infinite range of the interactions, systems will exhibit mean-field critical behavior, i.e., critical behavior not affected by fluctuations of the order parameter. If the interaction range is finite, the critical behavior asymptotically close to the critical point is determined by fluctuations and the actual critical behavior depends on the particular universality class. A variety of systems, including fluids and anisotropic ferromagnets, belongs to the three-dimensional Ising universality class. Recent numerical studies of Ising models with different interaction ranges have revealed a spectacular crossover between the asymptotic fluctuation-induced critical behavior and mean-field-type critical behavior. In this work, we compare these numerical results with a crossover Landau model based on renormalization-group matching. For this purpose we consider an application of the crossover Landau model to the three-dimensional Ising model without fitting to any adjustable parameters. The crossover behavior of the critical susceptibility and of the order parameter is analyzed over a broad range (ten orders) of the scaled distance to the critical temperature. The dependence of the coupling constant on the interaction range, governing the crossover critical behavior, is discussed

cond-mat.stat-mech↗

arXiv2001

The boundary between long-range and short-range critical behavior

We investigate phase transitions of two-dimensional Ising models with power-law interactions, using an efficient Monte Carlo algorithm. For slow decay, the transition is of the mean-field type; for fast decay, it belongs to the short-range Ising universality class. We focus on the intermediate range, where the critical exponents depend continuously on the power law. We find that the boundary with short-range critical behavior occurs for interactions depending on distance r as r^{-15/4}. This answers a long-standing controversy between mutually conflicting renormalization-group analyses.

cond-mat.stat-mech↗

arXiv2001

Universality class of criticality in the restricted primitive model electrolyte

The 1:1 equisized hard-sphere electrolyte or restricted primitive model has been simulated via grand-canonical fine-discretization Monte Carlo. Newly devised unbiased finite-size extrapolation methods using temperature-density, (T, rho), loci of inflections, Q = ^2/ maxima, canonical and C_V criticality, yield estimates of (T_c, rho_c) to +- (0.04, 3)%. Extrapolated exponents and Q-ratio are (gamma, nu, Q_c) = [1.24(3), 0.63(3); 0.624(2)] which support Ising (n = 1) behavior with (1.23_9, 0.630_3; 0.623_6), but exclude classical, XY (n = 2), SAW (n = 0), and n = 1 criticality with potentials phi(r)>Phi/r^{4.9} when r \to \infty.

cond-mat.stat-mech↗

arXiv2001

On quantum effects near the liquid-vapor transition in helium

The liquid-vapor transition in He-3 and He-4 is investigated by means of path-integral molecular dynamics and the quantum virial expansion. Both methods are applied to the critical isobar and the critical isochore. While previous path-integral simulations have mainly considered the lambda transition and superfluid regime in He-4, we focus on the vicinity of the critical point and obtain good agreement with experimental results for the molar volume and the internal energy down to subcritical temperatures. We find that an effective classical potential that properly describes the two-particle radial distribution function exhibits a strong temperature dependence near the critical temperature. This contrasts with the behavior of essentially classical systems like xenon, where the effective potential is independent of temperature. It is conjectured that, owing to this difference in behavior between classical and quantum-mechanical systems, the crossover behavior observed for helium in the vicinity of the critical point differs qualitatively from that of other simple liquids.

cond-mat.soft↗

arXiv2001

Criticality in one dimension with inverse square-law potentials

It is demonstrated that the scaled order parameter for ferromagnetic Ising and three-state Potts chains with inverse-square interactions exhibits a universal critical jump, in analogy with the superfluid density in helium films. Renormalization-group arguments are combined with numerical simulations of systems containing up to one million lattice sites to accurately determine the critical properties of these models. In strong contrast with earlier work, compelling quantitative evidence for the Kosterlitz--Thouless-like character of the phase transition is provided.

cond-mat.stat-mech↗

arXiv2000

The heat capacity of the restricted primitive model electrolyte

The constant-volume heat capacity, C_V(T, rho), of the restricted primitive model (RPM) electrolyte is considered in the vicinity of its critical point. It is demonstrated that, despite claims, recent simulations for finite systems do not convincingly indicate the absence of a divergence in C_V(T, rho)--which would point to non-Ising-type criticality. The strong qualitative difference between C_V for the RPM and for a Lennard-Jones fluid is shown to result from the low critical density of the former. If one considers the theoretically preferable configurational heat-capacity density, C_V/V, the finite-size results for the two systems display qualitatively similar behavior on near-critical isotherms.

cond-mat.soft↗

arXiv2000

A Kac-potential treatment of nonintegrable interactions

We consider d-dimensional systems with nonintegrable, algebraically decaying pairwise interactions. It is shown that, upon introduction of periodic boundary conditions and a long-distance cutoff in the interaction range, the bulk thermodynamics can be obtained rigorously by means of a Kac-potential treatment, leading to an exact, mean-field-like theory. This explains various numerical results recently obtained for finite systems in the context of ``nonextensive thermodynamics,'' and in passing exposes a strong regulator dependence not discussed in these studies. Our findings imply that, contrary to some claims, Boltzmann-Gibbs statistics are sufficient for a standard description of this class of nonintegrable interactions.

cond-mat.stat-mech↗

arXiv2000

Crossover behavior in He3 and Xe near their liquid-vapor critical point

We present a detailed discussion of the crossover from mean-field to Ising critical behavior upon approach of the critical point, both for He3 and Xe. By combining different sets of experimental data, we are able to cover an unusually large temperature range on either side of the critical temperature T_c. Below T_c, we thus can make an accurate comparison with a recent calculation for the crossover of the coexistence curve. For the regime above T_c, an analysis of the compressibility demonstrates that the crossover regime in He3 is unexpectedly widened by a subtle interplay between quantum and critical fluctuations.

cond-mat↗