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J Horbach

Publications and source records attributed to J Horbach.

15 recordsLinked to original sources
PubMed2006

Phase diagram and structure of colloid-polymer mixtures confined between walls.

The influence of confinement, due to flat parallel structureless walls, on phase separation in colloid-polymer mixtures, is investigated by means of grand-canonical Monte Carlo simulations. Ultrathin films, with thicknesses between D=3-10 colloid diameters, are studied. The Asakura-Oosawa model [J. Chem. Phys. 22, 1255 (1954)] is used to describe the particle interactions. To simulate efficiently, a "cluster move" [J. Chem. Phys. 121, 3253 (2004)] is used in conjunction with successive umbrella sampling [J. Chem. Phys. 120, 10925 (2004)]. These techniques, when combined with finite size scaling, enable an accurate determination of the unmixing binodal. Our results show that the critical behavior of the confined mixture is described by "effective" critical exponents, which gradually develop from values near those of the two-dimensional Ising model, to those of the three-dimensional Ising model, as D increases. The scaling predictions of and Fisher and Nakanishi [J. Chem. Phys. 75, 5875 (1981)] for the shift of the critical point are compatible with our simulation results. Surprisingly, however, the colloid packing fraction at criticality approaches its bulk (D-->infinity) value nonmonotonically, as D is increased. Far from the critical point, our results are compatible with the simple Kelvin equation, implying a shift of order 1/D in the coexistence colloid chemical potential. We also present density profiles and pair distribution functions for a number of state points on the binodal, and the influence of the colloid-wall interaction is studied.

Journal Article↗
PubMed2006

Nonlinear effects in charge stabilized colloidal suspensions.

Molecular-dynamics simulations are used to study the effective interactions in charged stabilized colloidal suspensions. We focus on highly charged macroions in the limit of low salt concentrations. Within this regime, nonlinear corrections to Debye-Hückel (DH) theory have to be considered. For non-bulk-like systems, such as isolated pairs or triples of macroions, we show that nonlinear effects can become relevant, which cannot be described by the charge renormalization concept [S. Alexander, J. Chem. Phys. 80, 5776 (1984)]. For an isolated pair of macroions, we find an almost perfect qualitative agreement between our simulation data and DH theory. However, on a quantitative level, neither DH theory nor the charge renormalization concept can be confirmed in detail. This seems mainly to be related to the fact that for small ion concentrations, microionic layers can strongly overlap, whereas, simultaneously, excluded volume effects are less important. In the case of isolated triples, where we compare between coaxial and triangular geometries, we find attractive corrections to pairwise additivity in the limit of small macroion separations and salt concentrations. These triplet interactions arise if all three microionic layers around the macroions exhibit a significant overlap. In contrast to the case of two isolated colloids, the charge distribution around a macroion in a triple is found to be anisotropic.

Journal Article↗
PubMed2006

Critical behavior of a colloid-polymer mixture confined between walls.

We investigate the influence of confinement on phase separation in colloid-polymer mixtures. To describe the particle interactions, the colloid-polymer model of Asakura and Oosawa [J. Chem. Phys. 22, 1255 (1954)] is used. Grand canonical Monte Carlo simulations are then applied to this model confined between two parallel hard walls, separated by a distance D = 5 colloid diameters. We focus on the critical regime of the phase separation and look for signs of crossover from three-dimensional (3D) Ising to two-dimensional (2D) Ising universality. To extract the critical behavior, finite size scaling techniques are used, including the recently proposed algorithm of Kim et al [Phys. Rev. Lett. 91, 065701 (2003)]. Our results point to "effective" critical exponents that differ profoundly from 3D Ising values, and that are already very close to 2D Ising values. In particular, we observe that the critical exponent of the order parameter in the confined system is smaller than in 3D bulk, yielding a "flatter" binodal. Our results also show an increase in the critical colloid packing fraction in the confined system with respect to the bulk. The latter seems consistent with theoretical expectations, although subtleties due to singularities in the critical behavior of the coexistence diameter cannot be ruled out.

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PubMed2005

Capillary waves in a colloid-polymer interface.

The structure and the statistical fluctuations of interfaces between coexisting phases in the Asakura-Oosawa model [J. Chem. Phys. 22, 1255 (1954)] for a colloid-polymer mixture are analyzed by extensive Monte Carlo simulations. We make use of a recently developed grand canonical cluster move with an additional constraint stabilizing the existence of two interfaces in the (rectangular) box that is simulated. Choosing very large systems, of size L x L x D with L=60 and D=120, measured in units of the colloid radius, the spectrum of capillary wave-type interfacial excitations is analyzed in detail. The local position of the interface is defined in terms of a (local) Gibbs surface concept. For small wave vectors capillary wave theory is verified quantitatively, while for larger wave vectors pronounced deviations show up. When one analyzes the data in terms of the concept of a wave vector-dependent interfacial tension, a monotonous decrease of this quantity with increasing wave vector is found. Limitations of our analysis are critically discussed.

Journal Article↗
PubMed2005

O triclusters revisited: classical MD and quantum cluster results for glasses of composition (Al(2)O(3))2(SiO(2)).

The (17)O NMR spectrum of CaAl(2)Si(2)O(8) glass shows two types of O sites that are not present in the crystalline material. One of these, with (17)O NMR parameters C(Q) = 2.3 MHz and delta = +20 ppm, has been assigned to a "tricluster" O, a local geometry in which the O is coordinated to three tetrahedrally coordinated atoms, either Al or Si. For crystalline CaAl(4)O(7), a tricluster site (with three Al linkages to O, i.e., OAl(3)) has been characterized experimentally, with a C(Q) of 2.5 MHz and a delta of about +40 ppm. Thus, a C(Q) value of 2.5 MHz or less seems to be a characteristic of such sites, although they may show a range of delta values. However, several different quantum chemical cluster calculations employing energy-optimized geometries for various tricluster species have given C(Q) values considerably larger than that seen experimentally in the CaAl(2)Si(2)O(8) glass (with minimum C(Q) values of 3.0 MHz even for all Al species). We have recently shown that for edge-sharing geometries, in which the tricluster O atoms participate in "two-membered rings" of composition Al(2)O(2), the calculated C(Q) values are considerably lower, in the range identified in the glass. However, such two-membered ring geometries had been observed only in crystalline inorganic alumoxanes. Ab initio MD calculations on related compositions, such as the calcium aluminosilicate, CAS, (CaO)(0.21)(Al(2)O(3))(0.12)(SiO(2))(0.67), show a small percentage of O triclusters, but none in two-membered rings of the Al(2)O(2) type, and the calculated C(Q) values for the triclusters that do exist are higher than seen in the original experiments on CaAl(2)Si(2)O(8) glass and not significantly different from those for two-coordinate O in Si-O-Al sites. However, a classical MD simulation of the structure of glassy aluminum silicate AS2, (Al(2)O(3))2(SiO(2)), gave a predominance of O triclusters within two-membered rings, with structures much like those seen in the alumoxanes. We have now calculated (17)O nuclear quadrupole coupling constants and NMR shielding values for clusters extracted from these simulations, using standard quantum chemical methods. The calculated C(Q) values for these O triclusters are now in the range observed experimentally in the CaAl(2)Si(2)O(8) glass (around 2.3-2.6 MHz) when the tricluster O is surrounded by three Al, two of which are part of an Al(2)O(2) ring. This supports the experimentalists' contention that such tricluster O species do exist and have been seen by (17)O NMR.

Journal Article↗
PubMed2005

Critical phenomena in colloid-polymer mixtures: interfacial tension, order parameter, susceptibility, and coexistence diameter.

The critical behavior of a model colloid-polymer mixture, the so-called Asakura-Oosawa (AO) model, is studied using computer simulations and finite size scaling techniques. Investigated are the interfacial tension, the order parameter, the susceptibility, and the coexistence diameter. Our results clearly show that the interfacial tension vanishes at the critical point with exponent 2nu approximately 1.26 . This is in good agreement with the 3D Ising exponent. Also calculated are critical amplitude ratios, which are shown to be compatible with the corresponding 3D Ising values. We additionally identify a number of subtleties that are encountered when finite size scaling is applied to the AO model. In particular, we find that close to the critical point, the finite size extrapolation of the interfacial tension is most consistent when logarithmic size dependences are ignored.

Journal Article↗
PubMed2004

Grand canonical Monte Carlo simulation of a model colloid-polymer mixture: coexistence line, critical behavior, and interfacial tension.

Grand canonical Monte Carlo simulations are used to study phase separation in a simple colloid-polymer model, the so-called Asakura-Oosawa model. To overcome the problem of small acceptance rates of the grand-canonical moves, cluster moves are introduced. Successive umbrella sampling, recently introduced by Virnau and Muller [J. Chem. Phys. 120, 10925 (2004)], is used to access the phase-separated regime. The unmixing binodal and the interfacial tension are measured and compared to theoretical predictions. By means of finite-size scaling, the behavior close to the critical point is also investigated. Close to criticality, we observe substantial deviations from mean-field behavior.

Journal Article↗
PubMed2004

Channel formation and intermediate range order in sodium silicate melts and glasses.

We use inelastic neutron scattering and molecular dynamics simulation to investigate the interplay between the structure and the fast sodium ion diffusion in various sodium silicates. With increasing temperature and decreasing density the structure factors exhibit an emerging prepeak around 0.9 A(-1). We show that this prepeak has its origin in the formation of sodium rich channels in the static structure. The channels serve as preferential ion conducting pathways in the relative immobile Si-O matrix. On cooling below the glass transition this intermediate range order is frozen in.

Diffusion↗
PubMed2001

Lattice-Boltzmann method for the simulation of transport phenomena in charged colloids.

We present a simulation scheme based on the lattice-Boltzmann method to simulate the dynamics of charged colloids in an electrolyte. In our model we describe the electrostatics on the level of a Poisson-Boltzmann equation and the hydrodynamics of the fluid by the linearized Navier-Stokes equations. We verify our simulation scheme by means of a Chapman-Enskog expansion. Our method is applied to the calculation of the reduced sedimentation velocity U/U(0) for a cubic array of charged spheres in an electrolyte. We show that we recover the analytical solution first derived by Booth [F. Booth, J. Chem. Phys. 22, 1956 (1954)] for a weakly charged, isolated sphere in an unbounded electrolyte. The present method makes it possible to go beyond the Booth theory, and we discuss the dependence of the sedimentation velocity on the charge of the spheres. Finally we compare our results to experimental data.

Journal Article↗
PubMed2001

Relaxation dynamics of a viscous silica melt: the intermediate scattering functions.

We use molecular dynamics computer simulations to study the relaxation dynamics of a viscous melt of silica. The coherent and incoherent intermediate scattering functions, F(q,t) and F(s)(q,t), show a crossover from a nearly exponential decay at high temperatures to a two-step relaxation at low temperatures. Close to the critical temperature of mode-coupling theory (MCT) the correlators obey in the alpha regime the time temperature superposition principle (TTSP) and show a weak stretching. We determine the wave-vector dependence of the stretching parameter and find that for F(q,t) it shows oscillations that are in phase with the static structure factor. The temperature dependence of the alpha-relaxation times tau shows a crossover from an Arrhenius law at low temperatures to a weaker T dependence at intermediate and high temperatures. At the latter temperatures the T dependence is described well by the power law proposed by MCT with the same critical temperature that has previously been found for the diffusion constant D and the viscosity. We find that the exponent gamma of the power law for tau are significantly larger than the one for D. The wave-vector dependence of the alpha-relaxation times for F(q,t) oscillates around tau(q) for F(s)(q,t) and is in phase with the structure factor. Due to the strong vibrational component of the dynamics at short times the TTSP is not valid in the beta-relaxation regime. We show, however, that in this time window the shape of the curves is independent of the correlator and is given by a functional form proposed by MCT. We find that the value of the von Schweidler exponent and the value of gamma for finite q are compatible with the expression proposed by MCT. Finally we discuss the q dependence of the critical amplitude and the correction term and find that they are qualitatively similar to the ones for simple liquids and the prediction of MCT. We conclude that, in the temperature regime where the relaxation times are mesoscopic, many aspects of the dynamics of this strong glass former can be rationalized very well by MCT.

Journal Article↗
PubMed1989

Arrest of epidermal growth factor-dependent growth in fetal hepatocytes after ethanol exposure.

Exposure of the fetal rat hepatocyte to ethanol in vitro blocks epidermal growth factor (EGF)-dependent cell replication. To define possible mechanisms for this growth arrest, we determined the effects of ethanol on EGF binding and EGF receptor (EGF-R) levels. During a 24-h exposure to ethanol (1.7 mg/ml, 31 mM), cell replication was completely blocked while EGF binding per cell doubled. This effect was no specific for EGF, with variable degrees of increased binding noted for insulin, transferrin, and glucagon. Significantly increased EGF binding was seen after 6 h of ethanol exposure, and both growth arrest and enhanced EGF binding were reversed within 12 h of ethanol withdrawal. Increases in both "high" and "low" affinity sites were seen, with no changes in the apparent Kd's. Total RNA, beta-actin mRNA, and EGF-R mRNA were increased 50-70% in ethanol exposed cells. However, direct measurements of EGF-R synthesis rates by [35S]methionine incorporation revealed no differences between control and ethanol exposed cells. Internalization of EGF-R was significantly altered by ethanol exposure. A 2-h incubation resulted in the internalization of 57% of the ligand in control cells, while only 31% of bound EGF was internalized in the ethanol exposed cells. Thus, the enhanced EGF binding may be due to decreased efficiency of internalization.

Animals↗