arXiv subjects

Juergen Horbach

Publications and source records attributed to Juergen Horbach.

At least 19 recordsLinked to original sources

arXiv2016

Amorphization of a crystalline solid by plastic deformation

We demonstrate that plastic deformation in solids is associated with a dynamic transition that is reminiscent to the transition from a superconducting to a mixed phase in Type II superconductors. We report analytic calculations, extensive molecular dynamics and sequential umbrella sampling Monte Carlo simulations of a two dimensional triangular crystalline solid undergoing plastic deformation under strain. The solid consists of particles connected by harmonic springs. Non-affine displacement fluctuations of the solid are enhanced using an external field, causing a rich deformation behaviour. The external field leads to a mixed phase, where defect and stress-free crystallites are surrounded by a network of amorphous regions with large local internal stress --- a "stress Meissner" effect. The transition shows slow ageing dynamics caused by the presence of many competing, non-crystalline free-energy minima. Under uniform uniaxial strain, this amorphization transition gives rise to irreversible plastic deformation with the amorphous regions appearing as band-like structures. Our results may be checked by careful experiments on colloidal crystals using holographic optical tweezers.

cond-mat.soft↗

arXiv2013

Localization dynamics of fluids in random confinement

The dynamics of two-dimensional fluids confined within a random matrix of obstacles is investigated using both colloidal model experiments and molecular dynamics simulations. By varying fluid and matrix area fractions in the experiment, we find delocalized tracer particle dynamics at small matrix area fractions and localized motion of the tracers at high matrix area fractions. In the delocalized region, the dynamics is subdiffusive at intermediate times, and diffusive at long times, while in the localized regime, trapping in finite pockets of the matrix is observed. These observations are found to agree with the simulation of an ideal gas confined in a weakly correlated matrix. Our results show that Lorentz gas systems with soft interactions are exhibiting a smoothening of the critical dynamics and consequently a rounded delocalization-to-localization transition.

cond-mat.soft↗

arXiv2008

Statics and Dynamics of Colloid-Polymer Mixtures Near Their Critical Point of Phase Separation: A Computer Simulation Study of a Continuous AO Model

We propose a new coarse-grained model for the description of liquid-vapor phase separation of colloid-polymer mixtures. The hard-sphere repulsion between colloids and between colloids and polymers, which is used in the well-known Asakura-Oosawa (AO) model, is replaced by Weeks-Chandler-Anderson potentials. Similarly, a soft potential of height comparable to thermal energy is used for the polymer-polymer interaction, rather than treating polymers as ideal gas particles. It is shown by grand-canonical Monte Carlo simulations that this model leads to a coexistence curve that almost coincides with that of the AO model and the Ising critical behavior of static quantities is reproduced. Then the main advantage of the model is exploited - its suitability for Molecular Dynamics simulations - to study the dynamics of mean square displacements of the particles, transport coefficients such as the self-diffusion and interdiffusion coefficients, and dynamic structure factors. While the self-diffusion of polymers increases slightly when the critical point is approached, the self-diffusion of colloids decreases and at criticality the colloid self-diffusion coefficient is about a factor of 10 smaller than that of the polymers. Critical slowing down of interdiffusion is observed, which is qualitatively similar to symmetric binary Lennard-Jones mixtures, for which no dynamic asymmetry of self-diffusion coefficients occurs.

cond-mat.soft↗

arXiv2008

Confinement Effects on Phase Behavior of Soft Matter Systems

When systems that can undergo phase separation between two coexisting phases in the bulk are confined in thin film geometry between parallel walls, the phase behavior can be profoundly modified. These phenomena shall be described and exemplified by computer simulations of the Asakura-Oosawa model for colloid-polymer mixtures, but applications to other soft matter systems (e.g. confined polymer blends) will also be mentioned. Typically a wall will prefer one of the phases, and hence the composition of the system in the direction perpendicular to the walls will not be homogeneous. If both walls are of the same kind, this effect leads to a distortion of the phase diagram of the system in thin film geometry, in comparison with the bulk, analogous to the phenomenon of "capillary condensation" of simple fluids in thin capillaries. In the case of "competing walls", where both walls prefer different phases of the two phases coexisting in the bulk, a state with an interface parallel to the walls gets stabilized. The transition from the disordered phase to this "soft mode phase" is rounded by the finite thickness of the film and not a sharp phase transition. However, a sharp transition can occur where this interface gets localized at (one of) the walls. The relation of this interface localization transition to wetting phenomena is discussed. Finally, an outlook to related phenomena is given, such as the effects of confinement in cylindrical pores on the phase behavior, and more complicated ordering phenomena (lamellar mesophases of block copolymers or nematic phases of liquid crystals under confinement).

cond-mat.soft↗

arXiv2008

Molecular dynamics computer simulation of amorphous silica under high pressure

The structural and dynamic properties of silica melts under high pressure are studied using molecular dynamics (MD) computer simulation. The interactions between the ions are modeled by a pairwise-additive potential, the so-called CHIK potential, that has been recently proposed by Carre et al. The experimental equation of state is well-reproduced by the CHIK model. With increasing pressure (density), the structure changes from a tetrahedral network to a network containing a high number of five- and six-fold Si-O coordination. In the partial static structure factors, this change of the structure with increasing density is reflected by a shift of the first sharp diffraction peak towards higher wavenumbers q, eventually merging with the main peak at densities around 4.2 g/cm^3. The self-diffusion constants as a function of pressure show the experimentally-known maximum, occurring around a pressure of about 20 GPa.

cond-mat.dis-nn↗

arXiv2008

Comparative classical and ab initio Molecular Dynamics study of molten and glassy germanium dioxide

A Molecular Dynamics (MD) study of static and dynamic properties of molten and glassy germanium dioxide is presented. The interactions between the atoms are modelled by the classical pair potential proposed by Oeffner and Elliott (OE) [Oeffner R D and Elliott S R 1998, Phys. Rev. B, 58, 14791]. We compare our results to experiments and previous simulations. In addition, an ab initio method, the so-called Car-Parrinello Molecular Dynamics (CPMD), is applied to check the accuracy of the structural properties, as obtained by the classical MD simulations with the OE potential. As in a similar study for SiO2, the structure predicted by CPMD is only slightly softer than that resulting from the classical MD. In contrast to earlier simulations, both the static structure and dynamic properties are in very good agreement with pertinent experimental data. MD simulations with the OE potential are also used to study the relaxation dynamics. As previously found for SiO2, for high temperatures the dynamics of molten GeO2 is compatible with a description in terms of mode coupling theory.

cond-mat.dis-nn↗

arXiv2008

Molecular Dynamics Computer Simulation of Crystal Growth and Melting in Al50Ni50

The melting and crystallization of Al50Ni50} are studied by means of molecular dynamics computer simulations, using a potential of the embedded atom type to model the interactions between the particles. Systems in a slab geometry are simulated where the B2 phase of AlNi in the middle of an elongated simulation box is separated by two planar interfaces from the liquid phase, thereby considering the (100) crystal orientation. By determining the temperature dependence of the interface velocity, an accurate estimate of the melting temperature is provided. The value k=0.0025 m/s/K for the kinetic growth coefficient is found. This value is about two orders of magnitude smaller than that found in recent simulation studies of one-component metals. The classical Wilson-Frenkel model is not able to describe the crystal growth kinetics on a quantitative level. We argue that this is due to the neglect of diffusion processes in the liquid-crystal interface.

cond-mat.mtrl-sci↗

arXiv2008

New fitting scheme to obtain effective potential from Car-Parrinello molecular dynamics simulations: Application to silica

A fitting scheme is proposed to obtain effective potentials from Car-Parrinello molecular dynamics (CPMD) simulations. It is used to parameterize a new pair potential for silica. MD simulations with this new potential are done to determine structural and dynamic properties and to compare these properties to those obtained from CPMD and a MD simulation using the so-called BKS potential. The new potential reproduces accurately the liquid structure generated by the CPMD trajectories, the experimental activation energies for the self-diffusion constants and the experimental density of amorphous silica. Also lattice parameters and elastic constants of alpha-quartz are well-reproduced, showing the transferability of the new potential.

cond-mat.dis-nn↗

arXiv2007

Amorphous silica modeled with truncated and screened Coulomb interactions: A molecular dynamics simulation study

We show that finite-range alternatives to the standard long-range BKS pair potential for silica might be used in molecular dynamics simulations. We study two such models that can be efficiently simulated since no Ewald summation is required. We first consider the Wolf method, where the Coulomb interactions are truncated at a cutoff distance r_c such that the requirement of charge neutrality holds. Various static and dynamic quantities are computed and compared to results from simulations using Ewald summations. We find very good agreement for r_c ~ 10 Angstroms. For lower values of r_c, the long--range structure is affected which is accompanied by a slight acceleration of dynamic properties. In a second approach, the Coulomb interaction is replaced by an effective Yukawa interaction with two new parameters determined by a force fitting procedure. The same trend as for the Wolf method is seen. However, slightly larger cutoffs have to be used in order to obtain the same accuracy with respect to static and dynamic quantities as for the Wolf method.

cond-mat.stat-mech↗

arXiv2007

Self-diffusion and Interdiffusion in Al80Ni20 Melts: Simulation and Experiment

A combination of experimental techniques and molecular dynamics (MD) computer simulation is used to investigate the diffusion dynamics in Al80Ni20 melts. Experimentally, the self-diffusion coefficient of Ni is measured by the long-capillary (LC) method and by quasielastic neutron scattering. The LC method yields also the interdiffusion coefficient. Whereas the experiments were done in the normal liquid state, the simulations provided the determination of both self-diffusion and interdiffusion constants in the undercooled regime as well. The simulation results show good agreement with the experimental data. In the temperature range 3000 K >= T >= 715 K, the interdiffusion coefficient is larger than the self-diffusion constants. Furthermore the simulation shows that this difference becomes larger in the undercooled regime. This result can be refered to a relatively strong temperature dependence of the thermodynamic factor Φ, which describes the thermodynamic driving force for interdiffusion. The simulations also indicate that the Darken equation is a good approximation, even in the undercooled regime. This implies that dynamic cross correlations play a minor role for the temperature range under consideration.

cond-mat.dis-nn↗

arXiv2006

Mobile particles in an immobile environment: Molecular Dynamics simulation of a binary Yukawa mixture

Molecular dynamics computer simulations are used to investigate thedynamics of a binary mixture of charged (Yukawa) particles with a size-ratio of 1:5. We find that the system undergoes a phase transition where the large particles crystallize while the small particles remain in a fluid-like (delocalized) phase. Upon decreasing temperature below the transition, the small particles become increasingly localized on intermediate time scales. This is reflected in the incoherent intermediate scattering functions by the appearance of a plateau with a growing height. At long times, the small particles show a diffusive hopping motion. We find that these transport properties are related to structural correlations and the single-particle potential energy distribution of the small particles.

cond-mat.dis-nn↗

arXiv2006

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. Ultra-thin 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 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 ($D \to \infty$) value non-monotonically, 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.

cond-mat.soft↗

arXiv2006

Critical Dynamics in a Binary Fluid: Simulations and Finite-size Scaling

We report comprehensive simulations of the critical dynamics of a symmetric binary Lennard-Jones mixture near its consolute point. The self-diffusion coefficient exhibits no detectable anomaly. The data for the shear viscosity and the mutual-diffusion coefficient are fully consistent with the asymptotic power laws and amplitudes predicted by renormalization-group and mode-coupling theories {\it provided} finite-size effects and the background contribution to the relevant Onsager coefficient are suitably accounted for. This resolves a controversy raised by recent molecular simulations.

cond-mat.stat-mech↗

arXiv2006

Static and Dynamic Critical Behavior of a Symmetrical Binary Fluid: A Computer Simulation

A symmetrical binary, A+B Lennard-Jones mixture is studied by a combination of semi-grandcanonical Monte Carlo (SGMC) and Molecular Dynamics (MD) methods near a liquid-liquid critical temperature $T_c$. Choosing equal chemical potentials for the two species, the SGMC switches identities (${\rm A} \to {\rm B} \to {\rm A}$) to generate well-equilibrated configurations of the system on the coexistence curve for $T T_c$. A finite-size scaling analysis of the concentration susceptibility above $T_c$ and of the order parameter below $T_c$ is performed, varying the number of particles from N=400 to 12800. The data are fully compatible with the expected critical exponents of the three-dimensional Ising universality class. The equilibrium configurations from the SGMC runs are used as initial states for microcanonical MD runs, from which transport coefficients are extracted. Self-diffusion coefficients are obtained from the Einstein relation, while the interdiffusion coefficient and the shear viscosity are estimated from Green-Kubo expressions. As expected, the self-diffusion constant does not display a detectable critical anomaly. With appropriate finite-size scaling analysis, we show that the simulation data for the shear viscosity and the mutual diffusion constant are quite consistent both with the theoretically predicted behavior, including the critical exponents and amplitudes, and with the most accurate experimental evidence.

cond-mat.stat-mech↗

arXiv2005

Structure and transport properties of amorphous aluminium silicates: computer simulation studies

The structure and transport properties of SiO2-Al2O3 melts containing 13 mol% and 47 mol% Al2O3 are investigated by means of large scale molecular dynamics computer simulations. The interactions between the atoms are modelled by a pair potential which is a modified version of the one proposed by Kramer et al. [J. Am. Chem. Soc. 64, 6435 (1991)]. Fully equilibrated melts in the temperature range 6000 K >= T > 2000 K are considered as well as glass configurations, that were obtained by a rapid quench from the lowest melt temperatures. Each system is simulated at two different densities in order to study the effect of pressure on structural and dynamic properties. We find that the Al atoms are, like the Si atoms, mainly four-fold coordinated by oxygen. However, the packing of the AlO4 tetrahedra is very different from that of the SiO4 tetrahedra, which is reflected by the presence of triclusters (O atoms surrounded by three cations) and edge--sharing AlO4 tetrahedra. On larger length scales, a micro-segregation occurs, resulting in an Al-rich network percolating through the Si-O network. This is reflected in a prepeak of concentration-concentration structure factors around 0.5 A^-1 (both in the system with 47 mol% and 13 mol% Al2O3!). We also address the interplay between structure and mass transport. To this end, the behavior of the selfdiffusion constants for the different compositions and densities is studied.

cond-mat.dis-nn↗

arXiv2005

Slow Dynamics in Ion-Conducting Sodium Silicate Melts: Simulation and Mode-Coupling Theory

A combination of molecular-dynamics (MD) computer simulation and mode-coupling theory (MCT) is used to elucidate the structure-dynamics relation in sodium-silicate melts (NSx) of varying sodium concentration. Using only the partial static structure factors from the MD as an input, MCT reproduces the large separation in relaxation time scales of the sodium and the silicon/oxygen components. This confirms the idea of sodium diffusion channels which are reflected by a prepeak in the static structure factors around 0.95 A^-1, and shows that it is possible to explain the fast sodium-ion dynamics peculiar to these mixtures using a microscopic theory.

cond-mat.dis-nn↗

arXiv2005

Kinetics of Phase Separation in Thin Films: Simulations for the Diffusive Case

We study the diffusion-driven kinetics of phase separation of a symmetric binary mixture (AB), confined in a thin-film geometry between two parallel walls. We consider cases where (a) both walls preferentially attract the same component (A), and (b) one wall attracts A and the other wall attracts B (with the same strength). We focus on the interplay of phase separation and wetting at the walls, which is referred to as {\it surface-directed spinodal decomposition} (SDSD). The formation of SDSD waves at the two surfaces, with wave-vectors oriented perpendicular to them, often results in a metastable layered state (also referred to as ``stratified morphology''). This state is reminiscent of the situation where the thin film is still in the one-phase region but the surfaces are completely wet, and hence coated with thick wetting layers. This metastable state decays by spinodal fluctuations and crosses over to an asymptotic growth regime characterized by the lateral coarsening of pancake-like domains. These pancakes may or may not be coated by precursors of wetting layers. We use Langevin simulations to study this crossover and the growth kinetics in the asymptotic coarsening regime.

cond-mat.soft↗

arXiv2005

Combining Molecular Dynamics with Lattice-Boltzmann: A Hybrid Method for the Simulation of (Charged) Colloidal Systems

We present a hybrid method for the simulation of colloidal systems, that combines molecular dynamics (MD) with the Lattice-Boltzmann (LB) scheme. The LB method is used as a model for the solvent in order to take into account the hydrodynamic mass and momentum transport through the solvent. The colloidal particles are propagated via MD and they are coupled to the LB fluid by viscous forces. With respect to the LB fluid, the colloids are represented by uniformly distributed points on a sphere. Each such point (with a velocity V(r) at any off-lattice position r is interacting with the neighboring eight LB nodes by a frictional force F=ξ_0(V(r)-u(r)) with ξ_0 being a friction force and u(r) being the velocity of the fluid at the position r. Thermal fluctuations are introduced in the framework of fluctuating hydrodynamics. This coupling scheme has been proposed recently for polymer systems by Ahlrichs and D"unweg [J. Chem. Phys. 111, 8225 (1999)]. We investigate several properties of a single colloidal particle in a LB fluid, namely the effective Stokes friction and long time tails in the autocorrelation functions for the translational and rotational velocity. Moreover, a charged colloidal system is considered consisting of a macroion, counterions and coions that are coupled to a LB fluid. We study the behavior of the ions in a constant electric field. In particular, an estimate of the effective charge of the macroion is yielded from the number of counterions that move with the macroion in the direction of the electric field.

cond-mat.soft↗