arXiv subjects

Juergen Horbach

Publications and source records attributed to Juergen Horbach.

24 records · Page 2Linked to original sources

arXiv2005

Water adsorption on amorphous silica surfaces: A Car-Parrinello simulation study

A combination of classical molecular dynamics (MD) and ab initio Car-Parrinello molecular dynamics (CPMD) simulations is used to investigate the adsorption of water on a free amorphous silica surface. From the classical MD SiO_2 configurations with a free surface are generated which are then used as starting configurations for the CPMD.We study the reaction of a water molecule with a two-membered ring at the temperature T=300K. We show that the result of this reaction is the formation of two silanol groups on the surface. The activation energy of the reaction is estimated and it is shown that the reaction is exothermic.

cond-mat.dis-nn↗

arXiv2003

Amorphous silica at surfaces and interfaces: simulation studies

The structure of surfaces and interfaces of silica (SiO2) is investigated by large scale molecular dynamics computer simulations. In the case of a free silica surface, the results of a classical molecular dynamics simulation are compared to those of an ab initio method, the Car-Parrinello molecular dynamics. This comparative study allows to check the accuracy of the model potential that underlies the classical simulation. By means of a pure classical MD, the interface between amorphous and crystalline SiO2 is investigated, and as a third example the structure of a silica melt between walls is studied in equilibrium and under shear. We show that in the latter three examples important structural information such as ring size distributions can be gained from the computer simulation that is not accessible in experiments.

cond-mat.dis-nn↗

arXiv2002

Amorphous silica between confining walls and under shear: a computer simulation study

Molecular dynamics computer simulations are used to investigate a silica melt confined between walls at equilibrium and in a steady-state Poisseuille flow. The walls consist of point particles forming a rigid face-centered cubic lattice and the interaction of the walls with the melt atoms is modelled such that the wall particles have only a weak bonding to those in the melt, i.e. much weaker than the covalent bonding of a Si-O unit. We observe a pronounced layering of the melt near the walls. This layering, as seen in the total density profile, has a very irregular character which can be attributed to a preferred orientational ordering of SiO4 tetrahedra near the wall. On intermediate length scales, the structure of the melt at the walls can be well distinguished from that of the bulk by means of the ring size distribution. Whereas essentially no structural changes occur in the bulk under the influence of the shear fields considered, strong structural rearrangements in the ring size distribution are present at the walls as far as there is a slip motion. For the sheared system, parabolic velocity profiles are found in the bulk region as expected from hydrodynamics and the values for the shear viscosity as extracted from those profiles are in good agreement with those obtained in pure bulk simulations from the appropriate Green-Kubo formula.

cond-mat.stat-mech↗

arXiv2001

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

We present a new 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.

cond-mat.dis-nn↗

arXiv2001

The dynamics of sodium in sodium disilicate: Channel relaxation and sodium diffusion

We use molecular dynamics computer simulations to study the dynamics of amorphous (Na_2O)2(SiO_2). We find that the Na ions move in channels embedded in a SiO_2 matrix. The characteristic distance between these channels gives rise to a prepeak in the structure factor at around q=0.95 A^-1. The dynamics of sodium is given by a fast process which can be seen in the incoherent scattering function and a slow process which is seen in the coherent function. The relaxation time of the latter coincides with the alpha-relaxation time of the matrix. The Kohlrausch exponent of the fast process for q>1.6 A^1 is the same as the von Schweidler exponent for the slow one, demonstrating that the two processes are closely related.

cond-mat.stat-mech↗

arXiv1997

The Dynamics of Supercooled Silica: Acoustic modes and Boson peak

Using molecular dynamics computer simulations we investigate the dynamics of supercooled silica in the frequency range 0.5-20~THz and the wave-vector range 0.13-1.1Å^{-1}. We find that for small wave-vectors the dispersion relations are in very good agreement with the ones found in experiments and that the frequency at which the boson-peak is observed shows a maximum at around 0.39Å^{-1}.

cond-mat.stat-mech↗