Lothar Brendel

Contact dynamics simulations of compacting cohesive granular systems

The modelling of cohesion on the particle scale in granular matter [Vermeer et al., 2001] [1] has recently received increasing attention, because various important applications call for a microscopic foundation for the existing phenomenological continuum theories. Specifically for understanding the different behaviour of nano-powders and ordinary granular matter cohesion has to be taken into account. An example in this context is the so-called sinter-forging of nano-powders, i.e. pressure sintering at relatively low temperatures [Skandan et al., 1994] [2]. We model the corresponding compaction by the rearrangement of a large number of grains by means of contact-dynamics. For this purpose we introduce an attractive force (cohesion) as well as rolling friction between the (round) grains. The latter turns out to be crucial for the stabilization of force chains.

Pore Stabilization in Cohesive Granular Systems

Cohesive powders tend to form porous aggregates which can be compacted by applying an external pressure. This process is modelled using the Contact Dynamics method supplemented with a cohesion law and rolling friction. Starting with ballistic deposits of varying density, we investigate how the porosity of the compacted sample depends on the cohesion strength and the friction coefficients. This allows to explain different pore stabilization mechanisms. The final porosity depends on the cohesion force scaled by the external pressure and on the lateral distance between branches of the ballistic deposit. Even if cohesion is switched off, pores can be stabilized by Coulomb friction alone. This effect is weak for round particles, as long as the friction coefficient is smaller than 1. However, for non-spherical particles the effect is much stronger.

Elastic behavior in contact dynamics of rigid particles

The systematic errors due to the practical implementation of the contact dynamics method for simulation of dense granular media are examined. It is shown that, using the usual iterative solver to simulate a chain of rigid particles, effective elasticity and sound propagation with a finite velocity occur. The characteristics of these phenomena are investigated analytically and numerically in order to assess the limits of applicability of this simulation method and to compare it with soft particle molecular dynamics.

Investigation of homoepitaxial growth on bcc surfaces with STM and kinetic Monte Carlo simulation

Abstract Time-resolved in situ STM has been applied to the system Fe on Fe(110) to study the characteristics of homoepitaxial growth on bcc surfaces. Additionally the growth of W on W(110) has been investigated which shows the same behaviour at 500°C as iron at room temperature. The resulting data are compared with kinetic Monte Carlo simulation which include the correct symmetry of the bcc(110) surface. It is found that islands grow strongly anisotropically elongated in [100] which is due to a hindered diffusion at step edges along [001]. At room temperature the inter-layer diffusion is suppressed by a step edge barrier which leads at higher coverages to a kinetic roughening and a complete faceting of the surface. The developing stable facets were investigated by SPA-LEED.

Trapping of interacting propelled colloidal particles in inhomogeneous media.

A trapping mechanism for propelled colloidal particles based on an inhomogeneous drive is presented and studied by means of computer simulations. In experiments this method can be realized using photophoretic Janus particles driven by a light source, which is partially blocked by a shading mask. This leads to an accumulation of particles in the passive part. An equation for an accumulation parameter is derived using the effective inhomogeneous diffusion constant generated by the inhomogeneous drive. The impact of particle interaction on the trapping mechanism is studied, as well as the interplay between passivity-induced trapping and the emergent self-clustering of systems containing a high density of active particles. The combination of both effects makes the clusters more controllable for applications.

A three-dimensional self-learning kinetic Monte Carlo model: application to Ag(111)

The reliability of kinetic Monte Carlo (KMC) simulations depends on accurate transition rates. The self-learning KMC method (Trushin et al 2005 Phys. Rev. B 72 115401) combines the accuracy of rates calculated from a realistic potential with the efficiency of a rate catalog, using a pattern recognition scheme. This work expands the original two-dimensional method to three dimensions. The concomitant huge increase in the number of rate calculations on the fly needed can be avoided by setting up an initial database, containing exact activation energies calculated for processes gathered from a simpler KMC model. To provide two representative examples, the model is applied to the diffusion of Ag monolayer islands on Ag(111), and the homoepitaxial growth of Ag on Ag(111) at low temperatures.

An adaptive hierarchical domain decomposition method for parallel contact dynamics simulations of granular materials

A fully parallel version of the contact dynamics (CD) method is presented in this paper. For large enough systems, 100% efficiency has been demonstrated for up to 256 processors using a hierarchical domain decomposition with dynamic load balancing. The iterative scheme to calculate the contact forces is left domain-wise sequential, with data exchange after each iteration step, which ensures its stability. The number of additional iterations required for convergence by the partially parallel updates at the domain boundaries becomes negligible with increasing number of particles, which allows for an effective parallelization. Compared to the sequential implementation, we found no influence of the parallelization on simulation results.

Spin excitations in a monolayer scanned by a magnetic tip

Energy dissipation via spin excitations is investigated for a hard ferromagnetic tip scanning a soft magnetic monolayer. We use the classical Heisenberg model with Landau-Lifshitz-Gilbert (LLG) dynamics including a stochastic field representing finite temperatures. The friction force depends linearly on the velocity (provided it is small enough) for all temperatures. For low temperatures, the corresponding friction coefficient is proportional to the phenomenological damping constant of the LLG equation. This dependence is lost at high temperatures, where the friction coefficient decreases exponentially. These findings can be explained by properties of the spin polarisation cloud dragged along with the tip.

Damping of Oscillations in Layer-by-Layer Growth

We present a theory for the damping of layer-by-layer growth oscillations in molecular beam epitaxy. The surface becomes rough on distances larger than a layer coherence length which is substantially larger than the diffusion length. The damping time can be calculated by a comparison of the competing roughening and smoothening mechanisms. The dependence on the growth conditions, temperature and deposition rate, is characterized to be a power law. The theoretical results are confirmed by computer simulations.

Visualization of Shear Motions of Cohesive Powders in the True Biaxial Shear Tester

Steady-state flow of powders is defined as a continuous deformation of the material without volume change while the stresses at the specimens boundaries remain constant. Recent investigations have shown that this state, especially for cohesive powders, is not always as steady as it should be by definition. In this article a recent extension of the true biaxial shear tester is introduced that allows a view of the shear motion of the brick-shaped powder specimen inside the tester. By applying a dark-colored powder pattern onto a light powder sample, the movement of the powder can be captured using a CCD camera. Development of shear bands and inhomogeneities of the sample can be visualized. Experiments with a cohesive powder with purely strain-controlled, volume-preserving shear cycles, as well as mixed stress-strain controlled experiments, are presented. The recorded images as well as stress-strain data from discrete elements simulations are compared to the experimental results.