Campus C 6.3
Saarbrücken D-66123
Tel: +49 681 302 57453
Fax: +49 681 302 57454


Contact mechanics challenge announcement

We call for a modeling competition in contact mechanics and invite theorists and modelers alike to make predictions on a mathematically well-defined problem. New deadline: June 30, 2016.

The problem can be specified as follows:
- linear elasticity and small-slope approximation
- short range adhesion
- self-affine rough surface, topography provided numerically in real space and in Fourier space.

Participants are asked to make predictions that we can test by solving the problem numerically. You can participate by sending your predictions in a PDF by e-mail. For more details…

Proceedings of Force Fields 2014

Proceedings of the Workshop on Force Fields: From atoms to materials, which was held from November 3-5, 2014, at Forschungszentrum Jülich are now published at
Modelling and Simulation in Materials Science and Engineering.

Dissipation of polymer brushes

Lubrication of biological systems often involves the presence of polymers that are end-anchored to rubbing surfaces. The polymers swell in a solvent to form brushes, which then act as low-viscosity bearings when surfaces slide against a counter face.

In a recent article, featured on the cover of Soft Matter and picked as one of July’s hot papers by RSC, we investigated a commonly  assumed hypothesis: Friction between rubbing surfaces is predominantly induced by the mutual penetration of opposing polymer brushes. (more…)

Contact mechanics and seal leakage

Seals are devices to close a gap and to prevent fluid flow through a joint. Although seals play a crucial role in many modern engineering devices, inexpensive elastomeric seals such as O-rings are often used. The failure of seals can have serious ramifications ranging from energy loss, environmental pollution, expensive and time-consuming replacement procedures all the way to catastrophic events like the Challenger disaster. Thus, seal systems should be handled as thoroughly in the design process as any other crucial machine element, and not like a secondary accessory.

In our simulations, we study fluid flow at the interfaces between elastic solids with randomly rough, self-affine surfaces. A trademark of our work is (more…)

Friction mechanisms

Friction is a well-known, but poorly understood, phenomenon that affects virtually all aspects of daily life. In some cases, friction is desirable, e.g., high friction in clutches leads to the effective transmission of forces between an automobile’s engine and its wheels, while in other cases friction is a significant drawback, e.g., friction between the piston and cylinder wall decreases the efficiency of automobile engines. Although macroscopic friction laws were introduced a few centuries ago, and the existence of friction was recognized long before that, the underlying atomic-level mechanisms leading to friction have remained elusive. The identification of these mechanisms (more…)

Proceedings of Hybrid 2013

Proceedings of the Workshop on Hybrid Particle-Continuum Methods in Computational Physics, which was held from March 4-7, 2013, at Forschungszentrum Jülich can now be downloaded as PDFs. The goal of the workshop was to foster the exchange of ideas between the communities working on complex fluids and on complex solids. 65 participants learned about the new developments in hybrid particle-continuum methods from 15 invited and 10 contributed talks as well as from 30 posters.

Accurate Interatomic Potentials

PotentialsThe picture shows the electrostatic field of a d-fructose molecule which must be reproduced in an efficient manner in a molecular dynamics simulation. Electrostatic forces dominate the long-range interactions between molecules and must be accounted for in atomistic simulations of large systems. Since these forces are required at each step in a molecular dynamics or Monte Carlo simulation, it is important to develop efficient methods that accurately reproduce the electrostatic fields of molecules. Traditionally, charge-equilibration, or fluctuating-charge techniques have been used to generate the electrostatic fields of molecules, yet their application remains limited due to their inaccuracy. We have developed (more…)

Phase Change Materials

picture of phase change materialsPhase change materials are class of materials, which can remain stable in a crystal or amorphous phase at ambient conditions requiring a significant amount of heat for a phase change. PCMs are gaining application in memory devices, where a bit of information is stored by changing the phase of a material region with help of heat, long pulse – recrystallization, short pulse – amorphization. We found that (more…)

Polymer diffusion on surfaces

PolymerThe lateral diffusion coefficient for strongly adsorbed mono-dispersed polymer/oligomer chains shows an anomalous behavior, which is a result of structural transition from single to double layer. The transition gives polymer more geometrical flexibility to adopt to the surface corrugation and hence, within the Arrhenius type activation picture, is responsible for much reduced lateral mobility at large concentrations. The study showed, (more…)

Simulations of collective quantum phenomena

DispersionThe image shows the phonon spectrum of a harmonic chain (modeling for instance a charge-density wave) in an external random potential at zero temperature (Ph.D. thesis F. Krajewski). The springs are weak compared to the maximum curvature of the external potential, which explains the occurrence of a main phonon branch (yellow line). The existence of the second branch is non-trivial. It is due to a collective tunneling of the particles constituting the chain. At zero wavevector q , the probability of zero frequencies appears finite. This in turn implies that the chain can be moved with arbitrarily small external fields.

Thermal and mechanical properties of silica

SiliziumA full description of quartz’s phase transitions and its interesting dielectric properties (such as piezoelectricity and optical activity) requires a realistic incorporation of the local electric fields and polarizations into the force fields. One of my students, Daniel Herzbach, incorporated these features into the simulations and the role of the dipoles (more…)

Smart Materials

smart materialPhosphates are successfully used in motor oils as antiwear agents. They reduce wear by forming a protective metal phosphate film on the rubbing surface. The origin of the extraordinary antiwear properties of the phosphates remains a matter of debates for decades. Our research indicates that unique antiwear properties of the phosphate films might be related to their smart materials behavior. Low coordination metal phosphates create open networks, which remain soft at low loads and stiffen rapidly upon loading. Material exhibiting such a “smart” effect can replace a system of layers (more…)