Project II – Simulation of adhesion, layering and friction

Contributor: B.Persson

The group of B. Persson has developed a computer code for the simulation of adhesion, layering and friction. The system consists of two elastic slabs with surfaces of arbitrary curvature separated by a fluid. This is the first computer code I am aware of which include curved surfaces and long range elasticity. The code has recently been applied to the squeezing out of a (lubrication) fluid between two surfaces (the fluid wets the surfaces). For thin fluid layers (say, less than 5 molecular layers) this process is a thermally activated nucleation process which, with high probability, start in the region where the pressure is maximal. This is an extremely interesting and important result and explains early experimental results of Tabor and Rabinowicz.

The next application will be to study sliding friction. The research will be focussed on very low sliding velocities (boundary lubrication) to study the very rapid processes which must occur in the lubrication film, even when the driving velocity is very low. The transition from boundary lubrication to hydrodynamic lubrication with increasing sliding velocity will be considered. Since the computer code can be applied to materials with any elasticity. The sliding of elastically very soft solids will be also considered, in particular rubber, where it may be possible for elastic instability waves to propagate on the solid surfaces. Such waves (so called Schallamach waves) have been observed for rubber on a macroscopic length scale, but nobody knows if they also occur on much shorter (molecular) length scales. The studies above will be performed with different model liquids (presently I a use a Lennard-Jones fluid) and for different fluid-solid interaction potentials (e.g., different spreading pressures and different lateral corrugation of the fluid-substrate interaction potential). These studies shall supply fundamental insight into the microscopic origin of sliding friction.

The outline above is a very specific project that I am certainwill result in many very interesting and novel results. However, our interest in physics is much broader, and other fields of physics including surface physics, biophysics, materials properties will be studied (including plastic deformation and brittle fracture, where B. Persson is particularly interested in understanding the transition from brittle to ductile fracture, which has been observed for many materials with increasing temperature), and the properties of small particle systems. The general approach to physics is to obtain simple physical pictures for the phenomena under investigation. Large-scale computer calculations are really meaningful for me only if they result in simple pictures, preferably based on simple analytical arguments, which explain the essence of the results of the computer calculations.