AVS 51st International Symposium
    Surface Science Thursday Sessions
       Session SS2-ThM

Invited Paper SS2-ThM5
Frictional Forces and Amontons' Law: From the Molecular to the Macroscopic Scale

Thursday, November 18, 2004, 9:40 am, Room 210C

Session: Tribology, Adhesion, and Friction
Presenter: U. Landman, Georgia Institute of Technology
Authors: U. Landman, Georgia Institute of Technology
J. Gao, Georgia Institute of Technology
W.D. Luedtke, Georgia Institute of Technology
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Amontons' law, which was already known to Leonardo da Vinci, states that the friction force is directly proportional to the (normal) applied load, with a constant of proportionality - the friction coefficient - that is constant and independent of the contact area, the surface roughness and the sliding velocity. No theory has yet satisfacorily explained this surprisingly general law, all attempts being model or system dependent. On the basis of large-scale molecular dynamics simulations pertaining to lubricated adhesive and non-adhesive junctions, with morphologically rough (as well as crystallographically flat) confining solid surfaces, and in conjunction with recent experiments, we show that the local energy-dissipation mechanisms are not 'mechanical', as assumed in most models, but â?othermodynamicâ? in nature. We show that a local analysis of the simulation results, based on division of the system into small cells, leads to a natural description in terms of the Weibull distribution. For the dynamic. non-equilibrium, energy-dissipating process that we study, this long-tail distribution serves a similar purpose as the Boltzmann distribution for classical systems at equilibrium. While Amontons' law does not hold on the local scale, it is recovered on the global scale, with the spatio-temporal averaging utilizing the Weibull distribtion of the local friction forces. Interestingly, the concept of "area of contact", often used in frictional studies, does not enter into our analysis. J. Gao, W.D. Luedtke, D. Gourdon, M Ruths, J.N. Israelachvili, U. Landman, Feature Article, in J. Phys. Chem. B 108, 3480 (2004).