Physics and Astronomy Calendar

Friction affects many aspects of everyday life and has played a central
role in technology dating from the creation of fire by rubbing sticks
together to current efforts to make nanodevices with moving parts.
The friction "laws" we teach today date from empirical relationships
observed by da Vinci and Amontons centuries ago. However, the microscopic
origins of these laws have remained unclear, because friction is a
complex multiscale phenomenon that depends on both atomic interactions
in contacts, and the macroscopic elastic and plastic deformation that
determine the morphology and stress distribution within these contacts.
The talk will begin with continuum studies of contact between elastic and
plastic surfaces with self-affine surface roughness. The results show
that the area of real contact generally increases linearly with load,
but reveal inconsistencies in the continuum approach and common
assumptions. Next, the fundamental limits of continuum theory
are explored using molecular dynamics. The most important factor is not
discreteness within the solids, but the surface roughness present on any
surface composed of atoms. The above results show that the area of
contact between surfaces depends on many factors that do not influence
measured friction forces. The talk will conclude by showing that a simple
mechanism based on the presence of debris between surfaces naturally
explains Amontons' laws and common exceptions to them.

Selected References:
G. He, M. H. Muser and M. O. Robbins, Science 284, 1650 (1999).
J. Ringlein and M. O. Robbins, Am. J. Phys. 72, 884 (2004).
S. Hyun, L. Pei, J.-F. Molinari and M. O. Robbins, Phys. Rev. E70, 026117
(2004).
B. Luan and M. O. Robbins, Nature 435, 929-932 (2005).