FRICTION

There are many exceptions to the "laws" of
friction we teach in introductory physics and the molecular origins of the
successes and failures of these laws have only begun to unravel in the last
decades [115]. The challenge is that friction forces come
from atomic interactions between atoms on opposing surfaces, but the number of
such atoms and the pressure pushing them together depend on elastic and plastic
deformation over the entire sample. Indeed one common explanation of why friction
is proportional to load is that the area of intimate contact between surfaces
is proportional to load. This leaves
open why the friction is proportional to area, and atomistic simulations
generally find a more complication connection between friction and area [91]. One possible
resolution is that the dirt and debris present on surfaces exposed to air locks
them together to produce a friction that is linear in load [85].

Some talks can be found here.

Recent research in the group has:

1) Calculated the area of contact between self-affine
fractal surfaces using continuum theory for elastic [118] and plastic [126] surfaces and
found simple equations that capture the behavior of all surfaces.

From left to
right the results above are for elastic and plastic calculations and the
traditional continuum approximation.

2) Examined the spatial correlations between contacting
regions and shown they are very different than assumed in traditional theories
for contacting surfaces [140] .

3) Studied the breakdown of continuum mechanics at atomic
scales [125]
and the implications for atomic force microscope experiments.

Plots above show atomistic tips and resulting
pressures.

4) Developed a multiscale
method that allows atomistic interactions to be used near the surface, while
using more efficient continuum methods farther away [144].

5) Examined the meaning of contact at atomic scales and
shown that thermal vibrations mean atoms only rarely touch until the pressure
is comparable to the hardness [150]. Even when surfaces are atomically flat, the
area of contact can have very complicated dependence on time and atomic scale
geometry (report).

6) Explored the effect of adsorbed molecules and other
debris that are present on any surface exposed to air. For both flat surfaces [85] and
single asperities [145]
this debris naturally leads to a friction that scales with load and is
insensitive to parameters that are not controlled in experiments.