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.