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Images of a monolayer of alkysilane molecules put under a series of loads (imposed by the probe of an atomic force microscope)---(a) 10 nanonewtons, (b) 25 nanonewtons, (c 35 nanonewtons. The brigher colors correspond to higher regions. The images show that the compression of these molecular "islands" is not uniform. (d) shows the height profiles, in angstroms, for the middle of the islands.
Schematic model of the alkyl chain molecules tilting under increasing frictional loads. Only certain angles are allowed owing to the zigzag arrangement of the carbon atoms in the chains. This discreteness in tilt at the molecular level translates into a stepwise value for friction at the macroscopic level.
This research is reported by LBL physcists in an upcoming issue of Physical Review Letters.