WHERE DOES FRICTIONAL HEAT GO? Rub your hands together and they get warm. How does this come about, at the atomic level? Miquel Salmeron and his colleagues at LBL (510-486-6230, salmeron2stm.lbl.gov) addressed this problem by running a nanoscopic hoe (an atomic force microscope probe) through a field of pliant stalks (a monolayer of closely packed, upright alkylsilane molecules, strandlike molecules used in lubrication) self- assembled on a mica prairie. When the probe pushed harder, the contact area increased and so did the friction. The surface molecules had to tilt and to do that they had to unlock from each other, and this consumes energy, energy which is not recovered when the probe passes, allowing the molecules to untilt. This is the energy of friction. (The same AFM tip bends and images the sample; see figures at Physics News Graphics.) By adjusting the loading force of the probe, the researchers could get the molecules to tilt in discrete steps (cramped atoms displacing into new notches along the molecule, one at a time) resulting in a quantized form of friction. The exertion felt by the probe provides a measure of this energy dissipation, thus quantifying, for the first time, the direct relation between physics at the molecular level and macroscopic friction. (Barrena et al., Physical Review Letters, 5 April 1999)