Anisotropic friction on a silicon surface

The frictional force we experience in our day-to-day activities arises from atomic-scale surface roughness. When two surfaces rub against each other, thousands of atomic projections, or asperities, interact. Those interactions have now been explored at the single-atom scale by Franz Giessibl (University of Regensburg, Germany), his postdoctoral fellow Jay Weymouth, and colleagues. In particular, the team observed anisotropy in the friction force opposing motion over a silicon crystal whose surface is saturated with hydrogen. As the figure shows (beige is Si; white is H), each terrace on the Si surface presents rows of aligned Si dimers, but the rows in neighboring terraces are orthogonal. The researchers measured how the oscillation frequency of a lateral force microscope cantilever tip varied as they dragged it parallel to the dimers on one terrace and perpendicular to the dimers on another. Those frequency changes mirror changes in the spring constant of the cantilever and so allow one to determine the force experienced by the microscope tip. The deduced forces clearly distinguished between motions parallel and perpendicular to dimers. Moreover, collaborating theorists led by Pavel Jelinek (Academy of Sciences of the Czech Republic) ran first-principles simulations whose results agreed well with the measured forces. Interestingly, the simulations reveal that the Si dimers respond to the encroaching cantilever tip by rocking back and forth like a seesaw. (A. J. Weymouth et al., Phys. Rev. Lett., in press.)