Friction between two sliding objects involves the conversion of kinetic energy into heat, which is essentially the vibration of atoms that make up the materials. Robert Carpick of the University of Pennsylvania and colleagues have gained new insight into how this conversion occurs by sliding an atomic force microscope (AFM) tip along single-crystal diamond and silicon surfaces. They measured the force of friction between the tip and surfaces covered by either a single layer of hydrogen or deuterium atoms. Deuterium has the same chemical properties as hydrogen but is twice as heavy – allowing the team to study the effect of atomic mass on friction without having to worry about chemical effects.

The group, which includes researchers from the University of Wisconsin-Madison and the University of Houston, found that the larger the mass of terminating atoms at the surface (in this case deuterium), the lower the energy loss via friction. "The larger atomic mass of deuterium results in a lower natural vibration frequency of the atoms," explains Carpick. "These atoms collide less frequently with the tip sliding over it and thus energy is dissipated away from the contact at a lower rate."

Modelling studies performed by the team suggest that the lower frequency of the deuterium lowers the rate at which kinetic energy from the AFM tip is transformed into vibrations. The layer of atoms on the surface effectively acts an energy transfer medium and absorbs kinetic energy from the tip. The amount of energy absorbed depends on the natural vibration frequency of the surface atoms, with lighter atoms absorbing energy faster than heavier ones.

The results provide a better fundamental understanding of friction, which still lacks a comprehensive model. "We know how some properties -- adhesion, roughness and material stiffness, for example -- contribute to friction over several length scales, but this work reveals now truly atomic-scale phenomena can and do play a meaningful role," adds team member Matthew Brukman, who is at the University of Pennsylvania.