The study found that the larger mass of the terminating atoms at the surface, in this case deuterium, led to less energy lost to heat in the system, Robert Carpick, associate professor of mechanical engineering and applied mechanics at Penn, said. The larger atomic mass of deuterium results in a lower natural vibration frequency of the atoms. These atoms collide less frequently with the tip sliding over it, and thus energy is more slowly dissipated away from the contact.

The single layer of atoms at the surface of each crystal acts as an energy transfer medium, absorbing kinetic energy from the tip of the atomic force microscope. The tips were less than 50 nm in radius at their ends. How much energy is absorbed is dependent, researchers found, on the adsorbates natural atomic vibration frequencies. The heavier an atom, the lower its vibrational frequency. The lighter an atom, the faster the vibrations and thus the faster the dissipation of energy from the contact in the sample. Keeping the atoms chemically similar avoided any changes arising from chemical bonding.

The Penn findings provide a better understanding of the nature of friction, which lacks a comprehensive model at the fundamental level.

We know how some properties—adhesion, roughness and material stiffness for example—contribute to friction over several length scales, but this work reveals how truly atomic-scale phenomena can and do play a meaningful role, Matthew Brukman, a contributor to the research, said.

Industry has long been concerned with ways to reduce friction between objects, both to maintain the energy of the system as well as to reduce heat-generation and wear, which can weaken machinery and materials to the breaking point. The authors note that improved friction models can be used for the opposite effect; makers of some mechanical components such as automobile clutches may be interested in techniques to increase friction without changing the wear or adhesion of materials.

Even in the absence of rough edges or wear between sliding bodies, friction between the atoms at the surface causes vibrations which dissipate energy, but the exact mechanisms of this process remain unresolved. Scientists continue to explore the details of friction, and other open questions include the effects of environmental variables such as temperature and atmosphere.

The research was performed by Carpick and Brukman of the Department of Materials Science and Engineering in Penn's School of Engineering and Applied Science; Rachel J. Cannara, now of the IBM Zurich Research Laboratory; Anirudha V. Sumant, now at Argonne National Laboratory; and Steven Baldelli and Katherine Cimatu of the University of Houston.

The research was supported by the National Science Foundation, an NSF Graduate Research Fellowship and the Air Force Office of Scientific Research.

Resources

• Rachel J. Cannara, Matthew J. Brukman, Katherine Cimatu, Anirudha V. Sumant, Steven Baldelli, Robert W. Carpick. “Nanoscale Friction Varied by Isotopic Shifting of Surface Vibrational Frequencies” Science 2 November 2007: Vol. 318. no. 5851, pp. 780 - 783 DOI: 10.1126/science.1147550