Friction can be a big problem for small objects. Nanosized devices have very high surface-to-volume ratios, which means that their surfaces quickly wear out or even spontaneously stick together as they come into contact. To be able to control friction at the nanoscale, scientists have to first understand where friction comes from.

Current models of nanoscale friction assume that nanoscale surfaces are perfectly smooth, when in fact they are rough – rather like a mountain range, where peaks correspond to individual atoms or molecules. Nanomaterials in contact therefore behave like large rough objects that rub across each other.

Collection of atoms
Izabela Szlufarska and colleagues have performed computer simulations that look at nanoscale materials as a collection of atoms. The researchers monitor the positions and interactions of the atoms as the materials slide across each other. This is the first time that atomistic simulations of sliding friction have shown quantitative agreement with experiments carried out on the same material systems – sliding diamond-like carbon AFM tips on hydrogen-terminated diamond surfaces, says Szlufarska.

The team discovered that the force of friction is proportional to the number of atoms that interact between two nanoscale surfaces. "Our discovery provides a new and simple framework for interpreting nanoscale friction experiments, which up to now had been interpreted by continuum mechanics models," Szlufarska told "And, our demonstration that roughness theories also apply at the nanoscale provides a foundation for building unified friction laws across all length scales."

The researchers would ultimately like to use their simulations to build quantitative theories of friction that would allow them to predict the coefficient of friction for given materials and experimental conditions.

The work was published in Nature.