Friction or Slip? A nanoscale explanation –

At what level can the surface of a material cause friction? Scientists from the Laboratory of Tribology and System Dynamics and the American Johns Hopkins University have shown for the first time that such interactions already take place in the presence of surface-adsorbed molecules on the nanometer and molecular scale with sufficient effect to control macroscopic friction. This work, published in the journal ACS Nano, combines experiments, theoretical modeling and numerical simulations. Structure at the interface of a multi-contact friction.
Left macroscopic contact between two rough surfaces made in the ATLAS tribometer.
Right simulation of a roughness in the nanometer range, covered with adsorbed molecules, also nanometric.
© Lucas Frerot

Tribology, the science of friction, deals with how two objects slide past each other or not. This big question does not remain unanswered in applications, since friction causes wear and energy loss or, on the contrary, serves to hold structures in place. About a quarter of the energy produced worldwide is lost through friction, and this ubiquitous phenomenon is even found in earthquakes. It is also this friction that can save an athlete a tenth of a second over their opponents or even avoid skidding on an ice surface. Although perceptible, the phenomena acting in close contact with the surfaces that generate this sliding are not yet known.

Researchers from the Tribology and Systems Dynamics Laboratory (LTDS, CNRS/Centrale Lyon/ENTPE) and Johns-Hopkins University (Baltimore, USA) have shown for the first time the existence of a link at the molecular level between the physical interactions of contact and macroscopic friction in the presence of molecules adsorbed on the surface. This connection is the origin of the universality of the laws of friction.

Two touching sliding surfaces form a macroscopic contact. This consists of a series of micrometric contact transitions resulting from nanometric roughness. In practice, the materials are not perfectly clean and layers of molecules are naturally present and adsorbed on the surfaces. On contact, they penetrate each other like two nanometer-thick matte brushes facing each other. This research, published in the journal ACS Nano, shows that these local dynamics within these junctions determine friction. This work also shows that the presence of geometric surface defects is necessary for the occurrence of a stiction peak that prevents or slows the sliding of objects.

The team began a series of experiments using the LTDS Atlas Tribometer, the only device in the world capable of measuring the frictional response of multi-asperity contacts with a displacement resolution of less than 0.1 nanometers under moderate contact pressure. The conditions there are perfectly controlled, also for the deposition of molecules adsorbed by the surfaces. These results formed the basis of a theoretical model that predicted the need to account for the geometry of nanometer-scale surfaces. This idea was then confirmed by numerical simulations in molecular dynamics developed at Johns Hopkins University, which made it possible to couple phenomena at different scales, both spatially and temporally.

From molecular to multiasperity contacts: How roughness bridges the friction scale gap.
Lucas Frérot, Alexia Crespo, Jaafar A. El-Awady, Mark O. Robbins, Juliette Cayer-Barrioz, and Denis Mazuyer.
ACS Nano 2023.