Engineering surfaces are heterogeneous and can be slippery or sticky locally depending on surface roughness and chemistry. The variation of surface properties affects the interaction between the wall and lubricant and determines friction. Slip can be used to control friction in lubricated devices.
In hydrodynamic theory, the slip is quantified by the Navier slip length : a distance from the wall, where the flow profile would become zero if extrapolated linearly. If the thickness of lubricant film , then the effect of slip is negligible. If on the other hand , slip cannot be ignored. The slip length was reported to reach up to , depending on the material properties and applied conditions, which is comparable to the hydrodynamic films developed in many situations.
A team of researchers from Karlsruhe Institute of Technology and Fraunhofer Institute for Mechanics of Materials considered a theoretical surface with periodic slip/no-slip pattern sliding against a regular surface with a lubricant film in between. Using Molecular Dynamic simulations (MD), they showed that switch from no-slip to slip conditions gives rise to a pressure gradient, which can lead to cavitation. Cavitation, as well known in hydrodynamic theory, is necessary for load carrying capacity, as it was also confirmed by the simulations in the article.
Using continuum mechanics and Reynold’s approximation, the team obtained a closed form solution for the pressure gradient and pressure drop in the presence of slip/no-slip areas on the surface and an excellent agreement with MD simulations was observed. Obtained relation implies that the pressure drop increases with the increase of the slip/no-slip periodicity length, viscosity, sliding speed and slip length and decreases quadratically with lubricant film (). The relation was then applied to the randomly distributed patches of slipping areas and it was concluded that the relation still holds and is in agreement with MD simulation results.
The slip effect is important not only for artificially patterned surfaces, but also for the regular ones. Many of them are naturally patterned by slip areas of various slip length. Examples are steel surfaces with random grain orientation, diamond surfaces with mixed hydrogen and oxygen terminations, etc. Since , with the decrease of the film , as in the case of transition from hydrodynamic to boundary lubrication ( ) in sliding contacts, the cavitation due to variation of the slip length may be unavoidable in many systems. This effect then may control friction and wear in the sliding contacts. Cavitation due to slip length variation as a main feature of the boundary lubrication proposed by the researchers is different from the current theories and may open a new path of its further exploration.
Further details can be found in the original article: “Boundary lubrication of heterogeneous surfaces and the onset of cavitation in frictional contacts”, D. Savio, L. Pastewka, P. Gumbsh.