The transition from Elasto‐Hydrodynamic to Mixed Regimes in Lubricated Friction of Soft Solid Surfaces

Abstract

Lubricated contacts in soft materials are common in various engineering and natural settings, such as tires, haptic applications, contact lenses, and the fabrication of soft electronic devices. Two major regimes are elasto‐hydrodynamic lubrication (EHL), in which solid surfaces are fully separated by a fluid film, and mixed lubrication (ML), in which there is partial solid‐to‐solid contact. The transition between these regimes governs the minimum sliding friction achievable and is thus very important. Generally, the transition from EHL to ML regimes is believed to occur when the thickness of the lubricant layer is comparable with the amplitude of surface roughness. Here, it is reported that in lubricated sliding experiments on smooth, soft, poly(dimethylsiloxane) substrates, the transition can occur when the thickness of the liquid layer is much larger than the height of the asperities. Direct visualization of the “contact” region shows that the transition corresponds to the formation of wave‐like surface wrinkles at the leading contact edge and associated instabilities at the trailing contact edge, which are believed to trigger the transition to the mixed regime. These results change the understanding of what governs the important EHL–ML transition in the lubricated sliding of soft solids.

Fluid-Elastic Interactions Near Contact at Low Reynolds Number

“…The mixed regime is presumed to comprise interspersed regions of solid-solid contact and lubricated contact, and is connected to surface roughness features (asperities) that are usually ignored in lubrication theory. Experimental data for hard materials suggest that the mixed regime is relevant when the film thickness is a few times the characteristic asperity size, though recent experiments show that this transition may occur for much thicker films and is associated with the formation of surface wrinkles (Dong et al 2023).…”

“…Descriptions of the mixed lubrication regime typically split the normal load between solidsolid and thin-film contact in a relative proportion that depends on the film thickness (see, e.g., Persson & Scaraggi 2009). While such models qualitatively capture the variation of friction versus sliding speed (the Stribeck curve), the detailed dynamics by which the lubricating fluid film breaks to produce solid contact has been the subject of debate for both rigid and soft geometries (see, e.g., Jamali & Brady 2019, Jamali et al 2020, Dong et al 2023. Transients bring soft surfaces temporarily closer to each other than at steady state, but without any finite-time solid contact (Weekley et al 2006).…”