Transition from Elastohydrodynamic to Mixed Regimes in Lubricated Friction of Soft Solid Surfaces.

Lubricated contacts in soft materials are common in various engineering and natural systems. Two major regimes are elastohydrodynamic 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. Transition between these regimes governs the minimum sliding friction achievable and is thus very important in various engineering and natural settings such as tires, haptic applications, bio-inspired systems, contact lenses, and the fabrication of soft electronic devices. 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. We performed lubricated sliding experiments on smooth polydimethylsiloxane (PDMS) substrates under various normal load, sliding velocity, and lubricant conditions. We found that for smooth soft surfaces, the transition from EHL to ML regimes can occur when the thickness of the liquid layer is much larger than the height of asperities, that is, the conventional criterion is highly inaccurate. By direct visualization of the "contact" region during sliding experiments, we demonstrate 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. We believe these instabilities are peculiar to soft solids and trigger transition to the mixed regime. Our results change the fundamental understanding of what governs the important EHL-ML transition in lubricated sliding of soft solids. This article is protected by copyright. All rights reserved.