(671e) Lubricated Friction of Soft Solid Surfaces: Transition from Elastohydrodynamic to Mixed Regime

Lubricated contacts are ubiquitous in various engineering systems related to soft materials. Two major regimes are elastohydrodynamic lubrication (EHL) in which solid surfaces are fully separated by a fluid film and mixed lubrication in which there is solid to solid contact. Transition between these regimes is very important and has been widely studied for soft materials. Contributing factors of the lubrication regime transition such as hydrophilicity of contact pairs, types of lubricants, surface pattern, stiffness of substrates, sliding velocity and normal loading have been considered. Generally, the lubrication regime transition is believed to happen when the thickness of the lubricant layer is comparable with the amplitude of surface roughness. In this work, we perform lubricated sliding experiments on smooth polydimethylsiloxane (PDMS) substrates under various load, sliding velocity, and lubricant conditions. We find that the transition in lubrication regime happens in various systems. In general, the system transitions from the EHL to mixed regime with lower velocity, higher load, larger Young’s modulus and less viscous lubricant. More significantly, combining simulation, direct visualization of flow by fluorescent particle tracking, and film thickness measurement, we demonstrate that for smooth surfaces, the system can transition from EHL to mixed regime even if the thickness of liquid layer is much larger than the height of asperities. For most cases, especially for high viscosity lubricants, the film thickness at the transition point is greater than 500 nm, much larger than the surface square root mean roughness of around 3 nm. That the lubrication regime transition is triggered while the liquid layer is still thick implies that surface roughness is not the determining factor for the transition in soft solids. We therefore argue that the conventional lubrication regime transition models need to be re-considered for soft solids. To explore the mechanism of the lubrication regime transition, we apply fluorescein O,O′-diacrylate to dye the PDMS solid surface. By spin-coating dyed PDMS precursor onto a pre-cured PDMS substrate, we obtain a double-deck PDMS substrate with a dyed thin layer (~40 μm) on top and without significant change of bulk mechanical properties. Exploiting the two-layer structured PDMS dyed by fluorescein to visualize the sliding experiment, we find that in the transition region and mixed regime, the soft-solid surface is subject to mechanical instabilities. Surface creases can form at the leading front while wave-like surface instabilities with contact area shape change can be observed at the trailing edge. Moreover, if the system starts to move from a stationary state and is slowly accelerated to a high speed, the system goes through a lubrication regime transition from the mixed to the EHL regime. On the contrary, starting at a steady state in EHL regime, there is neither surface wrinkling nor elastic instability present on the PDMS surface even if the system is gradually decelerated to zero speed. The hysteresis phenomenon suggests the existence of an energy barrier between the mixed and EHL regimes. Multiple parameters are coupled in lubrication regime transition and this work opens the way to develop physical models for it, for which we offer some hypotheses.