CPOS Seminar: Controlling Lubricated Sliding Friction for Soft Solids
Speaker: Hao Dong, Postdoctoral Researcher, UC Santa Barbara, Megan Valentine and Chris Bates Groups
Lubricated contacts in soft materials are common in various engineering and natural systems. Three major regimes are elastohydrodynamic lubrication (EHL), in which solid surfaces are fully separated by a fluid film, mixed lubrication (ML), in which there is partial solid-to-solid contact and boundary regime, in which the contact region is dry. ML-boundary and EHL-ML regime transitions govern the maximum and minimum sliding friction achievable for a certain system and are 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. However, the insufficient understanding of transitions between these regimes impedes desired control of lubricated sliding friction for soft solids.
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 fingering instabilities at the trailing contact edge. We believe these instabilities are peculiar to soft solids and is accompanied by the transition from EHL to ML regime. Our results change the fundamental understanding of what governs the important EHL-ML transition in lubricated sliding of soft solids.
To further investigate the mechanisms controlling lubrication regime transitions, we explored the roles of wettability and adhesion played in the lubricated sliding for soft materials. We adopted the ultraviolet light-ozone (UV-Ozone) cleaner to change the wettability of glycerol on polydimethylsiloxane (PDMS) surface and adhesion between glass indenter and PDMS. By combining friction tests and visualizations, we demonstrate that the transition from ML to BL regime is dominated by the wettability of the lubricant as increasing wettability of glycerol can maintain a liquid layer more easily in the contact region. EHL-ML transition is related to a series of events with increasing normal load, which are thinning of lubricant layer, sudden contact between a glass indenter over a few hundreds of nanometers, and elastic instabilities. The morphology of elastic instabilities is determined by the competition between the wettability of the liquid and the adhesion of two solid surfaces. These results provide a comprehensive understanding of the frictional behavior of a soft material and elaborate the maximum and minimum friction achievable of a soft system.
