Abstract
Classical lubrication theory is unable to explain a variety of phenomena and experimental observations involving soft viscoelastic materials, which are ubiquitous and increasingly used in e.g. engineering and biomedical applications. These include unexpected ruptures of the lubricating film and a friction-speed dependence, which cannot be elucidated by means of conventional models, based on time-independent stress-strain constitutive laws for the lubricated solids. A new modeling framework, corroborated through experimental measurements enabled via an interferometric technique, is proposed to address these issues: Solid/fluid interactions are captured thanks to a coupling strategy that makes it possible to study the effect that solid viscoelasticity has on fluid film lubrication. It is shown that a newly defined visco-elasto-hydrodynamic lubrication (VEHL) regime can be experienced depending on the degree of coupling between the fluid flow and the solid hysteretic response. Pressure distributions show a marked asymmetry with a peak at the flow inlet, and correspondingly, the film thickness reveals a pronounced shrinkage at the flow outlet; friction is heavily influenced by the viscoelastic hysteresis which is experienced in addition to the viscous losses. These features show significant differences with respect to the classical elasto-hydrodynamic lubrication (EHL) regime response that would be predicted when solid viscoelasticity is neglected. A simple yet powerful criterion to assess the importance of viscoelastic solid contributions to soft matter lubrication is finally proposed.
Highlights
Soft matter mechanics has recently become the focal point of much research in engineering and material science
We have shown that when dealing with the lubrication of soft solids that exhibit a viscoelastic rheology, the role of viscoelasticity has to be carefully accounted for since it can be responsible for phenomena that cannot be predicted by classical lubrication models
It has been shown that the rheology of the solid can have profound effects on both the fluid film thickness and pressure distribution, in turn affecting the capacity to predict the behavior of soft systems under lubricated conditions
Summary
Soft matter mechanics has recently become the focal point of much research in engineering and material science. Additional surprising experimental evidence linked to the interplay between solids and fluids in soft contact problems is provided in ref 24 where, in the presence of strongly viscoelastic materials, the rupture of the fluid film is shown to occur at the flow inlet of the lubricated interface This suggests that a point of minimum for the film thickness occurs there: Such a result is very hard to explain in the absence of time-dependent deformations, and is unexpected in classical lubrication, where, because of the flow conservation, a minimum is usually predicted close to the flow outlet. The fundamental understanding gained in introducing the effect of solid viscoelasticity in the solution of lubrication problems will play an essential role in overcoming some of the limitations of classical lubrication theory and, in providing a new tool to investigate the mechanisms governing some of the aforementioned phenomena
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