Soft Matter Lubrication: Does Solid Viscoelasticity Matter?

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.

Laser Microtextured Surfaces for Friction Reduction: Does the Pattern Matter?

“…At the same time in the y-direction, there is no symmetry and the distance continuously increases from the top downwards. The positions of 180 µm diameter dimples have been identified by Equations (8) and (9). The geometric void ratio of the Hydrostep texture was 20% (see Figure 3f).…”

“…Furthermore, LST has been proven to be highly flexible, adaptable and ready for rapid reconfiguration. In order to tailor the friction properties to the specific applications, very complex textures can be generated on a large class of materials, including different metals [6,7] and rubber-based composites [8] which often exhibit non-linearly elastic properties [9][10][11]. For these reasons, LST is increasingly becoming a valid alternative to other more widespread processes, such as the ion beam, chemical attack techniques, etc.…”

Reciprocal theorem for the prediction of the normal force induced on a particle translating parallel to an elastic membrane

“…As an example, the reversibility of the Stokes equations implies that no lift force is exerted on a rigid symmetric object, such as a sphere or a circular cylinder, that translates parallel to a flat hard wall 24,25 . However, this reversibility can be broken by introducing nonlinear effects due to inertia [26][27][28] , viscoelasticity of the surrounding fluid [29][30][31][32] , or the elastic nature of either or both of the translating object and the interface. For instance, a capsule that is enclosed by an elastic membrane in a wall-bounded shear flow experiences a net non-inertial lateral migration in which the lift velocity increases with the shear rate and decreases with distance from the wall [33][34][35] .…”

Transient viscoelastic lubrication analyses of UHMWPE hip replacements

“…Moreover, through solving the line contact lubrication problem, Elsharkawy [11], Hooke et al [12], Scaraggi et al [13] and Pandey et al [14] all found that the viscoelastic effects of the bearing materials may considerably affect the lubrication performance of the system. Furthermore, using a point contact lubrication model, Putignano et al [15] confirmed that if the viscoelasticity of the bearing surfaces was considered, the pressure distributions, film thickness and friction of a lubrication system were significant different from the classical EHL regime response. However, the viscoelastic effects of the UHMWPE liner on the lubrication performance of soft artificial hip joints remain unclear due to the lack of an effective numerical process.…”