Soft lubrication: the elastohydrodynamics of non-conforming and conforming contacts

TL;DR

This paper investigates how elastic deformation in soft interfaces influences lubrication, revealing optimal conditions for maximizing normal forces across various geometries and material responses, with implications for soft contact mechanics.

Contribution

It introduces a unified analysis of elastohydrodynamic lubrication in soft interfaces across multiple geometries, including elastic and poroelastic responses, identifying optimal deformation conditions.

Findings

Elastic deformation couples forces and generates lift.

Optimal parameters maximize normal force based on softness ratio.

Different geometries exhibit similar generic behavior.

Abstract

We study the lubrication of fluid-immersed soft interfaces and show that elastic deformation couples tangential and normal forces and thus generates lift. We consider materials that deform easily, due to either geometry (e.g. a shell) or constitutive properties (e.g. a gel or a rubber), so that the effects of pressure and temperature on the fluid properties may be neglected. Four different system geometries are considered: a rigid cylinder moving parallel to a soft layer coating a rigid substrate; a soft cylinder moving parallel to a rigid substrate; a cylindrical shell moving parallel to a rigid substrate; and finally a cylindrical conforming journal bearing coated with a thin soft layer. In addition, for the particular case of a soft layer coating a rigid substrate we consider both elastic and poroelastic material responses. For all these cases we find the same generic behavior: there is an optimal combination of geometric and material parameters that maximizes the dimensionless normal force as a function of the softness parameter = hydrodynamic pressure/elastic stiffness = surface deflection/gap thickness which characterizes the fluid-induced deformation of the interface. The corresponding cases for a spherical slider are treated using scaling concepts.