Surface Mechanics of Soft Matter: Lubrication, Adhesion and Surface Tension

Abstract

This thesis presents an effort to understand three topics of surface mechanics of soft matter, which are surface lubrication, adhesion, surface tension. For low modulus solids such as polymer or rubber, the mechanics of surface lubrication, adhesion and surface tension can be quite different from the conventional hard solid. For example, in lubricated sliding the compliant substrate’s deformation and viscoelasticity could alter the lubricant layer shape and thickness, which dramatically changes the hydrodynamic performance of the sliding objects. Also, for extremely soft solids, the surface adhesion and surface stress of the solid cannot be neglectable. The adhesion and surface stress could compete with the bulk’s response under external loadings or even dominates over that. Understanding the interplay between adhesion and surface stress is of great significance to a lot of physical settings and real-world applications.The first part of the thesis, we focus on surface lubrication in which an intervening liquid layer separates two solid surfaces. We consider a special regime of elasto-hydrodynamic lubrication where the liquid film is sufficiently thin so that a “Hertz-like” effective contact region forms. In this regime, much of the fluid is squeezed out of the contact region although enough is retained to keep the solid surfaces fully separated. The behavior of such soft lubricated contacts is controlled by a single dimensionless parameter that can be interpreted as a normalized sliding velocity. Solving this fundamental soft-lubrication problem poses significant computational difficulty for large , which is the limit relevant for soft solids We present a new solution of this soft lubrication problem focusing on the “Hertz” limit. We study how hydrodynamic pressure, film thickness and hydrodynamic friction vary with . In addition, we extend our theory by accounting for the substrate’s viscoelasticity. The key finding is that the lubricated sliding in a viscoelastic substrate can be well approximately by two limiting cases. The first of these is the dry sliding limit in which the liquid layer is absent – the cylinder and viscoelastic substrate are in direct contact. The second is the elastic lubrication limit in which the viscoelastic substrate is replaced by an elastic material. This is followed by developing a theory on how structured surface can be used to control surface lubrication. Our results provide insight into the mechanism of friction enhancement and can guide design of surface architectures to control friction during lubricated sliding. The second part of this dissertation aims to study the adhesion of structured surface. We focus on a specially fabricated structured surface called film-terminated ridge-channel surface structure. We have experimentally showed that rolling this structured surface against a flat rigid plate showed significant enhancement in rolling resistance. We developed a finite element method (FEM) to simulate the rolling process. Rolling can be viewed as a combination of opening and closing of two interface cracks. Our simulation captures qualitatively the key experimental observation and help to reveal the structured surface adhesion enhancement mechanism. Finally, we studied how surface stresses affect the mechanics of load transmission in soft solids. We focused on the transmission of a vertical line load on a hyperelastic substrate with surface elasticity. We studied how the conventional normal force balance break down in the large normal load regime and how this breakdown would be rescued by additionally considering the surface elasticity (surface stiffening).