DEFORMATION OFCOMPLIANT COATINGS DUE TO LUBRICATION FORCES

Elastohydrodynamic;Surface forces measurement;Drainage;Adhesion;Colloids and Interface;Chemical & Biomolecular Engineering

Wang, YumoWeihs, Timothy P. ;

Johns Hopkins University

关键词: Elastohydrodynamic;    Surface forces measurement;    Drainage;    Adhesion;    Colloids and Interface;    Chemical & Biomolecular Engineering;   

Others  :  https://jscholarship.library.jhu.edu/bitstream/handle/1774.2/44678/WANG-DISSERTATION-2017.pdf?sequence=1&isAllowed=y

瑞士|英语

来源: JOHNS HOPKINS DSpace Repository

【 摘 要 】

Elastic deformation due to viscous forces play an important role in soft matter where materials such as gels, biological tissues, or elastomers can deform during motion in a fluid at relatively low velocities and viscosities.For example, elastic deformation will affect the collision and rheological response of soft colloidal particles, coalescence of bubbles/drops, and lubrication in joint cartilage.In particular, rigid materials with soft coatings (stratified materials) are ubiquitous in tribology, biology, or rheology, and elastic deformation of stratified materials due to lubrication forces can prevent contact formation between approaching surface and can alter the interpretation of dynamic surface forces measurements as those encountered with the atomic force microscope (AFM) or the surface forces apparatus (SFA). Due to its broad practical significance, a better understanding of the impact of deformation on hydrodynamic interactions is necessary, and harnessing the coupling between lubrication pressure and elasticity can provides materials design strategies for a number of applications such as adhesives, coatings, microsensors, and biomaterials. When studying elastohydrodynamics in soft matter, it is a challenge to measure simultaneously the hydrodynamic forces and the deformation, both necessary to understand how contact is reached and the coupling between deformation and viscous dissipation. For example, the absence of absolute measurement of spatiotemporal separation brings uncertainties to the role played by elasticity on hydrodynamic interactions, especially for the case of thin elastic coatings where our understanding is more limited. In addition, previous works have been mostly focusing on specific cases, for example surface deformation of an elastic half-space or elastic foundation, but the universal and general cases of compliant coating with finite film thickness are largely ignored. Specifically, treatment for cases where the stratified materials have a compliant coating with thickness comparable to the hydrodynamic radius, have largely been ignored. To the best of our knowledge, no experiment has investigated systematically the role of coating thickness on fluid drainage.To understand the dynamic of fluid drainage past a compliant stratified boundary, In this dissertation, a development in experimental techniques is first proposed to characterize the spatiotemporal deformation of a thick elastic film during the radial drainage of fluid from a narrowing gap. The results validate classic half-space theory, with a dimple formed and full contacted prevented during drainage. With thinner elastic film the stress becomes increasingly supported by the underlying rigid substrate and the dimple formation is suppressed, which allows the surfaces to reach full contact. The lag due to viscoelasticity on the surface profiles is highlighted. For a given fluid film thickness, elastic deformation leads to stronger hydrodynamic forces than for rigid surfaces, while for a given time, the hydrodynamic forces is weaker between compliant surface due to deformation. Subsequently, a theoretical framework combining lubrication theory and solid linear elasticity to describe the dynamic of fluid drainage past a compliant stratified boundary is proposed. The analysis presented covers the full range of coating thicknesses, from an elastic foundation to a half-space. The individual contributions of the coating thickness and material properties on the elastic deformation, hydrodynamic forces, and fluid film thickness are decoupled. For a rigid surface with compliant coating, a finite fluid film remains trapped between approaching surfaces, and that the thickness of this fluid film has a non-linear dependence with the coating thickness. The numerical results predict how the thickness and material properties of the elastic coating shifts the force curves as they would be measured using surface force measurements with a surface forces apparatus (SFA) or with colloidal probe microscopy with an atomic force microscope (AFM). The generalized contour maps that can be employed directly to estimate the elastic deformation present in any dynamic surface force measurements are shown and explained.The stratified model for elastohydrodynamic deformation directly predict the role of elastic coating thickness on dynamic normal drainage. More specifically, a rigid surface continuously driven to another surface with an elastic coating of intermediate thickness results in a unique ;;wimple” morphology. The systematic experimental studies on role of coating thickness are executed using surface forces apparatus for varying thicknesses, and direct validation of the numerical results is presented. An effects of surface slip due to roughness is characterized and investigated together with interpretation of surface stratification. The knowledge of layered drainage presented in this thesis can guide the study of complex soft matter, help us understand biologic multiphase system and facilitate new avenues to engineering innovations.