Deformation and Adhesion of Soft Composite Systems for Bio-inspired Adhesives and Wrinkled Surface Fabrication

The study of soft material deformation and adhesion has broad applicability to industries ranging from automobile tires to medical prosthetics and implants. When a mechanical load is imposed on a soft material system, a variety of issues can arise, including non-linear deformations at interfaces between soft and rigid components. The work presented in this dissertation embraces the occurrence of these non-linear deformations, leading to the design of functional systems that incorporate a soft elastomer layer with application to bio-inspired adhesives and wrinkled surface fabrication. Understanding the deformation of a soft elastomer layer and how the system loading and geometry influence non-linear mechanical transitions, including interfacial failure and surface buckling, are crucial for predicting the performance of the mechanical system. This dissertation focuses on three soft composite systems of particular interest: (1) a multi-component, multiple adhesive contact surface device that allows for control of reversible adhesive force with geometric arrangement, (2) a confined isolated shear contact and an elastomeric coating, where the deformation and adhesion scale with the degree of confinement, and (3) a thin film lamination technique involving a soft substrate, where surface wrinkles are created and tuned in a continuous manner by controlling interfacial strains via applied contact load and substrate curvature. We first study the deformation and adhesion of a multi-component fabric-elastomer system with multiple adhesive contacts, or digits . We conduct lap adhesion experiments in a model three digit system, finding that increasing angular spacing between adhesive digits increases system compliance and attenuates adhesive force capacity. To describe these findings we develop several relationships between system loading, materials properties, and geometry. We develop an equation which describes the relationship of system compliance with individual digit compliance and angular spacing between adhesive digits that agrees well with experimental data. Additionally, we derive equations for adhesive force capacity in a multiple adhesive contact system that agree well with experimental data. These explicit equations not only relate angular spacing with force capacity, but include critical strain energy release rate, digit compliance, and contact area. The equations derived and verified in this study will lead to more complex adhesive device design, as well as provide a foundation for studying the biomechanics of animals that use adhesion for locomotion. Next, we examine the deformation and adhesion of a rigid punch contacting and shearing a thin elastic coating. Using experiment we find that increasing confinement leads to a decrease in compliance and an increase in adhesive force capacity. We develop an explicit, semi-empirical equation with the help of finite element analysis to describe the influence of confinement ratio on shear compliance. This derived equation agrees with our experimental data, with the exception of a few data points that deviate due to a pronounced normal force component. Additionally, we derive an equation for adhesive force capacity as a function of confinement, elastic coating modulus, and critical strain energy release rate. We find experimentally that an increase in adhesive force capacity was largely dictated by an increase in confinement, with some additional contributions attributed to dissipative processes confined to the adhesive crack tip. These equations will serve as a guide for decoupling the contributions of geometry and materials parameters to adhesive force in systems involving a thin elastic layer. Lastly, we develop a fabrication technique that transforms the existing manufacturing process of film lamination to create tunable wrinkled surfaces in a thin film/soft elastomer composite. We conduct experiments to find that the process parameters of applied contact load and roller curvature can be used to control wrinkle aspect ratio. Our experimental results convey that increasing applied contact load and decreasing roller radius lead to an increase in wrinkle amplitude. Using both experimental results and finite element analysis, we develop a relation between wrinkle aspect ratio and the process parameters of applied contact load and roller curvature. This explicit equation allows us to predict the change in wrinkle amplitude for a given materials system as process parameters are tuned using our modified film lamination technique. Wrinkled surface technology has been envisioned in many applications ranging from optoelectronics to enhanced adhesives. The technique presented here to tune wrinkle size in a continuous process can lead to the large scale manufacturing of these previously proposed wrinkling technologies