Graduate Thesis Or Dissertation
Rolling Contact Mechanics of Soft Elastomers on Engineered Surfaces Public Deposited
This report details the experimental work done to characterize the rolling contact mechanics of engineered surfaces against highly deformable, tissue-like elastomers. This work is motivated primarily by the design of medical devices, including robotic capsule endoscopes (RCEs), endoscopy balloons, stents, catheters, and bandages. Each of these devices presents different challenges regarding different tissue interactions and optimization goals, but all rely to some extent on carefully tuned adhesive responses. A better understanding of the adhesive response of these surfaces could lead to a better models and a subsequent narrowing of the design field regarding microstructured surfaces.
Chapter 1 of this work is focused on the historical development of the contact theories that will serve as the theoretical framework on which the remainder of the research is built. Beginning with Hertz' pioneering work and continuing through modern theories incorporating adhesion, this chapter is meant to provide a brief theoretical introduction relevant to the remaining work.
Chapter 2 will focus on biological adaptations for locomotion. Driven by the basic urges to find shelter, food, and mates, many species have co-evolved, through millenia of adaptive pressure, highly advanced locomotive structures. This chapter will introduce several species and will discuss the mechanisms behind their adaptations, as well as some attempts to mimic them with engineered surfaces.
Chapter 3 presents the first aim of this work: the design, construction, and validation of a tribometric device capable of characterizing the rolling contact of soft elastomers. A device was developed capable of measuring the normal and tractive forces and contact area of a cylindrical indenter rolling freely against a fixed substrate with fixed indentation or fixed normal force and controlled translational velocity. The device was validated using a rigid acrylic cylinder and rate-independent (polydimethylsiloxane (PDMS)) substrate, the results of which could be compared against classical contact theories. A second experimental setup, incorporating a thin (3 mm) shell of highly deformable and viscoelastic elastomer (polyvinyl chloride (PVC)) fixed to a rigid cylindrical core rolling on a flat PDMS substrate, highlights the novelty of this approach, in that the response varies greatly from analytical models due to high deformation, finite thickness corrections, and high viscoelasticity.
Chapter 4 presents the second aim of this work: the characterization of the rolling contact between elastomeric surfaces and a highly-deformable tissue-mimicking substrate. Using our tribometer, we conducted a series of rolling contact experiments involving a flat PDMS substrate and an indenter composed of a thin (3 mm) shell of PVC bonded to a rigid core. Using a range of normal forces and translational velocities, we observed tractive force dependencies on both velocity (due to the rate-dependent nature of interface energy) and normal force (due to friction caused by partial slippage of the interface). These results were compared to a finite element simulation using a Cohesive Zone Model (CZM) to simulate interfacial adhesion and a post-processing step relating normal and tractive surface tractions using Amonton's law for friction. The flat PDMS substrate was then replaced with a micropillared substrate and subjected to the same battery of tests. Through this, it was determined that the micropillars had only a modest effect on rolling contact, a finding we attribute to the high deformability of PVC leading to extensive backing layer contact between the micropillars.
Chapter 5 presents the final aim of this work: the use of readily-available manufacturing techniques for rapid prototyping of pillared surfaces in order to explore the pillar design space. The abundance of micro-manufacturing techniques, from micro-machining to lithography and laser etching molds, has created an effectively infinite design space regarding pillar shape, orientation, and aspect ratio. Because analytical solutions can rarely be determined for arbitraty pillar geometries, designers have two options for navigating the design space: simulation or developing and testing prototypes of varying pillar geometries. As many pillar shapes require the development of 3D simulations, necessitating either computationally-intensive whole-body models or the implementation of diffiult and potentially non-physical boundary conditions on representative elements, prototyping offers a possible alternative to test various geometries quickly. Because many micro-manufacturing processes are time consuming or require access to highly specialized equipment, we set out to use two readily-available techniques, 3D printing and laser printing, to develop sub-millimeter mesopillars and studied the effect of their geometry on the contact mechanics with both rigid and soft indenters.
Chapters 6 and 7 present a list of the conclusions of each aspect of this work, as well as concluding discussion.
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