Study on Buckling of Stiff Thin Films on Soft Substrates as Functional Materials

abstract: In engineering, buckling is mechanical instability of walls or columns under compression and usually is a problem that engineers try to prevent. In everyday life buckles (wrinkles) on different substrates are ubiquitous -- from human skin to a rotten apple they are a commonly observed phenomenon. It seems that buckles with macroscopic wavelengths are not technologically useful; over the past decade or so, however, thanks to the widespread availability of soft polymers and silicone materials micro-buckles with wavelengths in submicron to micron scale have received increasing attention because it is useful for generating well-ordered periodic microstructures spontaneously without conventional lithographic techniques. This thesis investigates the buckling behavior of thin stiff films on soft polymeric substrates and explores a variety of applications, ranging from optical gratings, optical masks, energy harvest to energy storage. A laser scanning technique is proposed to detect micro-strain induced by thermomechanical loads and a periodic buckling microstructure is employed as a diffraction grating with broad wavelength tunability, which is spontaneously generated from a metallic thin film on polymer substrates. A mechanical strategy is also presented for quantitatively buckling nanoribbons of piezoelectric material on polymer substrates involving the combined use of lithographically patterning surface adhesion sites and transfer printing technique. The precisely engineered buckling configurations provide a route to energy harvesters with extremely high levels of stretchability. This stiff-thin-film/polymer hybrid structure is further employed into electrochemical field to circumvent the electrochemically-driven stress issue in silicon-anode-based lithium ion batteries. It shows that the initial flat silicon-nanoribbon-anode on a polymer substrate tends to buckle to mitigate the lithiation-induced stress so as to avoid the pulverization of silicon anode. Spontaneously generated submicron buckles of film/polymer are also used as an optical mask to produce submicron periodic patterns with large filling ratio in contrast to generating only ~100 nm edge submicron patterns in conventional near-field soft contact photolithography. This thesis aims to deepen understanding of buckling behavior of thin films on compliant substrates and, in turn, to harness the fundamental properties of such instability for diverse applications.Dissertation/ThesisPh.D. Mechanical Engineering 201

Flexible piezoelectric nano-composite films for kinetic energy harvesting from textiles

This paper details the enhancements in the dielectric and piezoelectric properties of a low-temperature screen-printable piezoelectric nano-composite film on flexible plastic and textile substrates. These enhancements involved adding silver nano particles to the nano-composite material and using an additional cold isostatic pressing (CIP) post-processing procedure. These developments have resulted in a 18% increase in the free-standing piezoelectric charge coefficient d33 to a value of 98 pC/N. The increase in the dielectric constant of the piezoelectric film has, however, resulted in a decrease in the peak output voltage of the composite film. The potential for this material to be used to harvest mechanical energy from a variety of textiles under compressive and bending forces has been evaluated theoretically and experimentally. The maximum energy density of the enhanced piezoelectric material under 800 N compressive force was found to be 34 J/m3 on a Kermel textile. The maximum energy density of the enhanced piezoelectric material under bending was found to be 14.3 J/m3 on a cotton textile. These results agree very favourably with the theoretical predictions. For a 10x10 cm piezoelectric element 100 µm thick this equates to 38 μJ and 14.3 μJ of energy generated per mechanical action respectively which is a potentially useful amount of energy

PATTERNING AND MECHANICAL ANALYSIS OF FIBER-BASED MATERIALS

The ability to define and control the topography of a surface has been studied extensively due to its importance in a wide variety of applications. The control of a non-planar topography would be very valuable since a number of structures that are pervasive in artificial applications (e.g. fibers, lenses) are curved interfaces. This potential of enabling applications that incorporate non-planar geometries was the motivation for this thesis. The first study of this thesis comprises the study of patterning the circumference of micrometer sized fibers. Specifically, a unique technique was described to pattern the fiber with a periodic array of colloids. The effect of immobilizing fibers on different substrates and the parameters that govern a successful transfer of the colloidal array onto 7 mm diameter fibers were studied. Finally, replication of inverse submicrometer patterns onto the diameter of the fiber is completed with mild removal of the colloidal template. The second component of the thesis is the patterning of fabric assemblies of fibers. Composites of soft elastomer resins and rigid fiber materials are explored for their complimentary properties. Specifically, the organization of the fiber structure was contrasted with other homogenous materials. These composites were shown to possesses rigid in-plane strength, yet remain flexible to bending deformation. Furthermore, the carbon fiber fabric composites demonstrate superior tensile strength and greater flexibility than common homogenous materials such as PET and crosslinked elastomers. Finally, the use of a liquid resin permits submicrometer patterns to form on the periphery of the fabric assembly. The final component of the thesis is the use of the patterned fabric assemblies for adhesive applications. Carbon fiber-elastomer composites were patterned with submicrometer shear adhesion. The effects of the pattern size and orientation on the shear adhesion were studied. By varying the velocity of the sample testing, adhesion was observed to change for different patterned samples. We highlight the aspects of the fabric composite and the patterning that permits the features to alter the adhesion. Finally, we suggest how these results could be designed to improve the shear adhesion of reversible adhesives

Functional surface microstructures inspired by nature – From adhesion and wetting principles to sustainable new devices

In the course of evolution nature has arrived at startling materials solutions to ensure survival. Investigations into biological surfaces, ranging from plants, insects and geckos to aquatic animals, have inspired the design of intricate surface patterns to create useful functionalities. This paper reviews the fundamental interaction mechanisms of such micropatterns with liquids, solids, and soft matter such as skin for control of wetting, self-cleaning, anti-fouling, adhesion, skin adherence, and sensing. Compared to conventional chemical strategies, the paradigm of micropatterning enables solutions with superior resource efficiency and sustainability. Associated applications range from water management and robotics to future health monitoring devices. We finally provide an overview of the relevant patterning methods as an appendix

Functional surface microstructures inspired by nature : From adhesion and wetting principles to sustainable new devices

In the course of evolution nature has arrived at startling materials solutions to ensure survival. Investigations into biological surfaces, ranging from plants, insects and geckos to aquatic animals, have inspired the design of intricate surface patterns to create useful functionalities. This paper reviews the fundamental interaction mechanisms of such micropatterns with liquids, solids, and soft matter such as skin for control of wetting, self-cleaning, anti-fouling, adhesion, skin adherence, and sensing. Compared to conventional chemical strategies, the paradigm of micropatterning enables solutions with superior resource efficiency and sustainability. Associated applications range from water management and robotics to future health monitoring devices. We finally provide an overview of the relevant patterning methods as an appendix

TiNi-based thin films for MEMS applications

In this paper, some critical issues and problems in the development of TiNi thin films were discussed, including preparation and characterization considerations, residual stress and adhesion, frequency improvement, fatigue and stability, as well as functionally graded or composite thin film design. Different types of MEMS applications were reviewed and the prospects for future advances in fabrication process and device development were discussed.Singapore-MIT Alliance (SMA

Composite Posts For Enhanced And Tunable Adhesion

Tunable adhesion is the ability for the same surface to have high adhesion under one set of conditions and low adhesion under another. It has a variety of applications, including transfer printing of micro- and nano-scale components, climbing and perching robots, and material handling in manufacturing. Approaches to tunable adhesion, including the work in this dissertation, often rely on van der Waals forces to achieve dry adhesion. Previous strategies for dry tunable adhesives have generally exploited complex fibrillar structures that are inspired by nature. The work in this dissertation investigates a different strategy for enhanced and tunable adhesion based on composite structures with simple geometries. This dissertation examines the use of composite posts, consisting of stiff insets surrounded by a compliant shell, as an approach for achieving enhanced and tunable adhesion. This composite structure has a high effective adhesion strength under normal loading and low adhesion when shear is applied. Experiments as well as finite element (FE) analysis are used to understand the mechanics of these posts under both types of loading. The adhesion of composite posts is affected by the stress distribution at the contacting surface. Homogeneous posts have concentrated stress near the edge, facilitating crack initiation, while the composite post can result in a redistribution of this stress towards the center, resulting in higher adhesion. The basic mechanics of these posts are demonstrated through experiments on mm-scale posts. The composite mm-scale composite posts have 3x higher adhesion than homogeneous posts under normal loading and shear displacement was shown to significantly decrease the effective adhesion strength. Micro-scale posts are studied and used in micro-transfer printing applications. These posts have an effective adhesion strength of 1.5 MPa, and the pull-off force of the composite post is 9x that of a homogeneous post. In both the mm-scale and micro-scale studies, the experimental results are supported by FE simulations. Arrays of micro-scale posts were fabricated and their adhesion behavior characterized. In an array, the contact of each individual post becomes less critical and can contact diverse surfaces. This work established the mechanics of composite posts for achieving enhanced and tunable adhesion

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