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

Harness Surface Morphologies for Optical Applications

Industrial and technology in advanced promote the development of optical system towards accessible to be high-performance, robust and easy to be tunable. Nowadays, with the development of precision manufacturing, the surface morphologies can be modified in micro scale even nano-scale which are in conformity with the requirement of the optical systems or devices. In this dissertation, several surficial morphology-based compatible and tunable optical systems have been put forward with simple dynamic actuation with the potential applications in smart window, optical diffuser and optical grating.First of all, an optimal etching time has been explored in a wrinkling-cracking based tunable smart window application. With the increased UVO etching time, the thickness of top oxidized layer enhanced a lot. As the uniaxial strain applied, the surface morphology is variable. By the comparison of different UVO etched situations, the optimal UVO treating time for smart window sample with largest tunable transmittance has be raised. Moreover, the tunable transmittance range versus UVO treating time has been developed as a consequence.Additional discover is that the micro-scaled sinusoidal surface has a scattering effect for transmitted light source when we went through the first project. Therefore, a high-performance optical diffuser with novel screwthread-like wrinkling morphology is fabricated simply and cheaply based on two arrays of micro-scaled wrinkling patterns perpendicular to each other. Such optical diffuser can scatter the light uniformly in a rectangular region, with a high haze performance. Furthermore, two-sided surface modified optical diffuser is demonstrated to enhance the optical performance with a much larger scattering region. The scattering region can also be tuned from rectangular shape to one-dimensional line via simple mechanical actuation as well.Last but not the least, a series of hierarchical surface architectures are fabricated by combining the micro-scaled wrinkling pattern with sub-micro-scaled wavy structure. Such morphology cannot only diffuse the transmitting light but also perform the tunable diffraction phenomenon for potential optical grating application. In additional, a high-performance optical diffuser (scattering angle ~60°) is exhibited via two optical grating samples stacking together

Super-Flexible Sensors and Advanced 3D Morphing Actuators based on Elastic Instability

Super-flexible devices based on soft materials have the potential to sustain large mechanical deformations, enabling advanced applications such as flexible electronics, soft robots, artificial skin, and biomedical transducers. Subject to a large compression, materials may undergo different types of elastic instabilities such as wrinkles, creases, and folds. Despite recent growing interests in turning this usually unwanted phenomenon into useful engineering applications (e.g. tactile sensing), this topic remains relatively under-researched. Therefore, this thesis focuses on developing the control mechanisms of elastic instabilities, and their applications in sensing and actuation systems. Elastic instabilities induced strain-gated logic sensing technology is developed by research into micro structured metal-elastomer tri-layer system. The test structures are designed to study the deformation behaviour and to exploit the large strain sensing mechanism. The stepwise electrical signals are achieved (from ~1010 to ~120 Ω at first switching stage and then to ~50 Ω at second switching stage) that survived much higher than usual compressive strains of up to 60%. On the other hand, elastic instabilities induced topo-optical sensing strategy is created by patterning microstructure arrays within the tri-layer system. Two unwanted phenomena (creases/folds and oxygen quenching effect) are turned into a responsive and programmable 'fold to glitter' function through micro engineering, which can light up areas of an object or material by creating microscopic creases/folds within its surface. The signal-Noise-Ratio (SNR) contrast in optical pattern generation is improved by 6 folds due to the oxygen quenching effect. The numerical analysis by ABAQUS provides the fundamental theory on the mechanism of generating targeted folding through simulating the in-plane and out-of-plane strain energy localization. Different luminescent optical patterns are demonstrated under in-plane uniaxial or equi-biaxial compression. Apart from the surface deformation, the bulk deformation of heterogeneous layered structures of soft functional hydrogel is also developed to generate the controllable and reconfigurable 3D morphing device. The initial configurations with various shapes (“S”, “W” and “C”) are demonstrated due to the swelling ratio mismatch. The developed sensing and actuation technologies provide opportunities for future applications in flexible electronics, tuneable optics, soft robotics and bio-medical systems

Design and Synthesis of Polymer Brush Surfaces with Complex Molecular Architectures and Morphologies

The combination of surface-initiated polymerization (SIP) and postpolymerization modification (PPM) is a powerful technique for the fabrication of functional soft surfaces. Better understanding the influence of the aforementioned factors on the PPM effectiveness is valuable for fulfilling the potential of the PPM approach for the fabrication of functional soft surfaces. Specifically, by carefully balancing modification reactivity and limitation of mass transport, polymer brush with composition heterogeneity and gradient along the normal direction of the surface otherwise unattainable by methods of direct polymerization can be fabricated via the PPM approach which opens doors to new routes to polymer brush with complex functionality and morphology (i.e. buckling). This dissertation is focused on designing and synthesizing polymer brush surfaces with complex molecular architectures and morphologies with specific emphasis on improving the understanding of PPM effectiveness and the distribution of post modification moieties on grafted polymer chains. In the first study, microwave-assisted surface-initiated polymerization (μW-SIP) was developed and employed to demonstrate the synthesis of polymer brushes on silicon and quartz substrates. The μW-SIP approach shows significant enhancements in polymer brush thickness at reduced reaction times and monomer concentration. In the second study, the postpolymerization modification of a poly(2-isocyanatoethyl methacrylate) (pNCOMA) brush surfaces with deuterated thiols of different sizes was studied and the depth profiles of the distribution of the modified brush were drawn using neutron reflectometry analysis. By applying a sequential PPM strategy, polymer brush with tapered block copolymer architectures was synthesized. In the third study, a poly(styrene-alt-maleic anhydride) (pSMA) copolymer brush was synthesized in an effort to fabricate pendent polyfunctional thiols polymer brush for further thiol-ene modifications. Furthermore, the pSMA brush itself was found to be a stable and versatile platform for amine modification. In the last study, a straightforward PPM approach, utilizing the knowledge gained in previous studies, to engineer ultrathin polymer brush surfaces with tunable wrinkled morphologies was demonstrated by creating a modulus mismatch between the top layer and bottom of the polymer brush via selectively crosslinking of the outer layer of pSMA brushes by balancing the rate of PPM and reactive molecule diffusion

Mechanics study and application of micro-engineered smart surface

Naturally existing functional surfaces with micro-structure arose competing interests due to their potential application in engineering filed such as wetting control, optical control, micro-fluidic, tissue scaffolds, marine engineering, oil field, etc al. A patterned surface with stimuli responsive properties attracts considerable interest for its importance in advanced engineering, partly due to its reversibility, easy design and control, good compatibility and responsive behaviour to external stimuli. In this work, we have investigated various surface instabilities that enable a convenient strategy of micro-engineered structure impart reversible patterned feature to an elastic surface. We focus on the classic bi-layer system contains a stiff layer on a soft substrate that produces parallel harmonic wrinkles at uniaxial compression and ultimately develop into deep creases and fold. By introducing the microscale planar Bravais lattice holes, we guided these instabilities into various patterns to achieve an anisotropic manipulation of single liquid droplet by initialize localized surface morphologies. The Finite Element Analysis provided the fundamental theory on the surface instabilities evolution and development. The finding demonstrates considerable control over the threshold of a surface elastic instability and bi-axial switching of droplet shape that relevant to many novel applications including wearable electronic devices, bio-medical systems, micro-fluidics and optical devices

PROGRAMMABLE WRINKLE PATTERNING ON MICROPARTICLES

학위논문 (박사)-- 서울대학교 대학원 : 전기·컴퓨터공학부, 2017. 2. 권성훈.Wrinkles can be defined as sinusoidal topography with ridge and valley structures, and they commonly exist in various organisms like human skins. Many scientists have studied to understand the fundamental principles of the natural wrinkling phenomenon in various material systems. Moreover, engineers have also paid attention to these spontaneously generated wrinkle patterns found in nature, even with complex structures in micro/nano scale, because it is hard to fabricate them with conventional lithography technologies. Therefore, various bottom-up patterning methods based on the mechanical instability have been developed as alternatives to top-down patterning approaches. To utilize wrinkling as patterning purposes, appropriate control mechanisms are required in the fabrication processes due to the random nature of it. Although numerous patterning technologies with controllability have been developed by pre-patterning the substrates or films, engineering the stress states, and others, it was elusive to achieve both the flexible pattern design (e.g., precise control of in individual ridge to any geometry) and the high-throughput production of heterogeneously patterned structures, simultaneously. In this dissertation, a new wrinkle patterning platform based on the microparticle substrate is presented, which is able to realize them and thus to extend utility of the wrinkle patterns. For this purpose, polymeric microparticles coated with silica film were utilized for the unit structure, because the parameters to program the resulting wrinkle patterns (e.g., elastic modulus, film thickness, and geometry of the microparticle) could be dynamically tuned in each microparticle during the fabrication processes. By shrinking the homogeneously or heterogeneously programmed silica-coated microparticles, a few thousands of wrinkled microstructures could be constructed in a single fabrication process. First, the random wrinkle patterns were generated on plane, disk-type microparticles, and they were utilized as unclonable codes analogous to human fingerprint for anti-counterfeiting purposes. Using conventional fingerprint identification algorithms, the authentication system of these artificial fingerprints was demonstrated, and the uniqueness, individuality, and durability of them were verified. This application was the first functionalization of random wrinkle patterns. Next, several control techniques were applied to tune the degree of the pattern randomness or the directionality of ridges in low-level. Further, an elaborate wrinkle control mechanism was developed by pre-patterning the ridge guiding structures consisting of small grooves on the surface of the polymeric microparticles. This slightly modified patterning method allowed the self-organization of microstructures with precise control of the individual ridge orientation over the randomness. Not only the anisotropic, orthogonal, and hexagonal ridge patterns, but also the letter-shaped ridge patterns were realized. Although this dissertation focused on the polymeric microparticles covered by silica, the presented programmable wrinkle patterning concept could be also applied to other materials or substrates systems. It is expected that this patterning technology and the resulting structures could be utilized for various purposes other than the presented applications, including those for useful experimental platforms in studying mechanical instability.Chapter 1 Introduction 1 1.1 Principle of Wrinkling 2 1.2 Wrinkle Patterning Methods 4 1.2.1 Planar Substrates 5 1.2.2 Curved Substrates 12 1.3 Applications 15 1.4 Main Concept: Wrinkle Patterning on Microparticles 17 Chapter 2 Patterning Random Wrinkles 20 2.1 Microparticle Synthesis 21 2.2 Silica-Coating 24 2.3 Wrinkling Process 28 Chapter 3 Application: Artificial Microfingerprints 33 3.1 Anti-Counterfeiting Technologies 34 3.1.1 Taggant Systems 35 3.1.2 Physical Unclonable Function (PUF) 38 3.2 Concept of Artificial Fingerprints 42 3.3 Security Level Control 47 3.4 Individuality Analysis 57 3.5 Demonstration 66 Chapter 4 Patterning Controlled Wrinkles 78 4.1 Rough Control Methods 79 4.2 Sophisticated Control Method: Guided Wrinkling 83 4.3 Analysis of Orthogonal Ridge Patterns 87 4.4 Programming Ridge Directionality 93 Chapter 5 Conclusion 98 Bibliography 101 Abstract in Korean 109Docto

Instability and buckling analysis of stretchable silicon system

Ph.DDOCTOR OF PHILOSOPH