Ultra-Thin Polymer Films and Hierarchical Composites: Processing and Mechanical Properties

Properties and fabrications of ultra-thin polymer films and hierarchical composites are of great interest in packaging, electronics, separations, and other manufacturing fields. However, due to the inherently fragile nature of ultra-thin polymer films, measuring their properties has proven difficult. Additionally, variables controlling thin polymer patterns (e.g. substrate wetting property) and composites (weight percent of particulates in matrix) formation have not been fundamentally well understood. Within this spectrum, fundamental understanding of formation mechanisms of these patterns and composites are needed. Additionally, a new characterization technique is required to be able to measure the mechanical properties of fabricated composites and thin films. The pattern (Chapter 2) and composite (Chapter 3) formation presented in this thesis is based upon flexible blade flow-coating, an evaporative self-assembly method. The impact of substrate wetting, varying from being hydrophobic (water advancing contact angle 113°) to hydrophilic (water advancing contact angle 27°), on polymer pattern formation is examined here (Chapter 2). We observe a variety of polystyrene structures including dots, hyper-branched patterns, stripes and lines that can be deposited on substrates with a range of wetting properties. We propose the mechanism for these pattern formations as a balance between Marangoni instability and solute absorption. When adding quantum dot nanoparticles into the polymer (poly(methyl methacrylate) solution in the flow-coating process on hydrophilic substrates, we could obtain free-standing hierarchical nanocomposite films with alternating line and film structures (Chapter 3). The ability to guide assemblies of nanoparticles and polymers in designated areas in one step via flow-coating, provides new understanding of the flow competition of mixing two components which are both on the nanometer scale. Additionally, we introduce a method designated for ultra-thin film tensile testing (Chapter 4). We demonstrate the capability of this method by stretching two-dimensionally macroscopic, yet nanoscopically thin, polymer films on the surface of water. Through laser tracking of the force and displacement on the film, we characterize the full stress-strain response of brittle (polystyrene), ductile (polycarbonate), and rubbery (cross-linked polydimethylsiloxane) polymer thin films. In the brittle (polystyrene) films, we observe a precipitous decrease in mechanical properties (Young’s modulus, strain at failure, and nominal stress at failure) for film thicknesses approaching the size of an individual polymer chain (~ 25 nm) yielding insights into polymer chain entanglement theory. To verify our hypothesis in polymer chain entanglement theory for determining failure properties of thin polymer films, we further study the molecular weight effect (853, 490, 137 and 61.8 kg/mol) of polystyrene on failure properties (Chapter 5). We compare maximum tensile strain, maximum tensile stress, and modulus respectively as a function of molecular weight as well as film thickness. We support our hypothesis on polymer inter-chain entanglements theory in thin polymer films by this molecular weight study. This thesis provides direct measurements of failure properties of ultra-thin films. These findings have important implications for the design of materials used in wide range of applications, as well as for the pursuit of new fundamental understanding of polymer physics in confined states

Transitions in synchronization states of model cilia through basal-connection coupling

Despite evidence for a hydrodynamic origin of flagellar synchronization between different eukaryotic cells, recent experiments have shown that in single multi-flagellated organisms, coordination hinges instead on direct basal body connections. The mechanism by which these connections leads to coordination, however, is currently not understood. Here we focus on the model biflagellate {\it Chlamydomonas reinhardtii}, and propose a minimal model for the synchronization of its two flagella as a result of both hydrodynamic and direct mechanical coupling. A spectrum of different types of coordination can be selected, depending on small changes in the stiffness of intracellular couplings. These include prolonged in-phase and anti-phase synchronization, as well as a range of multistable states induced by spontaneous symmetry breaking of the system. Linking synchrony to intracellular stiffness could lead to the use of flagellar dynamics as a probe for the mechanical state of the cell.Comment: 14 pages, 9 figure

Crafting Concurrent Data Structures

Concurrent data structures lie at the heart of modern parallel programs. The design and implementation of concurrent data structures can be challenging due to the demand for good performance (low latency and high scalability) and strong progress guarantees. In this dissertation, we enrich the knowledge of concurrent data structure design by proposing new implementations, as well as general techniques to improve the performance of existing ones.The first part of the dissertation present an unordered linked list implementation that supports nonblocking insert, remove, and lookup operations. The algorithm is based on a novel ``enlist\u27\u27 technique that greatly simplifies the task of achieving wait-freedom. The value of our technique is also demonstrated in the creation of other wait-free data structures such as stacks and hash tables.The second data structure presented is a nonblocking hash table implementation which solves a long-standing design challenge by permitting the hash table to dynamically adjust its size in a nonblocking manner. Additionally, our hash table offers strong theoretical properties such as supporting unbounded memory. In our algorithm, we introduce a new ``freezable set\u27\u27 abstraction which allows us to achieve atomic migration of keys during a resize. The freezable set abstraction also enables highly efficient implementations which maximally exploit the processor cache locality. In experiments, we found our lock-free hash table performs consistently better than state-of-the-art implementations, such as the split-ordered list.The third data structure we present is a concurrent priority queue called the ``mound\u27\u27. Our implementations include nonblocking and lock-based variants. The mound employs randomization to reduce contention on concurrent insert operations, and decomposes a remove operation into smaller atomic operations so that multiple remove operations can execute in parallel within a pipeline. In experiments, we show that the mound can provide excellent latency at low thread counts.Lastly, we discuss how hardware transactional memory (HTM) can be used to accelerate existing nonblocking concurrent data structure implementations. We propose optimization techniques that can significantly improve the performance (1.5x to 3x speedups) of a variety of important concurrent data structures, such as binary search trees and hash tables. The optimizations also preserve the strong progress guarantees of the original implementations

Bearing fault diagnosis based on TEO and SVM

A fault method for bearing based on Teager energy operator (TEO) and support vector machine (SVM) is proposed in this paper. First, the total energy of the vibration signal of the bearing is estimated by the TEO technique, which has good time resolution for the instantaneous signal. Then, the Teager spectrums are obtained by applying fast Fourier transform (FFT) to the Teager energy signal. The feature frequencies of different fault modes, as well as the ratio of resonance frequency band energy to total energy in the Teager spectrum are extracted to form the feature vectors. Finally, these vectors are introduced into SVM to realize fault classification for the bearing. Experiments are conducted to verify the feasibility of the proposed method, the results show that the proposed method performs effectively to identify the failure mode of the bearing under variable conditions