Browsing by Author "Zhang, Bo"
Now showing 1 - 6 of 6
Results Per Page
Sort Options
Item Adaptive model selection in linear mixed models.(2009-08) Zhang, BoLinear mixed models are commonly used models in the analysis of correlated data, in which the observed data are grouped according to one or more clustering factors. The selection of covariates, the variance structure and the correlation structure is crucial to the accuracy of both estimation and prediction in linear mixed models. Information criteria such as Akaike's information criterion, Bayesian information criterion, and the risk inflation criterion are mostly applied to select linear mixed models. Most information criteria penalize an increase in the size of a model through a fixed penalization parameter. In this dissertation, we firstly derive the generalized degrees of freedom for linear mixed models. A resampling technique, data perturbation, is employed to estimate the generalized degrees of freedom of linear mixed models. Further, based upon the generalized degrees of freedom of linear mixed models, we develop an adaptive model selection procedure with a data-adaptive model complexity penalty for selecting linear mixed models. The asymptotic optimality of the adaptive model selection procedure in linear mixed models is shown over a class of information criteria. The performance of the adaptive model selection procedure in linear mixed models is studied by numerical simulations. Simulation results show that the adaptive model selection procedure outperforms information criteria such as Akaike's information criterion and Bayesian information criterion in selecting covariates, the variance structure and the correlation structure in linear mixed models. Finally, an application to diabetic retinopathy is examined.Item The Colloidal Glass Transition Under Confinement(2018-05) Zhang, BoUnderstanding the nature of the glass transition is one of the most challenging problems in condensed matter physics. Although ubiquitous and technically important, glasses still elude a universally accepted theoretical description. Here, we use colloidal particles as hard-sphere models and experimentally study particle dynamics of colloidal suspensions under different confinements near the glass transition. In three dimension (3D), we design a colloidal system, where particles are confined inside spherical cavities with an amorphous layer of particles pinned at the boundary. Using this novel system, we capture the amorphous-order particle clusters proposed in the framework of the random first-order transition (RFOT) theory and demonstrate the development of a static correlation near the glass transition. Moreover, by investigating the dynamics of spherically confined samples, we reveal a profound influence of the static correlation on the relaxation of colloidal liquids. In analogy to glass-forming liquids with randomly pinned particles, we propose a simple relation for the change of configurational entropy of confined colloidal liquids, which quantitatively explains our experimental findings and illustrates a divergent static length scale during the colloidal glass transition. In two dimension (2D), we prepare quasi-2D confined colloidal liquids with optical tweezers. We confirm the existence of a divergent static length in quasi-2D liquids. We further use the confinement as a tool to probe the Mermin-Wagner long-wavelength fluctuations. We find that the fluctuations have a logarithmic dependence on the system size in quasi-2D when the system approaches to the glass transition. Ellipsoidal and rodlike particles are also used to directly compare the translational and rotational dy- namics. We show a decoupling between translational and rotational dynamics and the decoupling is not affected by the confinement. What’s more, constant values of critical volume fractions are observed regardless of types of particle aspect ratios, measurement methods, fitting functions, and values of structural factors. Lastly, we have also conduct an experimental study on the 1D dynamic self-assembly of charged colloidal particles in microfluidic flows. Using high-speed confocal microscopy, we systematically investigate the influence of flow rates, electrostatics and particle poly- dispersity on the observed string structures. By studying the detailed dynamics of stable flow-driven particle pairs, we quantitatively characterize interparticle interac- tions. Based on the results, we construct a simple model that explains the intriguing non-equilibrium self-assembly process. Our study shows that the colloidal strings arise from a delicate balance between attractive hydrodynamic coupling and repulsive electro- static interaction between particles. Finally, we demonstrate that, with the assistance of transverse electric fields, a similar mechanism also leads to the formation of 2D colloidal walls. Our study provides key experimental evidences to support the development of RFOT theory to better understand the glass transition in both 3D and 2D. The fundamental differences of particle dynamics between 3D and 2D are also studied. In addition to providing experimental results for assessing general glass transition theories and par- ticle self-assembly, our studies also provide new insights into the dynamics of confined colloidal liquids and may shed light on the behavior of atomic/molecular liquids under nano-confinements.Item FMM-Yukawa: An Adaptive Fast Multipole Method for Screened Coulomb Interactions(University of Minnesota. Institute for Mathematics and Its Applications, 2009-06) Huang, Jingfang; Jia, Jun; Zhang, BoItem Nanomaterial based biosensor powered by solar cell(2014-11) Zhang, BoBiosensors development using nanomaterials provides promising approaches to offer high performance of sensors in resolution and detection limits. Renewable energy development is attracting interests as an alternative to other sources of energy such as fossil fuels and nuclear energy. Therefore, if biosensor systems can be integrated with nanomaterials and photovoltaics, this biosensor platform can detect various biotargets and support itself by solar energy harvesting with better performance and lower cost. It can reduce cost and pollution from battery or electrical power in a green strategy. It will bridge technological advances in multidiscipline to address fundamental emerging issues in applied science and engineering.A flexible biosensor based on "bottom up" layer-by-layer self-assembled graphene is investigated. This graphene biosensor can detect different concentrations of biotargets (e.g., glucose, vascular endothelial growth factor, acetylcholine) as a detection platform by measuring the conductance change of the self-assembled graphene. After optimizing of the biosensor structure and dimensions, the suspended graphene sensors are capable of detecting very low concentrations of prostate specific antigen down to 0.4 fg/ml (4×10-16 g/ml), showing a great advantage over conventional testing methods with only 0.4 ng/ml (4×10-10 g/ml) detection limit.To fabricate solar cell power source, a simple, rapid and robust approach to controllably create nanostructures on a shrink polymer substrate photocathode, demonstrating a 34.1% enhancement of energy conversion efficiency for dye-sensitized solar cells (DSSCs). Glass photoanodes are also replaced with patterned shrink polymer substrates to form the flexible all-polymer DSSCs. A low-cost shrink lithography technique with 21 nm resolution to support the nanostructure fabrication of biosensor and solar cell in a low-cost way. By using this novel lithography technique, a biosensor based on suspended graphene nanoribbon with only 50 nm width was successfully fabricated. This shrinkage strategy was extended to the fabrication of tunable micro/nano structures with very low cost. These shrink induced micro/nano structures are tunable and controllable on the material properties (e.g. conductance, surface wetting ability, surface morphology), which offering more controllable and flexible applications to biochemical detection and energy harvesting with simple and low cost strategy.Item Structure and Thermodynamics of Neutral and Charged Block Copolymer-Based Materials(2022-08) Zhang, BoNext-generation materials are often required to exhibit two (or more) orthogonal properties simultaneously. One example is polymer electrolytes, as both facile ion transport and mechanical robustness are desired. However, these orthogonal properties are hard to achieve in single-component systems, because ion transport usually requires high chain mobility while high chain rigidity or low chain mobility is desired for mechanical stability. One way to overcome this challenge is to develop co-continuous nanostructured materials, such that one domain provides ion transport while the other imparts mechanical robustness. A promising predictable and tunable co-continuous structure is the bicontinuous microemulsion from ternary blends of an AB diblock copolymer and the corresponding A and B homopolymers. However, the structure and thermodynamics of such ternary mixtures are not fully elucidated, even in the limit of neutral ternary blends. Moreover, little is known about ion-containing ternary blends. Therefore, the focus of this thesis work is to understand the fundamental phase behavior of these systems and to ultimately provide insight into the rational design of functional materials. In Chapter 2, we investigate the phase behavior of neutral ternary blends comprising a linear diblock copolymer and the corresponding homopolymers. The impacts of block copolymer compositional asymmetry on ordered, disordered, and macrophase-separated regions of the ternary phase prism are discussed. In Chapter 3, we expand the research to ternary mixtures involving a bottlebrush diblock copolymer and the corresponding linear homopolymers. The overall phase behavior closely resembles that of linear ternary mixtures, except for an unconventional spatial distribution of the homopolymers. Chapters 4 and 5 focus on the self-assembly of charged diblock copolymers, serving as the starting point for the investigation of charged ternary blend phase behavior. Chapter 4 details the phase behavior of a series of symmetric charged diblock copolymers, where the effective interaction parameter was found to increase linearly with the increase in charge fraction. Chapter 5 extends the work to a different model system with a relatively nonpolar charged block. A tilted, “chimney”-like order-disorder transition boundary was observed. However, the composition windows of the ordered phases remain nearly unchanged. Overall, the findings from this thesis research provide valuable insight into the structure and thermodynamics of neutral and charged polymer mixtures, and will inform the rational design of nanostructured polymer electrolytes with tunable structure and properties.Item Supporting data for Core−Shell Gyroid in ABC Bottlebrush Block Terpolymers(2023-05-25) Cui, Shuquan; Zhang, Bo; Shen, Liyang; Bates, Frank S; Lodge, Timothy P; lodge@umn.edu; Lodge, Timothy P; University of Minnesota Department of ChemistryThese files contain primary data supporting all results reported in Cui et al. "Core−shell gyroid in ABC bottlebrush block terpolymers." A series of bottlebrush block polymers containing 24 PEP-PS diblock copolymers and 109 PEP-PS-PEO triblock terpolymers were synthesized by ring-opening metathesis polymerization (ROMP) of norbornene-functionalized poly(ethylene-alt-propylene) (PEP), poly(styrene) (PS), and poly(ethylene oxide) (PEO) macromonomers. The molecular weights of the three macromonomers were around 1 kg/mol. The relatively modest total backbone degrees of polymerization ranged from ca. 20 to 40. Morphologies of these bottlebrush block polymers were characterized by small-angle X-ray scattering (SAXS). The PEP-PS diblocks exhibited only cylindrical (HEX) and lamellar (LAM) morphologies; the desired network phases did not appear in these materials, consistent with previous experimental studies. However, adding variable-length bottlebrush PEO blocks to diblocks containing 30% to 50% PS led to a substantial core-shell double gyroid (GYR) phase window in the ternary phase portrait. Encouragingly, the GYR unit cell dimensions increased almost linearly with the backbone degree of polymerization, portending the ability to access larger network dimensions than previously obtained with linear block polymers. This finding demonstrates a periodic network phase in bottlebrush block polymers for the first time and highlights extraordinary opportunities associated with applying facile ROMP chemistry to multiblock bottlebrush polymers.