Browsing by Subject "Liquid crystals"
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Item A laser-induced surface flow visualization technique using liquid crystal thermography(Texas Tech University, 2002-12) Hunt, Emily McFatherObservation of flow field characteristics such as flow separation and reattachment are important in many industries. Current methods for flow visualization can be difficult to implement, expensive, and highly intrusive. The objective of this project is to develop an inexpensive, user-friendly, non-intrusive measurement technique useful to engineers interested in surface flow visualization. This is accomplished using liquid crystals in conjunction with a laser heat source to generate a thermal tuft. The shape and size of the thermal tuft is used to characterize the flow field. Wind tunnel experiments are conducted to validate this concept and examine flow behavior over a flat plate in a low Reynolds number environment. The plate is coated with liquid crystals of one-degree and fivedegree bandwidths. A 150-mW infrared, diode laser provides a constant heat source and generates a high temperature thermal spot on the model. The results obtained during the wind tunnel experimentation show that an irradiated spot on a liquid crystal coated surface will produce a tuft. The shape and size of the thermal tuft is indicative of the direction and magnitude of the flow conditions. As the wind speed increases from 2 to 10 m/s, it was shown that the length of the thermal tuft increases linearly. The tail of the tuft was also found to follow the direction of flow. Turbulent and laminar flow conditions can be distinguished; however, the angle of attack could not be realized with this technique. Developing a technique for generating a matrix of heated spots on the model indicates that the results of using this method can be viewed over a large area. Overall, it was shown that this is an easy, inexpensive, and non-intrusive technique for visualizing flow on the surface of an object.Item Applications of self-assembly : liquid crystalline semiconductors and DNA-conjugated microparticles(2012-12) Tang, Hao, 1985-; Willson, C. Grant, 1939-Self-assembly provides an efficient way to build complex structures with great flexibility in terms of components and properties. This dissertation presents two different forms of self-assembly for technical applications. The first example is the molecular assembly of liquid crystals (LCs). Attaching appropriate side chains on anthracene, oligothiophene, and oligoarenethiophene successfully constructed liquid crystalline organic semiconductors. The phase transitions of the LC semiconductors were analyzed by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The effect of the LC phase change on charge transport was probed by the space-charge limited current (SCLC) method and the field-effect transistor (FET) method. Mobility in the LC phase rose in anthracenyl esters but decreased in oligothiophenes and oligoarenethiophenes. The different electronic behavior of LC semiconductors may be caused by the difference in domain size and/or the difference in response to electric field. The second example of self-assembly in this dissertation is DNA-guided self-assembly of micrometer-sized particles. Patternable bioconjugation polymers were synthesized to allow for lithographic patterning and DNA conjugation. The base pairing of DNA was then used to drive the self-assembly of DNA-conjugated particles. The DNA conjugation chemistry was studied in detail using a fluorescence-based reaction test platform. The conjugated DNA on the polymer surface retained its ability to hybridize with its complement and was efficient in binding microspheres with complementary strands. Highly specific bead-to-bead assembly was analyzed using imaging flow cytometry, and the fractions of self-assembly products were explained on the basis of chemical equilibrium. The process of particle fabrication using photolithography was successfully developed, and the self-assembly of lithographically-patterned particles was demonstrated. We envision that the technologies described in this dissertation will be useful in a variety of fields ranging from microelectronics to biotechnology.Item Discotic Colloids(2013-07-25) Mejia Mejia, Andres Fernando FernandoMany materials and biological systems in nature are suspensions composed of disks, such as clay, asphaltenes, and red blood cells. Despite their natural abundance and wide industrial application, disks are least studied compared to spheres and rods, due to the lack of model systems. In our research, disks at micro-scale were mass-produced with unprecedented uniformity in size and shape, and unique flexibility in the control of lateral size, lateral size polydispersity, shape, and aspect ratio (? = diameter/thickness). This dissertation focuses on two main areas: the study of the discotic colloidal liquid crystal phase transitions and the application of disk-like colloidal systems as Pickering emulsion and Pickering foam stabilizers. First, we engineered two discotic colloidal systems made from organic and inorganic materials. The former is made of ?-eicosene, which is an alkene of 20 carbons. The latter is composed of nano-sheets from exfoliated zirconium phosphate (?-ZrP). Both discotic systems were used to experimentally investigate the liquid crystalline phase transitions (Isotropic-Nematic, Isotropic-Cubic and Isotropic-Columnar). Also, the nematic crystalline phase was studied in detail by embedding it in a translucent and thermo-sensitive hydrogel. This was possible since nematic textures could be formed instantly by ZrP nano-sheets due to their high diameter-thickness ratio. Second, we developed Pickering emulsions and Pickering foams stabilized by high-aspect-ratio nano-sheets. We have also demonstrated for the first time the fabrication of the thinnest amphiphilic Janus and Gemini nano-sheets, which are either surface- or edge-modified plates with a thickness at atomic scale. These nano-sheets were obtained by exfoliating ?-ZrP crystals grafted with a coupling agent of hydrophobic molecules on their edges and outer surfaces. Extending this work, we studied crucial fundamental mechanisms that allow Pickering interfacial stabilization, including the effect on the adsorption properties of particle aspect ratio, concentration, and hydrophobicity. Our study is of great interest in the scientific community due to the difficulty in generating a discotic colloidal system of controllable parameters.Item Effects of nanoconfinement on structure and properties of side-chain liquid crystalline polymers(2013-12) Gonzalez Garza, Paola Anaid; Ellison, Christopher J.Semi-crystalline polymers have shown increased crystalline order and size when confined in multilayered films by coextrusion1. The resulting large crystals lead to dramatic improvements in gas barrier properties. Ordered polymers whose characteristics are between that of the liquid phase and the crystalline phase are known as liquid crystalline polymers. The highly ordered mesogens in liquid crystalline polymers contribute to their exceptional bulk properties. In this research, side-chain liquid crystalline polymers were confined in multilayered films, made by either multilayer coextrusion or spin coating, with a non-liquid crystalline polymer in an attempt to improve the ordering of the liquid crystalline mesogens. The liquid crystalline behavior and morphology was studied to understand the correlation between the confinement size and the properties of the multilayer films. Commercial main chain liquid crystalline polymers and hydrogen bonded liquid crystalline polymers were also explored in this research for their use in multilayer coextrusion.Item Organic materials for microelectronics : 157 nm photoresists and electrooptic liquid crystals(2001-12) Hung, Raymond Jui-pu, 1969-; Willson, C. G. (C. Grant), 1939-