Browsing by Subject "Colloids"
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Item Colloidal nanocrystals with near-infrared optical properties : synthesis, characterization, and applications(2011-12) Panthani, Matthew George; Korgel, Brian Allan, 1969-; Dodabalapur, Ananth; Chelikowsky, James; Mullins, C. Buddie; Manthiram, ArumugamColloidal nanocrystals with optical properties in the near-infrared (NIR) are of interest for many applications such as photovoltaic (PV) energy conversion, bioimaging, and therapeutics. For PVs and other electronic devices, challenges in using colloidal nanomaterials often deal with the surfaces. Because of the high surface-to-volume ratio of small nanocrystals, surfaces and interfaces play an enhanced role in the properties of nanocrystal films and devices. Organic ligand-capped CuInSe2 (CIS) and Cu(InXGa1-X)Se2 (CIGS) nanocrystals were synthesized and used as the absorber layer in prototype solar cells. By fabricating devices from spray-coated CuInSe nanocrystals under ambient conditions, solar-to-electric power conversion efficiencies as high as 3.1% were achieved. Many treatments of the nanocrystal films were explored. Although some treatments increased the conductivity of the nanocrystal films, the best devices were from untreated CIS films. By modifying the reaction chemistry, quantum-confined CuInSeXS2-X (CISS) nanocrystals were produced. The potential of the CISS nanocrystals for targeted bioimaging was demonstrated via oral delivery to mice and imaging of nanocrystal fluorescence. The size-dependent photoluminescence of Si nanocrystals was measured. Si nanocrystals supported on graphene were characterized by conventional transmission electron microscopy and spherical aberration (Cs)-corrected scanning transmission electron microscopy (STEM). Enhanced imaging contrast and resolution was achieved by using Cs-corrected STEM with a graphene support. In addition, clear imaging of defects and the organic-inorganic interface was enabled by utilizing this technique.Item Colloidal nanocrystals: synthesis and shape-control, interparticle interactions & self-assembly(2005) Saunders, Aaron Edward; Korgel, Brian A.Control over nanocrystal growth kinetics provides a powerful way of tailoring particle size and shape during synthesis. Investigations into the growth of gold nanocrystals demonstrated how reaction conditions can be adjusted to control the growth rate and produce monodisperse particles. Kinetic control during the synthesis of CdS, CdSe and CdTe nanoparticles allows the shape to be tuned, from rods to spheres, without modifying the reaction chemistry. The growth and optical properties of these shapeanisotropic semiconductor particles were studied, and these methods were extended to produce semiconductor heterostructure nanorods. Solvent-mediated interparticle interactions between nanocrystals dispersed in toluene and in supercritical carbon dioxide were also studied. Nanocrystal dispersions were characterized using small-angle X-ray scattering in order to obtain information about the pair interaction potential. In organic solvents, subtle differences in the concentration-dependent scattering from dispersions allowed second virial coefficients to be measured as a function of nanocrystal size. Interestingly, larger nanocrystals exhibited overall repulsive interactions, while smaller nanocrystals exhibited attractive interactions, which is likely due to differences in ligand coverage among the different sized particles. Nanocrystals coated with fluorinated ligands could be dispersed into supercritical carbon dioxide, and the relatively strong interparticle interactions were measured at different carbon dioxide densities. As expected, the interaction strength increased as the solvent density was lowered, due to a decreased ability of the solvent to solvate the capping ligands. The formation of metastable nanocrystal flocculates was also observed at all system conditions studied. The assembly of nanocrystals into ordered superlattices under equilibrium conditions is strongly influenced by nanocrystal interparticle interactions. The formation of binary superlattices was studied, and an ordered AB phase was observed from the coassembly of small gold and large iron nanocrystals. A non-equilibrium route, breath figure templating, was also used to produce nanocrystal films with hierarchal order and porous polymer films. Evaporation of a nanocrystal or polymer dispersion in a humid atmosphere causes water droplets to nucleate and grow at the solvent-air interface. The solute stabilizes the water droplets which assemble into ordered arrays to template the drying film. The design rules for producing macroporous nanocrystal and polymer films are discussed.Item Discotic Liquid Crystals and Polymersomes: Molecule Goniometers(2012-10-19) Chang, Ya-WenControlling the assembly of amphiphilic molecules and micron-sized, disk-shaped particles at different length scales into ordered structures enables bottom-up organization which is of great interest to emerging technologies based on structured materials. The primary object of this work is the investigation of structure forming components - Zirconium phosphate (ZrP) discotic particles and polymersomes/ amphiphiles on their self-assembly and interactions. The effect of bilayer architecture of polymersomes on surface reactivity was investigated via fluorescent probing method. Established through complementary experiments, correlation between reactivity and molecule diffusivity in polymer-rich environment revealed the mechanism of reduced reactivity when tethered reactive groups are located deeper within the hydrophilic polymer layer. The phase diagram of charged nanoplatelets was constructed as a function of particle concentration, surface cation moiety, and ionic strength. Influence of surface cation on the isotropic-nematic transition was done by measuring the transition boundaries of discotic suspensions prepared by acid-base exfoliation reaction with a series of exfoliating agents. Furthermore, a novel phase transition was found, where platelet-platelet interaction was influenced synergistically by ionic strength and ion exchange. At low pH, directional inter-platelet attractions lead to the formation of low volume fraction colloidal gels. Alternative surface modification approaches, including biomolecule deposition and alkyl chain grafting were explored. Finally, self-assembly of platelets in emulsions and oil-water interface was examined. Surface modification was applied to link surface properties to stable emulsion-forming ability in mixed surfactant-particle system. Emulsion uniformity was achieved by microfluidic flow focusing method. Surface engineering and interaction control was demonstrated throughout this work to be viable approaches to the fundamental understanding of collective behaviors of individual building blocks.Item Grain-scale mechanisms of particle retention in saturated and unsaturated granular materials(2010-12) Rodriguez-Pin, Elena; Bryant, Steven L.; Balhoff, Matthew; DiCarlo, David; Huh, Chun; Lloyd, Douglas R.The phenomenon of particle retention in granular materials has a wide range of implications. For agricultural operations, these particles can be contaminants transported through the ground that can eventually reach to aquifers, consequently contaminating the water. In oil reservoirs, these particles can be clays that get detached from the rock and migrate with the flow after a change of pressure, plugging the reservoir with the consequent reduction in permeability. These particles can also be traceable nanoparticles, introduced in the reservoir with the purpose of identifying bypassed oil. For all these reasons it is important to understand the mechanisms that contribute to the transport and retention of these particles. In this dissertation the retention of micro and nano size particles was investigated. In saturated model sediments (sphere packs), we analyzed the retention of particles by the mechanism of straining (size exclusion). The analysis focused on experiments reported in the literature in which particles smaller than the smallest pore throats were retained in the sediment. The analysis yields a mechanistic explanation of these observations, by indentifying the retention sites as gaps between pairs of sediment grains. A predictive model was developed that yields a relationship between the straining rate constant and particle size in agreement with the experimental observations. In unsaturated granular materials, the relative contributions of grain surfaces, interfacial areas and contact lines between phases to the retention of colloidal size particles were investigated. An important part of this analysis was the identification and calculation of the length of the contact lines between phases. This estimation of contact line lengths in porous media is the first of its kind. The algorithm developed to compute contact line length yielded values consistent with observations from beads pack and real rocks, which were obtained independently from analysis of high resolution images. Additionally, the predictions of interfacial areas in granular materials were consistent with an established thermodynamic theory of multiphase flow in porous media. Since there is a close relationship between interfacial areas and contact lines this supports the accuracy of the contact line length estimations. Predictions of contact line length and interfacial area in model sediments, combined with experimental values of retention of colloidal size particles in columns of glass beads suggested that it is plausible for interfacial area and contact line to contribute in the same proportion to the retention of particles. The mechanism of retention of surface treated nanoparticles in sedimentary rocks was also investigated, where it was found that retention is reversible and dominated by attractive van der Waals forces between the particles and the rock’s grain surfaces. The intricate combination of factors that affect retention makes the clear identification of the mechanism responsible for trapping a complex task. The work presented in this dissertation provides significant insight into the retention mechanisms in relevant scenarios.Item Laser direct-write of optical components prepared using the sol-gel process(2004) Ruizpalacios, Rodrigo; Wood, Kristin L.; Beaman, Joseph J.Item Neurotrophic factor combinations and extracellular matrix-based hydrogels for nerve regeneration(2006) Deister, Curt Andrew; Schmidt, Christine E.; Roy, KrishnenduItem Oral delivery of protein-transporter bioconjugates using intelligent complexation hydrogels(2008-12) Shofner, Justin Patrick, 1983-; Peppas, Nicholas A., 1948-; Brodbelt, Jennifer S.Several polymer systems including P(MAA-g-EG) and P(MAA-co-NVP) with crosslinking agents TEGDMA and PEGDMA1000, monomer-to-solvent ratios of 67:33, 60:40, and 50:50, and particle sizes of <75 microns, 90-150 microns, and 150-212 microns were synthesized for use with protein-transporter conjugates. All synthesized systems were characterized by SEM which demonstrated the visual size, surface features, and surface textures of the polymer microparticles. Insulin-transferrin and calcitonin-transferrin conjugates were successfully synthesized using the protein crosslinker SPDP, binding the two proteins with a disulfide bond. The multi-step conjugation reactions used to create the conjugates were analyzed by the use of UV spectroscopy and HPLC to ensure the quality of the final products. In both conjugation reactions, the final product yield was found to be over 70%. The in vitro loading and release characteristics for insulin-transferrin and calcitonin-transferrin were separately investigated. By testing loading and release using a number of different polymer systems with different synthesis parameters, it was possible to optimize the hydrogel carriers for use with each of the conjugates independently. Upon optimization, the ideal system for use with insulin-transferrin and calcitonin-transferrin was found to be P(MAA-g-EG) microparticles of <75 microns formed using a PEGDMA1000 crosslinker and a 50:50 monomer-to-solvent ratio for both conjugates through separate optimization processes. This optimized polymer carrier was found to release upwards of 50% of loaded insulin-transferrin conjugate and near 90% of loaded calcitonin-transferrin conjugate. The insulin-transferrin conjugate was further evaluated through the use of cellular and animal models. Using cellular models, the insulin-transferrin conjugate was shown to increase transport relative to insulin by a factor of 7, achieving an apparent permeability of 37 x 10⁹ cm/s. Also, in the presence of polymer microparticles, the insulin-transferrin conjugate increased transport by a factor of 14 times relative to insulin, achieve an apparent permeability of 72.8 x 10⁹ cm/s. The presence of the microparticles near the cells was found to improve conjugate transport by nearly 100%. The preliminary animal studies verified the successful synthesis of the insulin-transferrin conjugate as well as demonstrated the bioactivity of the insulin portion of the molecule by achieving a drop in blood glucose level upon subcutaneous injection.Item Reversible Attraction-Mediated Colloidal Crystallization on Patterned Substrates(2009-05-15) Fernandes, GregoryIn this dissertation we used tunable particle-particle and particle-substrate attraction to achieve reversible two-dimensional crystallization of colloids on homogeneous and patterned substrates. Total internal reflection and video microscopy techniques were used to quantify the interparticle and particle-substrate interactions in these colloidal systems. Equilibrium and dynamic simulations were then utilized to link these colloidal interactions to the experimental colloidal phase behaviour. The importance of the nature of the attractive interaction in successfully crystallizing colloids has also been documented. The first set of experiments demonstrates the use of temperature and specific ion effects to reversibly control the net particle-substrate van der Waals (vdW) attraction. Colloidal stabilization was achieved via the use of adsorbed polymer brush layers. By using evanescent wave microscopy, we directly and precisely measured how temperature and specific ion effects control the dimensions of adsorbed polymer layers and hence the net van der Waals attraction in between the colloids and the substrate. However, the magnitude of the van der Waals attraction decays very rapidly with increasing surface separation and is therefore not conducive to the self assembly of colloidal crystals. We successfully used thermoresponsive polymer nanoparticles to control the depletion attraction between micron sized silica particles and thereby induced reversible crystallization of the micron sized silica colloids on homogeneous substrates. Video and evanescent wave microscopy techniques were used to measure the nanoparticle-induced attractive interaction as a function of temperature. The experimentally observed phase behaviour was verified via simulations that utilized knowledge of the measured colloidal depletion interactions. Finally, patterned surface topologies were used to position attractive colloidal crystals. Simulations were used to link the measured colloidal interactions to experimental phase behaviour as well as substrate topology. An extension of the concepts developed in this dissertation might suggest a general strategy to assemble colloidal particles into robust and annealable crystals contributing to the fabrication of photonic bandgap materials.Item The self-assembly of colloidal particles into 2D arrays(2007-12) Rabideau, Brooks Douglas, 1979-; Bonnecaze, R. T. (Roger T.)As the feature size of new devices continues to decrease so too does the feasibility of top-down methods of patterning them. In many cases bottom-up methods are replacing the existing methods of assembly, as having building blocks self-organize into the desired structure appears, in many cases, to be a much more advantageous route. Self-assembled nanoparticulate films have a wide range of potential applications; high-density magnetic media, sensing arrays, meta-materials and as seeds for 3D photonic crystals to name a few. Thus, it is critical that we understand the fundamental dynamics of pattern formation on the nanoparticulate and colloidal scale so that we may have better control over the formation and final quality of these structures. We study computationally the self-organization of colloidal particles in 2D using both Monte Carlo and dynamic simulation We present 3 studies employing Monte Carlo simulation. In the first study, Monte Carlo simulations were used to understand the experimental observation of highlyordered 2D arrays of bidisperse, stabilized gold nanoparticles. It was shown that the LS lattice forms with the addition of interparticle forces and a simple compressive force, revealing that bidisperse lattice formation is, in fact, a dynamic process. It was evident that the LS lattice forms in large part because the particles within the lattice reside in their respective interparticle potential wells. In the second Monte Carlo study, this information was used to predict size-ratios and surface coverages for novel lattice structures. These predictions are intended to guide experimentalists in their search for these exciting new structures. In the third study it was shown that polydisperse amounts of amorphous-silicon nanoparticles could form 2D clusters exhibiting long-range orientational order even in the absence of translational order. Monte Carlo simulations were performed, which included lateral capillary forces and a simple stabilizing repulsion, resulting in structures that were strikingly similar to the experimentally observed In the fourth study we used dynamic simulation to study the hydrodynamicallyassisted self-organization of DNA-functionalized colloids in 2D. It was shown that hydrodynamic forces allow a more thorough sampling of phase space than through thermal or Brownian forces alone.Item Stokesian dynamic simulations and analyses of interfacial and bulk colloidal fluids(Texas A&M University, 2006-10-30) Anekal, Samartha GuhaUnderstanding dynamics of colloidal dispersions is important for several applications ranging from coatings such as paints to growing colloidal crystals for photonic bandgap materials. The research outlined in this dissertation describes the use of Monte Carlo and Stokesian Dynamic simulations to model colloidal dispersions, and the development of theoretical expressions to quantify and predict dynamics of colloidal dispersions. The emphasis is on accurately modeling conservative, Brownian, and hydrodynamic forces to model dynamics of colloidal dispersions. In addition, we develop theoretical expressions for quantifying self-diffusion in colloids interacting via different particle-particle and particle-wall potentials. Specifically, we have used simulations to quantitatively explain the observation of anomalous attraction between like-charged colloids, develop a new criterion for percolation in attractive colloidal fluids, and validate the use of analytical expressions for quantifying diffusion in interfacial colloidal fluids. The results of this work contribute to understanding dynamics in interfacial and bulk colloidal fluids.Item Structure and dynamics of fluids : from molecular to colloidal perspectives(2011-08) Pond, Mark Jeffrey; Truskett, Thomas Michael, 1973-; Ellison, Christopher J.; Ganesan, Venkat; Makarov, Dmitrii E.; Sanchez, Isaac C.Relationships between structure and dynamics have been well studied in molecular fluids, both in computer simulations and in experiments. However, the development of simple structure-dynamics relationships would also be useful in understanding colloidal fluids. Colloidal fluids display differentiated component dynamics, are made of polydisperse particles, have soft interactions and have a separation of length and time scales. In this dissertation work, we have used computer simulations to generalize some structure-dynamics scaling laws, originally formulated for molecular fluids, in a way that successfully accounts for these important aspects of colloidal suspensions. To begin, we examine a two-component mixture of ultrasoft Gaussian-core particles through molecular dynamics simulations. This fluid shows an anomalous dynamic crossover where the larger particles become more diffusive than the smaller particles. However, this dynamic crossover is accompanied by a corresponding structural crossover for a component-specific structural order metric. In the light of this structural order metric, the fluid is non-anomalous with respect to the relationship between static structuring and diffusivity. Next, we show that accounting for the many-component nature of even modestly polydisperse fluids is important for accurately characterizing their structure-dynamics relationships. We demonstrate this for colloids with short-range attractions through new Monte Carlo simulation techniques and through theoretical calculations carried out in the dilute limit. From here, we present a new generalized framework to non-dimensionalize diffusivity so that it will have an approximately one-to-one relationship with excess entropy. This method involves rescaling diffusivity with dilute-limit analyses that can be analytically and systematically executed. We tested this framework through a combination of molecular dynamics simulations, Brownian dynamics simulations and Monte Carlo simulations. The results of the simulations demonstrate that this framework can account for particle size asymmetry, particle additivity, interaction strength and some solvent effects. Finally, we present a new, simple equation that relates non-dimensionalized forms of diffusivity from molecular dynamics and Brownian dynamics simulations. This simple relationship is tested for inverse power law fluids, as well as a suite of ultrasoft fluids that show structural and dynamic anomalies.Item Structure formation in colloidal and nanoscale systems(2000-08) Gray, Jeffrey James; Bonnecaze, R. T. (Roger T.)In biotechnology, microelectronics, and materials science, many products require intimate attention to microscopic and sub-microscopic construction. Bulk properties of interest often depend on the system microstructure, leading researchers to strive to tailor custom microstructures and predict properties from microstructure—increasingly difficult tasks as component sizes shrink. A promising paradigm for engineering small systems is the idea of designing components which self-assemble into the structures desired, similar to the way that biological systems routinely build themselves from the molecular level up to the macroscopic. In this thesis, I use numerical simulation to study the structural evolution of colloidal and nanoscopic particulate systems. I focus on problems in rheology and adsorption. In the rheological study, I use Stokesian dynamics to investigate a transition where the shear rate qualitatively changes the trajectories of a lattice of particles and imparts a discontinuous, hysteretic viscosity jump. My model shows that a particular face-centered cubic crystal configuration is necessary to reproduce experimental findings. The adsorption studies are approached with two different models. First is a two-dimensional model for the random sequential adsorption of tethered nanoparticles. Tethers provide robust physical and/or electrical connections between particles and a substrate, but they also frustrate order. Hexatic and crystal structures form with surprisingly short tethers of one and four particle radii, respectively. Polydispersities of less than 5–7% (and sufficient tether length) are necessary to form crystal phases, and polydispersities of less than 7–8% are necessary to create hexatic phases. The second set of adsorption studies employs full three-dimensional Brownian dynamics simulations to model electrostatically-repulsive particles that are attracted to a substrate. The zeta-potential of the wall is the primary control of order formation on the surface, and the particle potentials are the primary control of surface coverage. Mixtures of particles that are bidisperse in surface zeta-potential can disrupt order for significant ratio of zeta-potentials, and at large ratios the process creates interesting patterns including dots, clusters, chains, and doped crystals. In each study, system history has a significant effect on the final state of the system; careful attention must be paid to the non-equilibrium process of assembling small systems.