Browsing by Subject "Colloids"

Now showing 1 - 3 of 3
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    Aerosol printing of colloidal nanocrystals by aerodynamic focusing.
    (2010-07) Qi, Lejun
    Colloidal synthesized semiconductor nanocrystals, or quantum dots, have shown promise as the active material in electronic and optoelectronic applications, because of their high quantum yield, narrow spectral emission band, size-tunable bandgap, chemical stability, and easy processibility. Meanwhile, it is still challenging to print patterns of nanocrystal films with desired linewidth and thickness, which is a critical step in fabrication of nanocrystal-based devices. In this thesis, a direct-write method of colloidal semiconductor nanocrystals has been developed. Like other direct-write techniques, this aerosol printing method also simplifies printing process and reduces the manufacturing cost, as it avoids mask screening, lithography, and pre-patterning of the substrate. Moreover, the aerosol printing with aerodynamic lenses needs neither microscale nozzles nor sheath gases, and is able to incorporate with the vacuum systems currently used in microelectronic fabrication. First, the synthesized colloidal nanocrystals in hexane were nebulized into compact and spherical agglomerates suspending in the carrier gas. The details about the impact dynamics of individual aerosolized nanocrystal agglomerates were investigated. As building blocks of printed nanocrystal films, the agglomerate exhibited cohesive and granular behaviors during impact deformation on the substrate. The strength of cohesion between nanocrystals in the agglomerates was adjustable by tuning the number concentration of colloidal nanocrystal dispersion. Second, ultrathin films of nanocrystals were obtained by printing monodisperse nanocrystal agglomerates. As the result of the granular property of nanocrystal agglomerates, it was found that the thickness of deposited agglomerates strongly depended on the size of agglomerates. A single monolayer film of nanocrystals was attained by aerodynamically focusing 40-nm nanocrystal agglomerates and translating the carbon substrate at a velocity of 10 µm/s. The formation of nanocrystal films during printing was found strongly influenced by the substrate surface wettability. Third, microscale towers, lines, and patterns were obtained by printing polydisperse nanocrystal agglomerates. The thickness and line width of the patterns were adjustable by altering experimental conditions. Micropatterns of linewidth of less than 10 µm were demonstrated. Upon exposure to near-UV illumination, the printed nanocrystals exhibited robust fluorescence in the visual, with the color depending on the diameter of the individual nanocrystals.
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    The Colloidal Glass Transition Under Confinement
    (2018-05) Zhang, Bo
    Understanding 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.
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    Complexation of DNA with Polycationic Micelles
    (2018-08) Jiang, Yaming
    Interpolyelectrolyte complexation is a ubiquitous phenomenon that plays vital roles in biological systems and in design of responsive materials. However, precise control of polyelectrolyte complexes (PEC) has been challenging, particular for DNA-polycation complexes designed for gene delivery applications. Incorporating polyionic micelles is a promising strategy to tune PEC properties, but has been under-utilized in designing polymeric gene delivery vehicles. Herein, cationic micelles self-assembled from amphiphilic block polymers are complexed with double stranded DNA. The structure, composition, and stability of the resulting "micelleplexes" are characterized to probe the fundamental physics that govern the formation and properties of micelleplexes. With cationic AB+ micelles, complexation of linear semiflexible DNA and flexible poly(styrenesulfonate) were compared and the influence of polyanion chain flexibility was extracted and discussed. DNA length was found to strongly influence the size, composition, and colloidal stability of micelleplexes, whereas DNA topology (linear or circular supercoiled) has minimal influence. To improve the colloidal stability and reduce the size of micelleplexes that are composed of multiple micelles connected by bridging DNA chains, AB+C micelles with hydrophilic nonionic outer coronas of varying length were designed. The addition of the outer nonionic corona dramatically improves the colloidal stability of micelleplexes over a much wider charge ratio, and the outer corona length closely correlates to micelleplex size, zeta potential, and the average number of micelles per micelleplex. In addition, AB+C micelleplexes adopt a beads-on-a-string structure that resembles the organization of DNA in chromatin. Lastly, structure, composition, and stability of micelleplexes were closely compared with those of another typically studied family of DNA complexes, "polyplexes", which form between DNA and cationic homopolymers or AB+ diblock copolymers with a hydrophilic nonionic A block. Compared to the polyplexes, micelleplexes showed more than a 4-fold increase in gene transfection efficiency, which was attributed to the high positive charge content of micelleplexes.