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Two-dimensional assembly of colloidal particles: fabrication and application
Gong, Jingjing
Gong, Jingjing
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2019
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Over the past decades, colloidal particles with different geometric shapes, interfacial properties, and chemical compositions have been synthesized. As a result, we are able to accurately control the strength, effective range, and direction of interparticle interactions in different ways. The study of colloidal particles and their assembly have significantly enriched our understanding about the organization of soft matter governed by thermodynamics or far-from-equilibrium. With these new building blocks and assembling tools, diverse colloidal structures have been fabricated with advanced functionalities. Abstract In this thesis, we primarily focused on the assembly of two-dimensional structures that have potential applications in biomedical and energy fields. We first developed an electric-field assisted approach to create and fix two-dimensional nonclose-packed colloidal arrays, which have applications ranging from plasmonic sensors, light trapping for photovoltaics, to transparent electrodes. A monolayer of nonclose-packed crystalline particle array was first created under a low-frequency alternating-current electric field in solution. We then applied a sequence of direct current pulses to fix the particle array onto the substrate so that it remained intact after field removal and solvent evaporation. Key process parameters such as the particle concentration, alternating-current (AC) field strength, direct current magnitude, and solvent evaporation rate that affect both array ordering and fixing process have been studied systematically. Our approach, in principle, can be conveniently adapted into the continuous convective coating process, thus making the fabrication of nonclose-packed colloidal arrays scalable. Abstract Two-dimensional structures can also be made for active motion of microscopic objects. By applying orthogonally oriented magnetic and electric fields, we presented two strategies to achieve reversible cell capture, transport, and release with both passive magnetic particles and active Janus magnetic micromotors. In the first strategy, we showed that hybrid particle-cell clusters can be assembled from yeast cells and magnetic particles, e.g., the Dynabeads, under a perpendicularly applied AC electric field due to included dipolar attraction. Interestingly, the hybrid clusters can propel under the same electric field due to unbalanced electrohydrodynamic flow surrounding them. To remotely steer and control the propulsion direction of the cell-particle cluster, we further applied a planar magnetic field, which oriented the magnetic module in the cluster and allowed directional transportation of the cells. By changing the electric field frequency, the hybrid cluster can disassemble and release the cell on-demand. In the second strategy, we replaced the isotropic and passive Dynabeads with Janus magnetic micromotors, which can actively navigate and selectively capture yeast or bacteria cells under AC electric fields. Again, the trajectory of the micromotor and cargo assembly can be achieved by pre-programming the planar magnetic fields. The strategies presented here can be further developed for making microfluidics-based diagnostic devices. Abstract We further extended our interest to two-dimensional colloidal arrays at the curved fluid-fluid interface. In particular, we investigated the impact of the wettability of silica nanoparticles on the formation and agglomeration of cyclopentane hydrates. Using a customized cooling cell, we observed the in situ formation of hydrates from Pickering water-in-oil emulsion droplets decorated by silica nanoparticles that were functionalized by different amounts hydrophobic silane molecules. We found that particles with intermediate hydrophobicity can stabilize emulsions, ice crystals, and hydrate particles at different temperatures. They represent a new type of anti-agglomerants for hydrate flow assurance, which are potentially non-toxic and recyclable.
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