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Motion of Particles as a Probe: Dynamics and Assembly in Gel Networks/Aqueous Media

Nanoparticles are of great interest with a wide variety of potential applications due to their unexpected but interesting physical properties which are different from bulk state, enable small length scale-driven transport through complex materials, and provide the building units for well-ordered structures. Observing the motion of nanoparticles provides information about surrounding microstructures, flow dynamics, and assembly processes by virtue of fluorescence of nanoparticles. However, the proper control of surface chemistry and the fluorescence of particles are both paramount and challenging to allow particles to be used in a quantitative and robust manner. This thesis describes the use of precisely-defined particles for characterizing and building complex structures. The research exploits advantages of the particle dynamics in three distinct studies: i) the tracking of single CdSe/ZnS core/shell QDs to characterize complex structures of hydrogels, ii) the transformation of dispersed QDs in bulk phase into unique ring assemblies at the air/liquid interface, and iii) the mapping of flow dynamics within an evaporating droplet. Chapter 2 describes the diffusion dynamics of single quantum dots (QDs) within polyacrylamide (PAAm) hydrogels to characterize the structural heterogeneity of gel networks by employing the single particle tracking (SPT) technique. Due to their photo-stable and highly fluorescent emission and its small size (4- 10 nm), individual QDs can be tracked by a fluorescence microscopy as they find pathways through structurally complex gel networks. This tracking provides information about spatiotemporal dynamics. The anomalous diffusion dynamics revealed by the motion of single QDs suggests that the structural heterogeneities of PAAm gels develop with increasing cross-linker content, and the length scales discovered are in a good agreement with the correlation length scale reported in the previous light scattering studies. Chapter 3 describes the assembly of QD rings at the air/water interface by ‘2-D Pickering emulsions’. This work emanated from the unexpected observation of QD rings on the droplet of QD solutions. These rings form from QDs adsorbed to the interfacial line of surfactant islands assembled at the interface, and the QDs mark islands, appearing as rings. This island assembly was found to occur only at a specific range of surfactant concentrations due to the phase transition. Uniformly dispersed QDs in the bulk phase affording the ring patterns exclusively at the air/water interface provides insight that the thermodynamic driving force arises at the interfacial line between three phases (air/water/surfactant islands). Finally, Chapter 4 details the radial flow dynamics within an evaporating droplet with a pinned contact line is investigated. By suspending and tracking fluorescent latex beads, the flow dynamics are quantified as a function of contact angle. This phenomenon, commonly called the “coffee ring effect”, is advantageous for patterning and depositing suspended solutes on substrates. To develop evaporative assembly as a scalable process, it is particularly important to understand the effect of contact angle on radial velocity. By tracking the motion of suspended particles in a droplet, we experimentally measured the flow dynamics, specifically the height averaged radial velocity, within an evaporating droplet in the range of contact angles 5-50o. We found that our experimental results are in a good agreement with the analytical prediction by Hu and Larson. Following the analytical predictions, we modified the original equation to a simplified equation that directly links radial velocity to contact angle and evaporation rate. This study provides insight into the manipulation of evaporative assembly processes on different substrates in terms of assembly kinetics and structural dimensions.
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