Date

2017

Document Type

Dissertation

Degree

Doctor of Philosophy

Department

Chemical Engineering

First Adviser

Gilchrist, James F.

Other advisers/committee members

Gilchrist, James F.; Snyder, Mark A.; Chaudhury, Manoj K.; Kretzschmar, Ilona

Abstract

Colloidal assembly is emerging in different avenues of modern technology like photonics, membrane, quantum dots, and molecular adsorption devices. Fabricating network of colloidal particle is governed by the surface energy and fluid flows. As you go smaller in size, fundamental understanding of the process is crucial. Previous research in our group was focused on the different applications of using the particle networks. Fabricating these structures on a desired substrate, with a choice of particles, and a desirable number of layers is a tedious task. It demands the thorough understanding of process parameters and different interactions in the thin colloidal film. In the following work, we have separated the convective deposition into two parts. One is governed by thin film stability of solvent, particle interactions with substrate and interface, and interfacial properties, on the other hand, the second part is governed by the solvent flow through a porous structure of particles network. Separating one process into two bits helps in probing for more details. Previous models of convective deposition capture the essential physics over narrow ranges of parameters, however, there is much room for developing robust models over broader deposition conditions and understanding instabilities in this system. In the convective deposition, the suspension flow is driven by solvent evaporation. The widely used Nagayama equation for the convective deposition defines the evaporation loss as the product of the evaporation rate (Je) and drying length ( ). Here, is the hydrodynamic length scale, which for many years’ have been assumed as a constant. Even though it is safe to assume a constant value for in a small region of substrate velocity, it fails to predict the coating thickness over wide velocity range. Following the work done by Y. Jung et al. we have derived an analytical expression for the drying length in the convective deposition for a more general geometry, treating a system as a Darcy flow. This analysis allows the prediction of coating thickness over a wider range of substrate velocities. In depositing a thicker or smaller particles colloidal crystals, the evaporation driven hydrodynamic stress in the thin film causes crack formation. In such cases, the crack spacing varies with the thickness. This phenomenon is not new and has been observed in all length scales. It has been shown that, in a colloidal crystal, a crack spacing scales up with a hydrodynamic stress. Interestingly, the simultaneous 1D drying and particle deposition in convective deposition results in highly linear uniform cracks. We have shown that the crack spacing can be easily tuned with the deposition speed or the substrate temperature. These fundamental studies enable optimizing deposition conditions to produce various thin film structures for a given particle size, solvent, and stabilizing agent. As is done in many previous studies of convective deposition and evaporating droplets, an addition of a surfactant can significantly change the mode of deposition to be nearly independent of the evaporation rate and deposition velocity by altering this length scale and thin film flow. At lower surfactant concentrations, the added surfactant had no effects on the assembly. At higher surfactant concentrations above the critical micelle concentration, the marangoni stresses become the main driving force for the flow inside the thin film. This Marangoni flow which can be much stronger than that driven by the evaporation may be tailored to produce the desired particle depositions over a wide range of velocities. The direction of marangoni flow is important. We have studied the mixture of water-ethanol as a choice of solvent, which allows completely reverse results that of the excess surfactant concentration. We also have studied the particle-particle and particle-substrate interactions, which play a crucial role in the convective deposition. Following results demonstrates the non-trivial effects of varying surface charge and ionic strength of monosized silica microspheres in the water on the quality of the deposited monolayer. The increase in particle surface charge results in a broader range of parameters that result in monolayer deposition which can be explained considering the particle-substrate electrostatic repulsion in solution. Resulting changes in the coating morphology and microstructure at different solution conditions were observed using confocal microscopy enabling correlation of order to disorder transitions with relative particle stability. The following work discusses the novel way of isolating the inherent streaking instabilities happening in the convective deposition. With hydrophobic treating substrate in periodic areas allows one to separate these instabilities, leaving an ideal coating in between. The final part of this thesis is devoted to a relatively new scale-up technique of continuous particle assembly. We have studied the limitations on microstructure at higher substrate velocities as well as a smaller improvement in microstructure using a bidisperse silica suspension.

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