Doctor of Philosophy
Gilchrist, James F.
Other advisers/committee members
Chaudhury, Manoj K.; Pearson, Raymond A.; Snyder, Mark A.
The assembly of colloidal particles has drawn great attention due to its fascinating impact on various applications. One simple but effective method for the deposition of well-ordered particles microstructure is convective deposition. Convective deposition translates a meniscus of particle suspension across a substrate leaving behind a thin film. As the liquid phase of the suspension evaporates, primarily from the thin film, particles are deposited and order through capillary interactions. In this thesis, we develop novel techniques for enhancing the microstructure of evaporation-driven convective deposition. Confocal laser scanning microscopy and image analysis are used for quantifying the quality of deposited particle microstructure through number of nearest neighbors, packing density, and local bond order. We have developed vibration-assisted convective deposition as an advanced technique for improving convective deposition. The original motivation for this work was to investigate how environmental disturbances would affect a deposition process in an industrial scale. With the application of lateral vibration of the substrate, drastic alterations of the interfacial liquid surface and the evaporation rate occur. Aqueous binary suspensions of colloidal microspheres and nanoparticles were used for studying effect of the amplitudes (0-250 µm) and the frequencies (0-50 Hz) of substrate vibration. This effect yields the unexpected result of opening the range of operating parameters from a single deposition velocity for a given set of conditions to a wide range of velocities that result in monolayer deposition. A large phase space of frequency and amplitude demonstrates a region where at moderate conditions enhancement occurs. At low amplitudes and high frequencies the particles do not organize into well-ordered monolayers and at high amplitudes and low frequencies the substrate motion does not yield long range monolayers because of the pseudo-steady motion of the substrate. It is also noted that this technique can be further extended to fabricate partially aligned (100) fcc thin film colloidal crystals. In two ways, the solution of the suspension has been manipulated to alter the dynamics of deposition. First, ionic strength of medium and particle surface charges are known as crucial parameters for understanding micro-scale mechanism of particle assembly. The presence of salt reduces the thickness of the electrical double layers (Debye length), which leads to pre-organized deposition and particle island formation. The electrostatic barrier as a function of the distance between particle and substrate as well as the effective separation distance as a function of the salt concentration are calculated using DLVO theory. On the contrary, the addition of NaOH provides additional electrostatic repulsion forces between particle-particle and particle-substrate, which results in the enhancement in particle deposition. Second, Marangoni flow, created by the surface tension gradient within binary liquid mixture of ethyl alcohol and water, offers the great promise in macro-scale defect suppression. Our preliminary results suggests significant enhancement of monolayer deposition at the concentration of ethyl alcohol between 30-50 %V. Droplet evaporation experiments give the detailed ideas of how surface tension-driven flow develops.
Muangnapoh, Tanyakorn, "Convective Assembly of Colloidal Monolayers: Mechanical and Chemical Approaches" (2015). Theses and Dissertations. 2735.