This thesis is on molecular dynamics studies of drying suspensions of bidisperse nanoparticle mixtures. I first use an explicit solvent model to investigate how the structure of the dry film depends on the evaporation rate of the solvent and the initial volume fractions of the nanoparticles. My simulation results show that the particle mixtures stratify according to their sizes when the suspensions are quickly dried, consistent with the prediction of recent theories. I further show that stratification can be controlled using thermophoresis induced by a thermal gradient imposed on the drying suspension. To model larger systems on longer time scales, I explore implicit solvent models of drying particle suspensions in which the solvent is treated as a uniform viscous background and the liquid-vapor interface is replaced by a potential barrier that confines all the solutes in the solution. Drying is then modeled as a process in which the location of the confining potential is moved. In order to clarify the physical foundation of this moving interface method, I analyze the meniscus on the outside of a circular cylinder and apply the results to understand the capillary force experienced by a spherical particle at a liquid-vapor interface. My analyses show that the capillary force is approximately linear with the displacement of the particle from its equilibrium location at the interface. An analytical expression is derived for the corresponding spring constant that depends on the surface tension and lateral span of the interface and the particle radius. I further show that with a careful mapping, both explicit and implicit solvent models yield similar stratification behavior for drying suspensions of bidisperse particles. Finally, I apply the moving interface method based on an implicit solvent to study the drying of various soft matter solutions, including a solution film of a mixture of polymers and nanoparticles, a suspension droplet of bidisperse nanoparticles, a solution droplet of a polymer blend, and a solution droplet of diblock copolymers. / PHD / Drying is a ubiquitous phenomenon. In this thesis, I use molecular dynamics methods to simulate the drying of a suspension of a bidisperse mixture of nanoparticles that have two different radii. First, I use a model in which the solvent is included explicitly as point particles and the nanoparticles are modeled as spheres with finite radii. Their trajectories are generated by numerically solving the Newtonian equations of motion for all the particles in the system. My simulations show that the bidisperse nanoparticle mixtures stratify according to their sizes after drying. For example, a “small-on-top” stratified film can be produced in which the smaller nanoparticles are distributed on top of the larger particles in the drying film. I further use a similar model to demonstrate that stratification can be controlled by imposing a thermal gradient on the drying suspension. I then map an explicit solvent system to an implicit one in which the solvent is treated as a uniform viscous background and only the nanoparticles are kept. The physical foundation of this mapping is clarified. I compare simulations using the explicit and implicit solvent models and show that similar stratification behavior emerge in both models. Therefore, the implicit solvent model can be applied to study much larger systems on longer time scales. Finally, I apply the implicit solvent model to study the drying of various soft matter solutions, including a solution film of a mixture of polymers and nanoparticles, a droplet of a bidisperse nanoparticle suspension, a solution droplet of a polymer blend, and a droplet of a diblock copolymer solution.

abstract: This study analyzes the thermoelectric phenomena of nanoparticle suspensions, which are composed of liquid and solid nanoparticles that show a relatively stable Seebeck coefficient as bulk solids near room temperature. The approach is to explore the thermoelectric character of the nanoparticle suspensions, predict the outcome of the experiment and compare the experimental data with anticipated results. In the experiment, the nanoparticle suspension is contained in a 15cm*2.5cm*2.5cm glass container, the temperature gradient ranges from 20 °C to 60 °C, and room temperature fluctuates from 20 °C to 23°C. The measured nanoparticles include multiwall carbon nanotubes, aluminum dioxide and bismuth telluride. A temperature gradient from 20 °C to 60 °C is imposed along the length of the container, and the resulting voltage (if any) is measured. Both heating and cooling processes are measured. With three different nanoparticle suspensions (carbon nano tubes, Al2O3 nanoparticles and Bi2Te3 nanoparticles), the correlation between temperature gradient and voltage is correspondingly 8%, 38% and 96%. A comparison of results calculated from the bulk Seebeck coefficients with our measured results indicate that the Seebeck coefficient measured for each suspension is much more than anticipated, which indicates that the thermophoresis effect could have enhanced the voltage. Further research with a closed-loop system might be able to affirm the results of this study. / Dissertation/Thesis / M.S. Mechanical Engineering 2010

Planetary ball milling (PBM) is a well-known technique for efficient size reduction and homogenization of materials that has been used for many decades in various engineering and industrial processes. More recently, it has emerged as a unique top-down nanofabrication approach for nanomaterials based on nanoscale grinding. However, its potential application in nanostructured thin film fabrication has not been fully explored, as only a limited number of studies have been carried out. In this work, the effects of different grinding parameters (speed, time and solvents) were used to create previously unstudied nanoscale grinding conditions for nanostructured thin film materials via PBM with distinct and novel properties: Nanoparticles of silicon, titanium disilicide (TiSi2) and zinc oxide (ZnO) ground in different solvents (deionized (DI) water/ ethylene glycol (EG)/isopropyl alcohol) resulted in colloidal suspensions (or nanoinks) that could be used to coat various substrates (wafers, glass, flexible substrates, etc.) via drop casting, doctor blading or dip coating. Thin film properties such as wettability, electrical conductivity and gas sensing behavior are studied. The fabricated thin film coating properties could be tuned depending on the combination of starting powder materials, grinding parameters and resulting nanoparticle size/geometry: The influence of surface chemistry, solvent type, particle geometry, surface roughness and defects was shown to alter the conductivity and surface wettability of the resultant films. Thus, thin films formed using PBM nanoinks allow varied and tunable properties for advanced multi-functional coatings and devices. To demonstrate the feasibility of PBM nanoinks for thin film device applications, ZnO nanoinks were used to create chemiresistive gas sensors that operate at room temperature. By varying grinding parameters (speed, time and solvent) thin film sensors with differing particle sizes and porosity were produced and tested with air/oxygen against hydrogen, argon and methane target gas species, in addition to relative humidity. Grinding speeds of up to 1000 rpm produced particle sizes and RMS thin film roughness below 100 nm, as measured by atomic force microscopy and scanning electron microscopy. Raman spectroscopy, photoluminescence and x-ray analysis confirmed the purity and structure of resulting films. The peak gas sensor response was found for grinding parameters of 400 rpm (average particle size 275 nm) and 30 minutes (average particle size 225 nm) in EG and DI water, respectively, which could be correlated to an increased film porosity and an enhanced electron concentration resulting from adsorption/desorption of oxygen ions on the surface of ZnO nanoparticles. Similarly, gas response and dynamic behavior were found to improve as the operating temperature was increased between 100 and 150 °C. These results demonstrate the use of low-cost PBM nanoinks to optimize the active materials for solution-processed thin film gas/humidity sensors that can operate at room temperature for use in environmental, medical, food packaging, laboratory, and industrial applications. Overall, the nanogrinding technique can produce large amounts of nanoparticle suspension with variable particle sizes for creating thin films with tunable properties. By adjusting grinding parameters, the nanoparticle shape/size and properties can be varied resulting in nanoparticle inks for inexpensive coatings on various substrates and for use in different applications. / Graduate