A. James Clark School of Engineering
Permanent URI for this communityhttp://hdl.handle.net/1903/1654
The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item Modeling and Experimental Measurement of Triboelectric Charging in Dielectric Granular Mixtures(2020) Carter, Dylan Patrick; Hartzell, Christine M; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Triboelectric charging, the phenomenon by which electrical charge is exchanged during contact between two surfaces, has been known to cause significant charge separation in granular mixtures, even between chemically identical grains. This charging is a stochastic process resulting from random collisions between grains, but creates clear charge segregation according to size in dielectric granular mixtures. Experiments in grain charging are frequently conducted with methods that may introduce additional charging mechanisms that would not be present in airless environments, and often aren't capable of measuring the precise charge of each grain. We resolved these issues through the development of a model that predicts the mean charge on grains of a particular size in an arbitrary mixture, and through experiments that do offer controlled measurement of precise grain charges. These results can be used to develop methods for electrostatic sorting to enable \textit{in situ} resource utilization of silica-based regoliths on airless extraterrestrial bodies. Beginning from a basic collision model for a mixture of hard spheres, we developed a robust semi-analytical model for making predictions about the charge distribution in a dielectric granular mixture. This model takes a set of assumptions about a mixture, including the continuous size distribution, collision frequencies, and charge transfered per collision, and calculates the mean charge acquired by grains of each size after all charges have been exchanged. This model allows us to explore experimental results through many different lenses. To test our predictions and provide a repeatable and flexible method for analyzing charging in a variety of granular mixtures, we designed and built our own experimental test stand. This device is housed entirely in a vacuum chamber, allowing us to induce tribocharging in dielectric grains in a controlled airless environment and measure individual charge and diameter of a grain by dropping samples through a transverse electric field. We observed that mixtures of zirconia-silica grains containing two primary size fractions exhibited size-dependent charge segregation when charged in vacuum. Unlike in other experiments with grains charged by fluidization with a gas, we consistently observed that the small grains charged predominantly positive, while the large grains were primarily negative. We considered a variety of charge transfer mechanisms and generated predicted charge distributions for each using the modeling framework we developed. Comparing these models to the collected data, we are able to assess the viability of each potential transfer mechanism by examining properties of its resulting distribution, including the relative charge magnitudes for each size fraction, the point at which the polarity changes, and the polarity and magnitude of the charge carrier density. The results of this work provide solid supporting evidence for the role of positive charge carriers in dielectric tribocharging. While some prior work has suggested positive ions from the atmosphere and/or adsorbed water are responsible, we have observed that even when these environmental factors are reduced or eliminated, silica-based materials still exhibit positive charge transfer. The modeling framework developed in search of a descriptive model for this effect is a useful, adaptable tool. The experimental apparatus itself, and especially in conjunction with these modeling tools, overcomes some of the more difficult challenges faced by experimentalists investigating granular tribocharging, enabling further investigation into tribocharging in regolith and other dielectric materials.Item Nanoconfined Polyelectrolyte Brushes: Thermodynamics, Electrostatics and Transport(2017) Chen, Guang; Das, Siddhartha; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Polyelectrolyte (PE) grafting on the solid-liquid interface of a nano-channel renders tremendous functionalities to the nano-channel. These grafted PE molecules attain “brush”-like configuration for large grafting density ($\sigma$), which makes the nano-channel (often denoted as soft nano-channel) capable of applications such as ion manipulation, ion sensing, current rectification, nano-fluidic diode action, and flow regulation. The present thesis focuses on the theoretical modeling of the thermodynamics, electrostatics and transport of such nano-confined PE brush systems. The thesis starts by developing new scaling laws to a) determine the phase space for the grafting density ($\sigma$) and the polymer size or number of monomers ($N_p$) of the grafted PE molecules that ensure that the PE chains can simultaneously adopt a “brush”-like configuration and do not exceed the nano-channel half height, and b) identify the regime where the elastic and the excluded volume effects of the chains can be decoupled from the electrostatic effects. The subsequent part of the thesis is divided into two broad parts. In the first part, the thermodynamics, electrostatics, and the transport of PE-brush-grafted nano-channels in the decoupled regime is probed. In the second part, however, the analysis is carried out to elucidate the physical picture of the PE-brush-grafted nano-channels in the coupled regime. For the analysis in the decoupled regime, firstly the electrostatics of such PE-brush-grafted nano-channels has been probed. These PE brushes are considered to exhibit pH-dependent charge density. The salient feature of the modeling is to account for the explicit hydrogen ion concentration in the corresponding electrostatics of the electric double layer (EDL) induced at the PE-brush-electrolyte interface. Results indicate profound influences of the hydrogen ion concentration, ionization constant of the PE brushes, salt concentration, and degree of confinement introduced by the nano-channel height in the overall electrostatics of the PE brushes. Secondly, continuum-based modeling is conducted to study the transport in such pH-responsive PE-brush-grafted nano-channels by quantifying the corresponding electric-field-driven electroosmotic (EOS) transport and the ionic current in the decoupled regime. Results reveal highly dominant ionic current and tremendously suppressed electroosmotic transport — both these findings are massively significant in designing of highly efficient and programmable soft nano-channels for sensing ions and analytes. The last part of the thesis is focused in studying the nano-confined PE brushes in the coupled regime, i.e., where the elastic and the excluded volume effects interplay with the electrostatic effects to determine the overall brush behavior. Firstly, mean field theory models are developed to probe the electrostatics and configuration of PE brushes grafting the nano-channel inner walls. Results indicate highly non-intuitive swelling-shrinking behavior of end-charged brush, while for backbone-charged brush, one can always witness swelling behavior due to the electrostatic effect. Detailed free energy analysis is subsequently invoked in order to explain these non-trivial results for the end-charged brushes. Secondly, ionic current and EOS transport in these end/backbone-charged-PE-brush-grafted nano-channels, with the brushes being described in the coupled regime, has been probed. Results indicate a most remarkable enhancement in the strength of the EOS transport. It completely reverses the standard understanding that the EOS transport is invariably suppressed in PE-brush-grafted nano-channels owing to the additional drag introduced by the brushes. Finally, we further quantify how the salt concentration and pH values of electrolyte effects the ionic and EOS transport in nano-channels grafted with end/backbone-charged brushes. We anticipate that the findings of the present thesis will provide completely new perspectives in understanding several unknown facets of PE-grafted nano-channels. These facets will be pivotal in not only designing soft nano-channels with novel functionalities that can potentially be applied in several disciplines ranging from nanotechnology to biomedical and biochemical engineering, but will also provide important clues to decipher the behavior of a myriad of biological and chemical systems (e.g., PE-grafted nanoparticles, sheathed bacteria, phage viruses, etc.) that bear certain geometric and physical resemblances to the PE-grafted nano-channel system.