UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a given thesis/dissertation in DRUM.
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Item ATOMISTIC EXPLORATION OF DENSELY-GRAFTED POLYELECTROLYTE BRUSHES: EFFECT OF APPLIED ELECTRIC FIELD AND MULTIVALENT SCREENING COUNTERIONS(2022) Pial, Md Turash Haque; Das, Siddhartha; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Polyelectrolyte (PE) or charged polymers are ubiquitous under biological and synthetic conditions, ranging from DNA to advanced technologies. PE chains can be grafted on a surface and they extend into solution to form a "brush"-like configuration if the grafting density is high. PE brushes respond to external stimuli by changing their conformation and chemical details, which make them very attractive for numerous applications. Multivalent counterions (neutralizing PE charges) and external electric fields are known to significantly affect the brush behavior. Obtaining fundamental insights into PE brush’s response to ions and electric filed is of utmost importance for both industrial and academic research. In this dissertation, we use atomistic tools to improve our understanding of the PE brushes grafted on a single surface and two inner walls of a nanochannel under these two stimuli.We start by developing an all-atom molecular dynamics simulation framework to test the behavior of the PE brushes (grafted on a single surface) in the presence of externally applied electric fields. It is discovered that the charge density of PE monomers can have significant influence on their response; a smaller monomer charge density helps the brush to tilts along the electric field, while the PE brush with higher monomer charge density bends and shrinks. We found that counterion condensation to PE chains has a substantial impact in controlling these responses. In the subsequent study we discuss the effect of counterion size and valence in dictating counterion mediated bridging interaction of two or more negative monomers. By examining the solvation behavior, we identify that bridging interactions are not a sole function of the counterion valence. Rather, it depends on the counterion condensation on the PE chain, as well as the size of the counterion solvation shell. We also test the dynamic properties of the counterions and associated bridges. Later, we proceeded to simulate PE brush-grafted nanochannels to explore equilibrium and flow behavior in presence of nanoconfinement. We identify the onset of overscreening: there are a greater number of coions than counterions in the bulk liquid outside the brush layer. This specific ion distribution ensures that the overall electroosmotic flow is along the direction of the coions. Furthermore, for a large electric field, some of the counterions leave the PE brush layer into the bulk, resulting in disappearance of overscreening. If the number of counterions is greater than coions, electroosmotic flow reverses its direction and follows the motion of counterions. Finally, we discover that counterion-monomer interactions control the ion distribution. As a result, a diverse range of electroosmotic flow is found for counterions with different valence and size.Item SEPARATIONS OF WATER-IN-OIL EMULSIONS BY ELECTROSTATIC FIELD AT THE ELEVATED TEMPERATURE(2019) Lee, Hak Seung; Yang, Bao; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Separation of water from oil has been a significant subject for a crude-oil purification in the petroleum industry and chemical processing. This dissertation reports theoretical and experimental studies on separation of water from water-in-oil emulsions under combined treatment of a radial electric field and elevated temperature. Compared to macroemulsion, there are fundamental differences when considering microemulsion. It is difficult to separate microemulsion by an electric field due to its tiny droplet size. Microemulsion can be transformed to macroemulsion state over the cloud point. Therefore, heating is applied to the microemulsion to change its phase, then an electric field is applied to the system to expedite the separation speed. The enhanced separation performance using the combined method can be mainly attributed to the reduced viscosity and dissociated surfactants at elevated temperature and the accelerated droplet coalescence under the radial electric field. Theoretical analysis shows that temperature and electric strength can strongly affect the movement of the water droplets, and these effects are also experimentally validated by water/oil separation tests. In our experiments, a cylindrical cone-shaped separation tube equipped with coaxial cylindrical electrodes was built to improve the separation using a non-uniform radial electric field. From the result of the numerical calculations, the precipitation and collision times of water droplets are rapidly reduced as the operating temperature increases. This theoretically expected values accurately predict the experimental results of the water-in-oil emulsion separation tests. It is experimentally observed that increasing the applied voltage and/or temperature can significantly reduce the separation time and the residual water concentration in the emulsion. Lastly, the dynamic water/oil separation system was designed using the combined method of heating and electric field. The effects of operating temperature and flow rate on the quality of the separated oil are investigated by executing several tests at different temperatures and flow rates. From the results of the dynamic separation tests, the residual water concentration in the separated oil reduces as the operating temperature increases under the boiling point. Also, the water concentration of the separated oil increases as the flow rate increases due to the reduced residence time.