Electrohydrodynamic (EHD) micropumps have been developed and used in many diverse applications such as in microscale liquid cooling and various microfluidic systems. The objective of this research is to investigate different methods of enhancing the performance of ion drag EHD micropumps. In particular, the effect of electrode surface topology, applied electric field and doping agent in the dielectric liquid were investigated. The effect of 3D sharp features on the electrodes on charge injection in HFE 7100 as dielectric fluid was studied under an applied DC electric field. Micro and nano-scale features with high aspect ratio were developed on smooth copper electrodes by chemical etching or through electrophoretic deposition of single walled carbon nanotube (SWCNT). The spacing between the electrodes was kept at 250 µm. A reduction factor of 5 was achieved for SWCNT electrodes compared to the smooth case for the onset of charge injection. This study was then extended to determine its effects on the performance of ion drag EHD micropumps with 100 pairs of interdigitated electrodes. The emitter electrodes (20 µm) were half the width of the collector electrodes (40 µm), with one pump having an inter-electrode spacing of 120 µm and the other with 40 µm. Each micropump had a width of 5 mm and a height of 100 µm. SWCNT was deposited on the emitter electrodes of the micropump to generate a maximum static pressure of 4.7 kPa at 900 V, which is a 5 fold increase compared to the pump with smooth electrodes. Flow rate at no back pressure condition was improved by a factor of 3. The effect of Ferrocene as a doping agent in the working fluid HFE 7100 was studied under DC voltages. A maximum static pressure of 6.7 kPa was achieved at 700 V with 0.2% weight based doping agent, 11 times higher than when there was no doping agent at the same applied voltage. When there was no back pressure the pump generated a maximum flow rate of 0.47 mL/min at 700 V with 0.05% doping agent which is 9 times greater than with no doping agent. The effect of pulsed voltage on the performance of ion drag EHD micropump has been studied to exploit the displacement current at the sudden change of applied voltage magnitude. A range of pulse repetition rate and duty cycle were found to significantly enhance the pump performance. Static pressure generation was up to 75% and 88% greater at an optimal pulse repetition rate and duty cycle, respectively, compared to the average of the two DC levels. The effect of external flow on the discharge characteristics of an injection micropump was studied with DC volts. Higher discharge current and lower threshold voltage for the onset of charge injection in case of co-flow compared to the static case was observed. There was an optimum flow rate to generate maximum current for both co and counter-flow cases. / Thesis / Doctor of Philosophy (PhD)

This work specifically aims to provide a fundamental framework, with some experimental validation, for understanding droplet emulsion dynamics in a microfluidic channel with an applied electric field. Electrification of fluids can result in several different modes of electrohydrodynamics (EHD). Several studies to date have provided theoretical, experimental, and numerical results for stationary droplet deformations and some flowing droplet configurations, but none have reported a method by which droplets of different diameters can be separated, binned and routed through the use of electric fields. It is therefore the goal of this work to fill that void and report a comprehensive understanding of how the electric field can affect flowing droplet dynamics. This work deals with two primary models used in electrohydrodynamics: the leaky dielectric model and the perfect dielectric model. The perfect dielectric model assumes that fluids with low conductivities do not react to any effects from the small amount of free charge they contain, and can be assumed as dielectrics, or electrical insulators. The leaky dielectric model suggests that even though the free charge is minimal in fluids with low conductivities, it is still is enough to affect droplet deformations. Finite element numerical results of stationary droplet deformations, implemented using the level set method, compare well both qualitatively (prolate/oblate and vortex directions), and quantitatively with results published by other researchers. Errors of less than 7.5% are found when comparing three-dimensional (3D) numerical results of this study to results predicted by the 3D leaky dielectric model, for a stationary high conductivity drop suspended in a slightly lower conductivity suspending medium. Droplet formations in a T-junction with no applied electric field are adequately predicted numerically using the level set finite element technique, as demonstrated by other researchers and verified in this study. For 3D models, droplet size is within 6%, and droplet production frequency is within 2.4% of experimental values found in the microfluidic Tjunction device. In order to reduce computational complexity, a larger scale model was solved first iii to obtain electrical potential distributions localized at the channel walls for the electrode placement configurations. Droplet deceleration and pinning is demonstrated, both experimentally and numerically, by applying steep gradients of electrical potential to the microchannel walls. As droplets flow over these electrical potential “steps,” they are pinned to the channel walls if the resulting electric forces are large enough to overcome the hydrodynamic forces. A balance between four dimensionless force ratios, the electric Euler number (Eue – ratio of inertial to electric forces), Mason number (M a – ratio of viscous to electric forces), electric pressure (P s – ratio of upstream pressure forces to electric forces), and the electric capillary number (Cae – ratio of electric to capillary forces) are used to quantify the magnitudes of each of these forces required to pin a droplet, and is consistent with a cubic dependency on the drop diameter. For larger drop diameters, effects of hydrodynamic forces become more prominent, and for smaller droplets, a greater electric forces is required due to the proximity of the droplet boundary with reference to the electrified channel wall. Droplet deceleration and pinning can be exploited to route droplets into different branches of a microfluidic T-junction. In addition, using steep electrical potential gradients placed strategically along a microchannel, droplets can even be passively binned by size into separate branches of the microfluidic device. These characteristics have been identified and demonstrated in this work.

Drying contributes a significantly important role in processing of agricultural products in Vietnam, particularly for high-value agricultural exports. Conventionally thermal-based drying techniques have remained critical disadvantages in term of enhancing product quality and process efficiency. The typical disadvantages include deterioration of organoleptic and nutritional properties, highenergy consumption, expensive costs yet low efficiency and hazards to environment change due to the consumption of fossil fuel sources. Electrohydrodynamics (EHD) drying technology has been demonstrated as an innovative solution for drying enhancement in various applications. This paper aims at an overview of the state-of-the-art EHD drying technology to enhance heat and mass transfer in agricultural drying processes. A case study is then presented to demonstrate an even better process efficiency, compared to the state-of-the-art EHD drying technology, and to shorten the gap “research-to-market”. Finally, this study shows obviously potential applications of this innovative technology in sustainable development of food and post-harvesting agricultural processing for Vietnam in the future. / Sấy khô đóng một vai trò quan trọng trong việc chế biến và bảo quản nông sản sau thu hoạch tại Việt Nam, đặc biệt đối với các mặt hàng nông sản cao cấp phục vụ xuất khẩu. Các phương pháp sấy khô bằng nhiệt truyền thống tồn tại nhiều nhược điểm trong việc nâng cao hiệu quả sấy và bảo đảm chất lượng nông sản. Những nhược điểm nổi bật bao gồm: biến đổi thành phần hóa học và giảm giá trị dinh dưỡng của nông sản sau sấy do sử dụng nhiệt trong quá trình sấy, tiêu hao nhiên liệu lớn, chi phí nhiên liệu, lắp đặt và duy trì hệ thống sấy cao nhưng hiệu suất thấp và đặc biệt ảnh hưởng tới môi trường do sử dụng các nguồn nhiên liệu hóa thạch. Công nghệ sấy cải tiến khíđiện động lực học (EHD) hiện tại đã cho thấy tiềm năng thay thế các phương pháp sấy truyền thống. Bài báo tập trung giới thiệu cơ chế sấy các sản phẩm nông sản bằng công nghệ EHD; qua đó tác giả giới thiệu một mô hình sấy đã chế tạo thành công cho hiệu quả thậm chí còn cao hơn các mô hình hiện tại, đồng thời dễ dàng triển khai trong ngành công nghiệp sấy khô nông sản. Nghiên cứu này chỉ ra tiềm năng ứng dụng to lớn của công nghệ EHD trong phát triển bền vững các quá trình sấy khô nông sản cao cấp tại Việt Nam trong tương lai.

Drying contributes a significantly important role in processing of agricultural products in Vietnam, particularly for high-value agricultural exports. Conventionally thermal-based drying techniques have remained critical disadvantages in term of enhancing product quality and process efficiency. The typical disadvantages include deterioration of organoleptic and nutritional properties, highenergy consumption, expensive costs yet low efficiency and hazards to environment change due to the consumption of fossil fuel sources. Electrohydrodynamics (EHD) drying technology has been demonstrated as an innovative solution for drying enhancement in various applications. This paper aims at an overview of the state-of-the-art EHD drying technology to enhance heat and mass transfer in agricultural drying processes. A case study is then presented to demonstrate an even better process efficiency, compared to the state-of-the-art EHD drying technology, and to shorten the gap “research-to-market”. Finally, this study shows obviously potential applications of this innovative technology in sustainable development of food and post-harvesting agricultural processing for Vietnam in the future. / Sấy khô đóng một vai trò quan trọng trong việc chế biến và bảo quản nông sản sau thu hoạch tại Việt Nam, đặc biệt đối với các mặt hàng nông sản cao cấp phục vụ xuất khẩu. Các phương pháp sấy khô bằng nhiệt truyền thống tồn tại nhiều nhược điểm trong việc nâng cao hiệu quả sấy và bảo đảm chất lượng nông sản. Những nhược điểm nổi bật bao gồm: biến đổi thành phần hóa học và giảm giá trị dinh dưỡng của nông sản sau sấy do sử dụng nhiệt trong quá trình sấy, tiêu hao nhiên liệu lớn, chi phí nhiên liệu, lắp đặt và duy trì hệ thống sấy cao nhưng hiệu suất thấp và đặc biệt ảnh hưởng tới môi trường do sử dụng các nguồn nhiên liệu hóa thạch. Công nghệ sấy cải tiến khíđiện động lực học (EHD) hiện tại đã cho thấy tiềm năng thay thế các phương pháp sấy truyền thống. Bài báo tập trung giới thiệu cơ chế sấy các sản phẩm nông sản bằng công nghệ EHD; qua đó tác giả giới thiệu một mô hình sấy đã chế tạo thành công cho hiệu quả thậm chí còn cao hơn các mô hình hiện tại, đồng thời dễ dàng triển khai trong ngành công nghiệp sấy khô nông sản. Nghiên cứu này chỉ ra tiềm năng ứng dụng to lớn của công nghệ EHD trong phát triển bền vững các quá trình sấy khô nông sản cao cấp tại Việt Nam trong tương lai.