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Title: Computational electrohydrodynamics for fabricating polymer microstructures
Author: Tonry, Catherine Elizabeth Henzell
ISNI:       0000 0004 6494 4772
Awarding Body: University of Greenwich
Current Institution: University of Greenwich
Date of Award: 2015
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The aim of the work presented in this thesis is the development of two computational models of two processes that can be used to shape molten polymers on a micro-scale, namely Electrohydrodynamic Induced Patterning (EHDIP) and Electric Field Assisted Capillarity (EFAC). These related processes both use the dielectric forces at the interface between a polymer and another dielectric such as air. When the molten polymers are placed in a shaped electric field the imbalance in these dielectric forces causes the polymer to flow in a controlled way creating shapes in the polymer melt, this is the basis for the EHDIP process. The shaped electric field is controlled by the morphology of the top mask which acts as an electrode. This process is further extended by introducing a heavily wetted surface on the top mask which results in capillary forces that cause the polymer melt to coat the top mask creating a fully enclosed shape. This process can be used to create enclosed micro-channels or micro-capsules. Thus results and discussion presented herein highlight several possible application routes for industrial manufacturing. The process is discussed here for microstructures of 1 µm to 200 µm in size. The range at which the processes work is not fully understood, however the EHDIP process has been shown to work at a nanoscale producing structures around 100 nm in size. From a comprehensive literature review, the underlying theory and mechanisms of this process were identified and the governing equations derived. Computational models were developed based on the underlying physics. These models were initially developed in PHYSICA version 3g and later they were implemented into COMSOL Multiphysics as the latter proved to be more stable. The results from the computational models were compared to the limited experimental data available. The results from the computational models show that the mask shape was found to have the largest effect on the final structure of the shaped poly-mer. Due to capillary forces the shape of the microstructure at the top mask mimics the shape of the mask. In the lower section of the enclosed microstructure there is a force balance between surface tension, dielectric forces and internal pressure, giving a rounded morphology. Furthermore, by wetting the lower mask, flat bottomed structures can be produced. By both shaping and wetting the lower mask the shape of the microstructure can be even further modified. However, sharp cornered masks are unsuitable for this process. The effects of other key parameters such as air gap, contact angle, polymer permittivity and applied voltage were investigated through a sensitivity analysis. Changing the permittivity is shown to have an effect on the final microstructure. The change is small; however the permittivity does affect the speed of the process. The contact angle between the top mask and the polymer modifies the thickness of the polymer at the top of the structures. Increasing the contact angle causes a decrease in polymer thickness due to a reduction in the capillary force. The depth of the structures can be altered by changing the air gap; hence a larger air gap gives a deeper structure. The initial polymer thickness has no effect on the top of the structure but determines the thickness, shape and curvature of the lower part of the structure. The applied voltage controls the electrostatic forces and hence the speed of the process. For a low voltage the electrostatic forces are not strong enough to initiate the process and an enclosed microstructure does not form. If the voltage is too high, the structure forms quickly and bubbles can be entrapped at the top mask. With the correct mask shapes the processes can produce a wide variety of microstructures. These would have a wide range of applications either in the communications sector as fibre-optical wave-uides or in the biomedical sector as microstructures used in BioMEMS. Further development of the process is required to ensure that the process can be controlled. The models presented here are initial investigations of this but further experimental work is required along with the expansion of the model into three-dimensions.
Supervisor: Patel, Mayur ; Bailey, Christopher Sponsor: Engineering and Physical Sciences Research Council (EPSRC) ; University of Greenwich
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QA Mathematics