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Title: High resolution additive manufacturing using electrohydrodynamic jet printing
Author: Mkhize, Nhlakanipho
ISNI:       0000 0004 8503 5607
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2019
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Electrohydrodynamic jet printing (EHD) has emerged in recent years as one of the most competitive high resolution, non-contact additive manufacturing techniques. EHD has been used to achieve fine structures with high aspect ratios in fields ranging from micro and nano-electronics to biomedical engineering and more. However, in order to achieve consistent and reliable printing, it is crucial to have a solid fundamental understanding of the properties of the materials used, as well as the parameters implemented whilst printing. Owing to the hydrodynamic instabilities associated with liquid jets, EHD is particularly sensitive to the properties of the ink used, the substrate being printed on, the environmental conditions and the nozzle configuration. In this thesis, I explore all of the aforementioned parameters in relation to EHD printing and highlight their physical significance and implications. Having achieved a comprehensive understanding of mechanisms relevant to EHD, I proceed to reduce the principles of EHD to practice by then building a printing system and characterizing its operation through various patterning and feature characterization experiments. Once satisfied that the system was operational, I proceed to answer three broad yet fundamental questions. Is it possible to achieve superior speed and resolution printing of functional inks? How does the electric field generated during EHD printing interact with agents used to control the wetting of surfaces on which printing is being done? Can EHD printing result in the improved performance of existing technologies, specifically looking at gas sensors? In the process of addressing the above, I demonstrate the ability to print a high resolution molecular template of up to 300 nm in width for subsequent nanoparticle assembly which could enable the design of nanoparticle electronics. The printing of such templates overcomes the disadvantages associated with contact printing, including the inflexibility of the design process and subsequent layer damage. I provide evidence of the deformational character of electric fields on the self-assembled monolayers used to modify surfaces. The integrity of the monolayers is crucial in the pursuit of high resolution EHD printing, but the influence of the electric field inherent with the process has not been characterized before. I use Kelvin Probe Microscopy to determine the work function imparted by self-assembled monolayers, both before and after an electric field is applied over the coated surface. Static contact angle measurements are used to measure the macroscopic effect of the field on the monolayers. Using computational modelling, the influence of the field is probed more finely. I find that the field does deform the conformation of the monolayers, and this degree of deformation is dependent on the magnitude of the monolayer molecule's dipole moment. Further, I demonstrate how the resolution of printing can be controlled on flexible substrates with a facile physical method. By inducing the formation of wrinkles on an elastomer via the deposition of a stiff thin film, anisotropic patterns are formed. The wavelength and amplitude of these wrinkles are governed by the thin film thickness. I find that coupling fast printing with wrinkles of high amplitude results in good resolution printing, compared to thinner films, or bare elastomer surfaces. Finally, I demonstrate how the discrete printing of chemiresistors using EHD can lead to the enhancement of ammonia sensing compared to dropcast thin film sensors. I show a more efficient sensing geometry on a substrate with interdigitated electrodes using doped polyaniline as the active material. By optimizing the dopant acid for the polyaniline sensing material, sensitivity of up to 200 ppb is demonstrated, far below the safe exposure limit of 25 ppm. Further, I demonstrate the printing of the polyaniline based sensor onto flexible substrates, demonstrating the versatility of EHD printing.
Supervisor: Bhaskaran, Harish Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available