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Electrohydrodynamic and Thermocapillary Instability of Thin Liquid Films
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- Author / Creator
- Nazaripoor, Hadi
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Electrically induced instability of thin liquid films is a contactless pattern transfer method, often called electrohydrodynamic (EHD) lithography, which has gained extensive attention due to its ability in creation of novel micro- and nano-sized structures. The need for powerful microprocessors and cheaper electronic memory has accelerated the research effort on finding novel, fast and inexpensive techniques for creation of nano-sized features. An electrostatic model is developed called an ionic liquid (IL) model which consider a finite diffuse electric layer with comparable thickness to the film. This overcomes the shortcoming of assuming infinitesimally large and small electric diffuse layer inherent in the perfect dielectric (PD) and leaky dielectric (LD) models respectively. The process of pattern formation is then numerically simulated by solving the nonlinear thin film equation using finite difference for the spatial domain and an adaptive time step solver for time. In single layer film, the total number of pillars formed (raised columnar structures called pillars) in 1 µm2 area of the domain in IL film is almost 5 times more than similar PD film for the conditions simulated. Replacing the flat electrode with the patterned one is found to result in more compact and well-ordered structures particularly when an electrode with square block protrusions is used. Structure size in PD films is reduced by a factor of four when it is replaced with IL films which results in nano-sized features with well-ordered pattern over the domain. In bilayer systems, an extensive numerical study is carried out to generate a map based on electric permittivity ratio of layers and the initial mean thickness of the lower layer. This map is used to predict the formation of various structures on PD-PD bilayers interface and provides a baseline for unstable IL-PD bilayers. The use of an IL layer is found to reduce the size of the structures, but results in polydispersed and disordered pillars spread over the domain. The numerical predictions follow similar trend of experimental observation in literature. To improve the electrically assisted patterning process and create smaller sized features with the higher active surface area, the combined thermocapillary (TC) and EHD instability of liquid nanofilms is considered using both linear stability and nonlinear analysis. The number density of pillars formed in 1 µm2 area is significantly increased compared to the EHD base-case and nano-sized pillars are created due to the TC effects. Relative interface area increases of up to 18% due to this pattern miniaturization are realized. It is also found that increase in the thermal conductivity ratio of layers changes the mechanism of pattern formation resulting in non-uniform and randomly distributed micro pillars being generated.
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- Graduation date
- Spring 2017
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- Type of Item
- Thesis
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- Degree
- Doctor of Philosophy
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- License
- This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.