Electric field based fabrication methods for multi-scale structured surfaces

Abstract

Control of micro/nano scale surface structures and properties is crucial to developing novel functional materials. From an engineering point of view, the development of scalable and economical micro/nano-fabrication methods has been in high demand. In this dissertation, electrophoretic deposition (EPD) and breakdown anodization (BDA) are examined for their potential to produce multi-scale structured surfaces. EPD uses electrophoresis to deposit thin films of nanoparticles, dispersed in suspension, onto charged or porous substrates. Depending upon the dispersion stability, the surface roughness can be modulated in order to affect the resulting wettability. BDA can be utilized to alter surface features by employing instabilities during high voltage anodization, which lead to micro scale topography. Different microporous structures are generated depending on electric potential and electrolyte temperature during BDA. A hybrid method employing EPD and BDA results in hierarchical surface structures with both nano/micro scale features. In this work EPD and BDA are utilized for the development of superhydrophobic and superhydrophilic surfaces; sample applications include anti-wetting fabric, capillarity driven flow design, and critical heat flux enhancement. In many applications it is critical to understand how moving liquid water droplets will behave when they encounter these modified surfaces. We investigate drop impingement on porous thin films produced by BDA and EPD in order to understand the effects of surface structure and chemical properties on droplet dynamics. Using dimensional analysis we've discovered a novel dimensionless parameter, named the Washburn- Reynolds number, which can predict the droplet impingement modes. Intriguingly we've also discovered that under certain conditions drop impingement results in gas trapped in the spreading droplet, leading to the generation of aerosol above the droplet when the gas bubbles burst. The Washburn-Reynolds number also largely dictates the aerosol generation process. Our results inform the understanding of dynamic interactions between porous surfaces and liquid drops for applications ranging from droplet microfluidics to aerosol generators. In summary, EPD and BDA provide promising micro and nano-scale fabrication technologies with reasonable control of surface morphology and properties in a cost-effective and time-effective and scalable.