PhD Oral Exam - Firoozeh Yeganehdoust, Mechanical Engineering
A Comparative Study on the Performance of Bio-inspired Slippery Lubricant Impregnated and Superhydrophobic surfaces in Droplet Impact and Shedding
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School of Graduate Studies
When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.
Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.
The development of robust surfaces that repel liquid droplets has a broad impact on enhancing surfaces in several applications of anti-icing/fogging, anti-biofouling, and anti-dewing. In superhydrophobic surfaces, the distinct physical configurations of the surface causes a water droplet to roll off easily. However, these surfaces may be ineffective when they are used in harsh environments. Inspiration from the natural non-wetting surfaces of the Lotus leaf and the slippery surface of the pitcher plant has led to the development of extremely slippery surfaces known as slippery lubricant impregnated surfaces (SLIPS/LIS) that have recently shown great promise. They are designed by creating micro/nano-structured surfaces that are infused with a low surface tension lubricant which can enhance droplet mobility for a wide range of liquids with low surface tension properties. Designing slippery lubricant impregnated surfaces is still ongoing research and it is more complicated compared to superhydrophobic surfaces as it involves several parameters. The physics of the impact and shedding of the liquid droplet on slippery surfaces remains elusive. In this regard, both experimental and numerical tools have been used in this work to explore the associated physics in slippery surfaces and their advantages and disadvantages compared to superhydrophobic surfaces.
The first goal was to evaluate the effect of an immiscible lubricant with different thicknesses on the impact of a millimeter-sized water droplet for different impact velocities. A three-phase flow numerical simulation based on a finite volume solution coupled with a volume of fluid method has been implemented. The numerical model showed that droplet spontaneous bouncing occurred due to the air entrapment because of the deformation in both droplet and liquid film surface in which the details of the gas layer thickness and dynamics of fluid motions were illustrated. It is observed that a liquid film surfaces can enhance the probability of droplet spontaneous bouncing.
The performance of superhydrophobic surfaces might fail their properties for micro-scale droplets as both the micro-structured surfaces and droplets are on the same scale. Accordingly, the anti-wetting performance of slippery lubricant impregnated surfaces compared to superhydrophobic surfaces for different surface morphologies have been numerically investigated during droplet impingement. The effect of the surface structure has been studied by considering different series of square-pillar arrays. To demonstrate the hydrodynamics of the three-phases of water droplet/lubricant/air during the impingement, a three-phase flow solver in conjunction with an accurate contact angle method has been implemented. It was observed that slippery surfaces with low--density micro-textured surfaces enhanced droplet mobility and repellency compared to superhydrophobic surfaces. Additionally, based on the quantitative characterization, the effect of droplet pinning decreased significantly compared to superhydrophobic surfaces.
In order to assess the mobility of droplets under the effect of air shear flow on slippery surfaces and superhydrophobic surfaces, an experimental study in conjunction with a numerical model has been conducted. Two different liquid oils have been considered to better examine the effect of lubricant viscosity on droplet movement. It is observed that the hydrodynamics of droplet motion is completely different on superhydrophobic compared to slippery surfaces. The wetting length and position of a droplet on all surfaces have been measured. Although similar trends are observed in quantitative measurements for slippery surfaces, the speed of droplets is greatly affected by the lubricant properties. A numerical simulation based on the VOF method coupled with the Large Eddy Simulation turbulent model in conjunction with the dynamic contact angle method has been used. A developed boundary condition is also implement to consider the effect of lubricant on slippery surfaces. The numerical simulations are compared with the experimental study to provide further information on the experimental results.