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Abstract

Superhydrophobic drag reduction has been a topic of research for the past two decades, but seldom explored is superoleophobic drag reduction. Oils encompass a large range of liquids that have varying liquid properties such as chemical species, surface tensions, and viscosities, and require a more thorough surface design to effectively repel. The defining property for drag reduction is the slip length which is theorized to increase proportionally with increasing fluid viscosity. However, oils typically violate this expected trend. In this work, a superoleophobic surface is fabricated from aluminum by generating a hierarchical structure from a wet chemical etch process followed by a boiling water treatment. The hierarchical structure is then coated in a fluorinated brush coating perfluoropolyether which provides the repellency characteristics. The repellency characteristics were studied using contact angle goniometry and the fabricated surface was found to be oleophobic down to a surface tension of ~38 mN/m. Drag reduction was evaluated against five probe liquids, polyethylene glycol 200 and 400, castor oil, glycerin, and polybutenes achieving approximately 6% drag reduction using a parallel plate rheometer system. Furthermore, the investigation of drag reduction when using a smooth surface as the reference case, as is typically done, versus using the wetted fabricated surface was explored. Slip length was evaluated by performing a linear regression over three gap heights for each of the probe liquids. Determining a trend in slip lengths from the measured results proved difficult and three scenarios were developed that provided varying slip lengths based on the measured data used. As the surface tensions and viscosities of the liquids varied, there was no discernible trend observed. This highlights the difficulty in developing a theoretical framework for a varying liquid group such as oils. Despite the difficulties encountered, the surface adequately reduced drag and was capable of maintaining a plastron with low surface tension liquids throughout repeated testing, with each test extending over 30 minutes.