UNS: Robust Superhydrophobic Surfaces for Enhanced Propulsive Performance and Maneuverability at Intermediate Reynolds Number

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#1510707Krueger, Paul S.The goal of this proposal is to improve propulsion by using superhydrophobic surfaces. A surface is defined as superhydrophobic when water droplets bead on the surface, and the angle between the drop and the surface is larger than 150 degrees. In cases like this, one can achieve drag reduction, since water slips on the superhydrophobic surface, and energy savings can be realized in propulsion. The introduction of apparent slip at the superhydrophobic surface using gas trapped near the surface by the microtexture of the surface can lead to drag reduction. However, it is known that such surfaces have their own challenges, including collapse of the gas film under increased hydrostatic or dynamic pressure, gradual depletion of the gas film by diffusion, and slip lengths limited by the spacing of the microtexture features. The proposed effort seeks to expand the applicability of superhydrophobic surfaces by pressurizing the gas film, which will allow for robust stabilization of the gas film against pressure disturbances and diffusion. Pneumatic stabilization has the additional advantages that changing the gas film pressure will allow mictotextured features to be spaced further apart, increasing the apparent slip, and dynamic variation of the pressure will allow for direct control of the surface behavior, including re-initialization of the gas film. Pneumatically controlled surfaces will be constructed from polydimethylsiloxane using laser micromachining and soft lithography techniques and tested on flapping-fin and pulsed-jet propulsion methods at intermediate Reynolds number (Re) flows. The effect of apparent slip introduced by the superhydrophobic surfaces on the propulsive performance of flow fields generated by flapping-fin and pulsed-jet propulsion will be investigated as a function of Re. The ability to dynamically actuate the gas film will also be investigated. A secondary outcome would be an improved technical capability in developing and sustaining superhydrophobic surfaces broadening the areas of their application to other cases of drag reduction and self-cleaning surfaces. Activities on undergraduate education through summer immersion were proposed and a web-based tutorial on mechanical and biological propulsion will be developed and made available to students and the community.