Surface modification to control drag is a topic of interest for a variety of civilian and military applications. By decreasing the drag across a surface caused by fluid flow, the required amount of energy needed to accomplish a task can be reduced, leading to significant financial gains. To reduce the frictional drag experienced by a surface due to fluid flow, a slip flow boundary condition is desired. A slip flow boundary condition is where fluid velocity at the wall is non-zero. While several methods have been used to control drag, superhydrophobic (water repellant) surfaces have been proven as an effective method for passively reducing frictional drag. Superhydrophobic surfaces are sought after in drag reduction research due to their ability to form an air layer (plastron) when submerged in water. The plastron acts as an air bearing between the fluid and surface to create a slip velocity at the interface.
Femtosecond laser surface processing (FLSP) can transition surfaces from superhydrophilic (water-attracting) to superhydrophobic (water-repelling) by coating the multiscale structures with a low surface energy material. Laminar (low Reynolds number) and turbulent (high Reynolds number) flow loops, shown below, are utilized in the Multiscale Heat Transfer Laboratory (MHTL) to measure the pressure drop and flow rate along a rectangular channel of varying height. Polished and FLSP-processed surfaces can be used on the top and bottom plates of the channel to measure the reduction in drag as a result of surface modification. Drag reduction up to 31% has been achieved using superhydrophobic FLSP surfaces in the laminar regime. We are currently researching drag reduction for superhydrophobic FLSP surfaces in the turbulent regime.
Laminar (a) and Turbulent (b) flow loop.