Drag Reduction by Air Layer on Superhydrophobic Surface in a Taylor-Couette Flow

When a superhydrophobic surface (SHPS) is immersed to an aqueous solution, an air layer is often formed depending on the surface characteristics. A slip condition formed at the interface of the air and the solution is known to reduce skin friction effectively. However, many previous studies on the drag reduction have been conducted at a low Reynolds number (Re) or in a narrow range of Re. We developed a Taylor-Couette flow device for precise and detailed measurements of torque and velocity vectors for Re ranging from 1,000 through 20,000 using a single working fluid. We also developed a SHPS with a porous structure that can retain an air layer against a high shear. Detailed results of torque measurements and preliminary velocity field data obtained by particle image velocimeter will be presented. In addition, we will discuss the drag reduction capabilities of the SHPS with increasing Re and compare mean and turbulent properties of the T-C flows with and without the SHPS.