Turbulent drag reduction on superhydrophobic surfaces confirmed by built-in shear sensing

A drag reduction by superhydrophobic (SHPo) surfaces is experimentally confirmed for the first time in a turbulent boundary-layer (TBL) flow, which represents the flows around moving vessels. The drag reduction is measured by comparing the displacement of a SHPo surface with that of a smooth hydrophobic surface in a given flow. Since the two surfaces each with an identical set of suspending beams share the same processing conditions during the monolithic microfabrication and the same flow conditions during the TBL flow experiment, the rate of reduction is measured accurately with no bias errors. Furthermore, the air pockets on the SHPo surface are visually confirmed throughout the flow experiments to prevent any rogue surface condition affecting the measurement. The turbulent drag reduction is found to increase with the gas fraction of the SHPo surface, reaching a reduction of skin friction by as high as 70%.