Skin friction reduction in tubes with hydrophobically structured surfaces

Skin friction drag is dominant in the total drag in a tube flow. In this study pore-type poly (dimethylsiloxane) microstructures are used to reduce the contact area between water and the tube wall. The water is lifted up by the microstructures with a hydrophobic nature so that the water flows on a solid/air composite surface. The solid/air composite surface results in skin friction reduction. The hydrophobicity of the tube wall surface is adjusted by the solid fraction of the surface. Three tube models with different hydrophobicities (contact angle: No Pore = 106°, Pore30 = 131°, Pore110 = 148°) were fabricated and their flow velocities according to the water height were measured. The water flow velocities in a tube with a higher contact angle were higher than those in a tube with a lower contact angle. The relative velocities of a Pore110 tube over a No Pore tube rapidly decrease as the water height increases, and they are saturated over Reynolds number(Re) = 1,400. On the other hand, the relative velocities of a Pore30 tube slowly decrease and they are saturated over Re = 900. A Pore110 tube with a superhydrophobic surface exhibited a high skin friction reduction in the low Reynolds number region (∼ 1,400). In the saturated relative velocity region, the velocities of the Pore110 and Pore30 tubes were on average 3.3% and 2.1% higher than that of the No Pore tube. These results indicate that a tube with a higher contact angle exhibits relatively lower skin friction and delays the velocity saturation point better than a tube with a lower contact angle in the laminar flow region.