Drag reduction and air restoration of streamwise hydrophobic microtrenches

International seaborne trade carries over 80% of the global merchandise. Over 60% of the propulsive power is consumed by friction drag. Hydrophobic surfaces, which can trap air in their surface cavities, can effectively reduce the friction drag. However, despite many successful drag reductions reported in the existing works, the optimal dimensions and hydrophobicity of the surface pattern to obtain drag reduction, maintain air stability, and air restoration seem to remain unclear. In this paper, we investigated the drag reduction and air restoration of the streamwise hydrophobic microtrenches. By analyzing the turbulent boundary layer property and air stability, the dimensions and hydrophobicity of the microtrenches were carefully determined. Microtrenches with a dimensionless width of 10 and hydrophobic coating with a static contact angle of 110.2° were prepared. The water tunnel experiments showed that the drag reduction rate of 19.9% was obtained, the air in the microtrenches was mechanically stable, and the wetted microtrenches could automatically dewet when they touched the new air. Our work implies that the streamwise hydrophobic microtrenches are capable of sustaining an effective drag reduction at a low supply of air.