Investigation of retention of gases in transverse hydrophobic microgrooved surfaces for drag reduction

• A dense and deep HTM surface was designed to achieve stable entrapped gas.
• We observed the entrapped gas by direct optical measurements.
• A persistent drag reducing efficiency, more than 13%, was achieved.
• The mechanism of reducing drag was attributed to the stable entrapped gas.

The drag reduction effect of superhydrophobic surface induced by trapped air is substantially weakened with water immersion time. The gradual disappearance of the trapped air pockets is known as the reason for this time dependent drag reduction effect. In this study, the transverse grates were designed to be dense and deep to sustain air pockets in the gaps of hydrophobic grooves for a long time. Direct optical measurements were conducted to observe the entrapped gas when water flowed over the surface in the perpendicular direction of grating pattern. Visualization of gas indicated that the gas could be hold in the designed structures within the water flowing time. When grooves were optimized, a drag reducing efficiency of more than 13% was achieved, which did not vary during the test lasting 1 h. The drag reduction mechanism of this specially designed surface was attributed to an “effective” slip which was generated by the steady gas in the microgrooves underwater.