Investigation on drag reduction of micro-structured PDMS film with self-replenishing bubble using water electrolysis

The "gas-liquid" interface at the boundary between air and water can reduce the frictional drag on the surface of ships and other navigation equipment, thereby increasing sailing speed and cruising range. However, practical implementation of this drag reduction (DR) approach is hindered by the high cost and difficulty in achieving the necessary persistent bubble stability and long-term effectiveness. This study addresses these issues by integrating bubble-based DR with electrolytic water reactions and micro-structured polydimethylsiloxane (PDMS) films. A reusable aluminum alloy mold featuring square micro-pillars was first fabricated via computer numerical control (CNC) micro-milling. Then, a simple three-step replication process transferred the microstructure to PDMS films, forming arrays of square micro-holes. These holes act as electrolysis channels, autonomously replenishing the gas-liquid interface by generating bubbles where depletion occurs. Treated PDMS films were adhered to circular metal samples for testing. Under static submerged condition, bubbles generated by 12V 1A electrolysis achieved full occupancy of micro-holes within 5 minutes, exhibiting independent growth per hole without cross-interference. DR performance was quantified on a rotating test platform under turbulent flow at six rotational speeds via torque measurements. Compared to smooth metal surfaces, the micro-structured film surface achieved a maximum drag reduction rate (DRR) of 7.19% at 100 r/min. This approach enables persistent, autonomous maintenance of the gas-liquid interface for effective turbulent friction reduction.