A novel approach for drag reduction using polymer coating

Abstract Polymer drag reduction (DR) remains challenging for marine applications due to the difficulties in introducing the polymer drag-reducer into external flows. We developed a novel coating that bonds drag-reducing polymers to metallic surfaces. The coating consists of a polydopamine (PDA) layer that can attach to any substrate. A layer of anionic polyacrylamide (APAM), which is a drag reducing agent, is then grafted on the PDA-coated surface. Owing to the covalent bond, the long chains of APAM polymer slowly dissolve from the PDA-coated surface into water. The drag of surfaces with 0.1–0.8 mg cm−2 of deposited APAM at three Reynolds numbers, Re, of 8 × 103, 11 × 103, and 16 × 103 were investigated in a turbulent channel flow. The maximum amount and duration of DR was achieved for an optimum APAM deposition of 0.4 mg cm−2 at Re of 16 × 103. A large DR of 19% occurred in the first 10 min of the tests, and then gradually reduced to zero within an hour. We also observed that increasing Re resulted in a greater initial DR that spanned over a shorter duration, potentially due to a faster dissolving rate of the polymer coating.