A novel approach for drag reduction using polymer coating

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 x 103, 11 x 103, and 16 x 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 x 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.

Automated summary

A novel coating that bonds drag-reducing polymers to metallic surfaces has been developed, achieving drag reduction by slowly dissolving in water. Maximum drag reduction was achieved with an optimum deposition of APAM at high Reynolds numbers, although the reduction gradually diminished over time.