Novel Passive and Active Approaches to Fluid Friction Reduction using Polymers & Plastrons

When a superhydrophobic (SH) textured surface is immersed in water, it traps a thin shiny layer of air within the texture that is known as a plastron. Contact line pinning stabilizes this Cassie-Baxter state and the patches of air trapped in the texture can act as shear-free regions that locally lower the frictional dissipation. Recent developments of scalable manufacturing methods for producing robust SH surfaces open new possibilities for using these textures in drag reduction applications. We use a newly-developed bespoke Taylor-Couette (TC) apparatus to compare and contrast frictional drag reduction by dilute polymer solutions, and superhydrophobic (SH) surfaces in turbulent flows for Reynolds numbers over a wide range (10,000 < Re < 100,000). By applying SH coatings to the inner rotating cylinder, we can evaluate the drag-reducing performance and robustness of different textures and calculate the effective slip length in turbulent flow using a suitably-modified Prandtl-von Kármán (PvK) analysis. We also investigate how these plastrons can be partially stabilized against turbulent pressure fluctuations using active heating as well as chemical methods to locally regenerate vapor. Additionally, we can use our apparatus to revisit an alternative drag-reducing strategy using dilute solutions of various high molecular weight polymers. We show that natural polysaccharides derived from plant mucilage can be an inexpensive and effective alternative to costly synthetic polymers, whilst still approaching the same maximum drag reduction (MDR) asymptote. Finally, we explore combinations of these two complementary drag reduction methods – one arising from wall slip and the other due to changes in turbulence dynamics in the bulk flow – and find that the two effects are not always additive; interestingly, the effectiveness of polymer drag reduction can actually be reduced in the presence of a SH coating on the wall.