Rheological Drag Reduction in Turbulent Taylor-Couette Flow using Polymers and Surfactants

It is well known that adding a small amount of long-chain polymers or surfactants to a turbulent liquid wall flow can produce a significant reduction in skin-friction drag. However, despite decades of research, the rheological mechanisms behind drag reduction (DR) remain unclear. This study links the rheology of dilute polymer and surfactant solutions to their drag-reducing performance in a custom turbulent Taylor-Couette flow (TTCF), where DR is quantified by the reduction in torque needed to maintain rotor speed. Aqueous solutions of flexible polymers (polyethylene oxide, polyacrylamide) and surfactants (tetradecyltrimethylammonium salicylate) are tested across a wide range of Reynolds numbers (1,000–100,000), spanning flow regimes from turbulent Taylor vortex flow to featureless turbulence. Rheological properties are measured using microfluidic viscometry and dripping-onto-substrate (DoS) extensional rheometry. Polymer solutions show large extensional relaxation times (>1 ms) and up to 14% DR. Surfactant solutions, at an exceptionally small concentration of 200 ppm — twice the critical micelle concentration (CMC) — achieve up to 26% DR, despite having no measurable extensional relaxation times from DoS rheometry and a Newtonian, water-like shear viscosity. These findings reveal that, while polymers and surfactants reduce drag through fundamentally different rheological mechanisms, surfactants offer a more effective and efficient route to turbulent drag reduction.