Chemical and rheological characterization of drag-reducing cationic surfactant systems

Surfactant drag-reducing additives are very promising for saving pumping energy in recirculation systems such as district heating and cooling systems because of their "self-repairability" after mechanical degradation compared to polymer drag-reducing additives, which degrade irreversibly. The effectiveness of cationic surfactant drag-reducing additives, as indicated by their effective drag-reduction temperatures and Reynolds number ranges, depends on the chemical structures and concentrations of the surfactants and counterions. In this paper, the effects of the surfactant (alkyl chain length, saturated/unsaturated chain, odd/even numbers of carbons, and surfactant headgroup) and counterion (size, polarity, etc.) chemical structures on surfactant drag reduction, as well as three physical properties postulated to be associated with surfactant drag reduction, are addressed. Shear-induced structure (SIS), viscoelasticity (in the forms of nonzero first normal stress difference, quick recoil, and stress overshoot), and high extensional/ shear viscosity ratios are three rheological characteristics found in many drag-reducing surfactant solutions. It has frequently been postulated that they are vital to surfactant drag reduction. Rheological experimental data on some drag-reducing surfactant solutions indicate, however, that SIS and viscoelasticity are not vital for surfactant drag reduction whereas the criterion of high extensional/shear viscosity ratio might be valid.