High-shear drag reduction of surfactant solutions

• A counterion-enhanced surfactant shows turbulent drag reduction ability at very high shear.
• Decrease in drag reduction at high shear is a function of shear stress ratio.
• We model the drag reduction based on our experiments.

Drag reducing surfactants produce micellar structures. Over critical shear stress, the break-up rate of these structures tends to be faster than the formation rate. An excess of counterions raises the critical shear stress, and enhances surfactant drag reduction up to higher shear in turbulent flow. We investigated this high-shear drag reduction for small circular cross-section tubes. The enhancement became saturated when the counterion to surfactant molar concentration ratio was higher than ten. This saturated condition allowed a 30% drag reduction for a wall shear stress of 103 Pa (approximately 106 s−1 shear rate) for a 1500 ppm surfactant solution. The frictional drag of the tube flow was dependent on a Reynolds number at low shear and obeyed the friction formulas of a laminar pipe flow and a limiting drag reduction flow. Over critical shear stress, high shear decreased the drag reduction because of break-up of drag reducing micellar structures. This disappearance of the drag reduction was found to be a function of wall shear stress and independent of the tube diameters under saturated counterion-effect conditions. A phenomenological formula was introduced to describe the flow friction for these saturated counterion-effect conditions.