Advection of shear-induced surfactant threads and turbulent drag reduction

Surfactant solutions containing wormlike micelles often show complex structural changes in laminar shear flow. These structural changes produce anomalous rheological phenomena in which the shear induced structure (hereafter SIS) increases the viscosity of surfactant solutions. In turbulent shear flow, dilute solutions of 5 nm diameter and micron length wormlike micelles reduce frictional drag. It has been speculated that this drag reduction is related to wormlike micelles or SIS, although to date there have been no studies that have made their direct link to drag reduction in turbulent flow. The mechanism of drag reduction and the role of the micellar structure remain unclear. Here, we identify SIS with a long (several centimeters), threadlike structure. The threads form a gel-like phase under shear, which is clearly separated from the surrounding fluid. The threads breakdown at high shear stresses and disappear at the same time as the drag reduction does. The correlation between the existence of surfactant threads and turbulent drag reduction is discussed. (C) 2013 The Society of Rheology.