Introduction
A spectacular reduction of energy losses in turbulent flows can be achieved by the addition of small amounts of certain polymers or surfactants [1, 2]. For polymers, drag reduction is due to the large elongational viscosity of the polymer solution; this stabilizes the turbulent boundary layer, leading to less turbulent energy generation, and hence less dissipation [3,4]. For surfactants, the phenomenon is still ill understood, in spite of the enormous attention the subject attracted over the past few decades, and the fact that due to their reversible degradation of the aggregates, surfactant solutions are better adapted to industrial applications than polymers that degrade irreversibly [5]. Surfactant drag reduction is uniquely found for systems forming wormlike micelles. These micelles are self-assembled structures in which the surfactants form tubular structures with a diameter roughly twice the size of a surfactant molecule and a length that can be thousands of times the molecular size. The flow behavior of dilute solutions of such wormlike micelles in both laminar and turbulent flows can be very different from that of the solvent alone, even at very low concentrations of surfactant [2, 6, 7].
