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Surfactant molecules can self-assemble into micellar networks and be used as efficient additives in turbulent drag reduction in aqueous solutions. The mechanism of turbulent drag reduction by these percolating structures is still unclear. In particular, the “viscoelasticity theory” postulates that the micellar network structures are capable of absorbing and releasing stress from turbulent kinetic energy which results in the decrease of the energy dissipation. Here, we focus on the single wormlike surfactant micelle which consists of cetyltrimethylammonium chloride (CTAC) and counter ion salts to study the energy variation during its deformation processes using MARTINI force field coarse-grained molecular dynamics (CGMD) simulations. The Muller-Plathe method was used to generate the small and large deformation stretching statuses of the wormlike micelle through adjusting the momenta exchange frequency. The simulation results show that the micelle can transfer its potential energy to water during its relaxation stage after being stretched. It is also found that the flow field of water is significantly influenced by the recoil of micelle and a vortex is observed in the area semi-enclosed by the bending micelle. Overall, the present study supports the hypothesis of “viscoelasticity theory” and provides further insight into the mechanism of turbulent drag reduction by surfactant additives.

The interactions of cationic surfactant cetyltrimethyl ammonium chloride (CTAC) and polyacrylamide (PAM) in drag reduction solutions are studied by experiments and simulations. Cryogenic Transmission Electron Microscopy shows that CTAC and PAM form aggregates with interconnected network structures, and the bridge effect from polymers can improve the network structures’ density. The associated potential of mean force (PMF) value indicates that surfactant/polymer attractions contribute to the formation of aggregates. Meanwhile, the mechanism of the enhanced drag reduction performance of surfactant/polymer solution is discussed by the extension energy of the aggregate structures. It is found that the interactions of surfactants and polymers help to improve the anti-shear performance of the surfactant micelles. Furthermore, polymers can balance micelles energy distribution, postpone the appearance of energy extreme point, and delay the rupture of micelles. This work fundamentally demonstrates that interactions and strengthened structures of micelle aggregates can intensify drag reduction in surfactant/polymer solutions.

Drag-reducing surfactant solution can provide a large-eddy environment for longitudinal microgrooves and may realize the complementarity between their drag-reduction mechanisms. In this work, the collaborative drag reduction performance of surfactant solution and longitudinal microgrooves was experimentally studied to verify the speculation about their complementary possibility. The mixture aqueous solution of cationic surfactant (cetyltrimethyl ammonium chloride) and counterion salt (NaSal) was tested in the smooth and two longitudinal microgroove channels respectively at the mass concentrations of 0.16–0.47 mmol/L. It was found that the drag reduction performance of surfactant solution was enhanced by the longitudinal microgrooves. The drag-reduction mechanisms of microgrooves in water and surfactant solution were illustrated by the competition between the “peak effect” and the “restriction effect” of microgroove. Moreover, the “second peak effect” was proposed to explain the drag-reduction enhancement mechanisms for surfactant flow in microgroove channels. The groove with a larger size and roughness which might increase the drag in water could still enhance the drag reduction effectiveness of surfactant flow, and had a lower critical temperature and critical Reynolds number in surfactant solution, indicating a promising application in the heat transfer and drag reduction field. Moreover, the results of particle image velocimetry of smooth channel indirectly verified that the drag-reducing mechanism of microgroove was related to the turbulent vortex scale and the restriction effect on near-wall vortices.

Surfactant drag reduction has wide applications in energy power and chemical industries. However, the drag reduction ability will be weakened or lost during long-term applications. We studied the surfactant drag reduction in turbulent pipe flow using coarse-grained molecular dynamics simulations and cryogenic transmission electron microscopy experiments. The results show that the weakening/loss of surfactant drag reduction ability occurs under the following conditions: weak alkalinity of the solution, tiny rust particles in the solution, and/or the rusty wall of the pipeline. Weak alkalinity conduces to irregular structures of surfactant micelles, charged tiny iron rust particles can adsorb the surfactant micelles and fill into micelle network gaps, and the rusty wall could adsorb part of the surfactant micelles and lessen the global micelle network structure. All the three can destroy the micelle network structure and result in partial or complete failure of the turbulent drag reduction.