We identify the physical mechanism through which newly developed quaternary ammonium salt (QAS) deposit control additives (DCAs) affect the rheological properties of cavitating turbulent flows, resulting in an increase in the volumetric efficiency of clean injectors fuelled with diesel or biodiesel fuels. Quaternary ammonium surfactants with appropriate counterions can be very effective in reducing the turbulent drag in aqueous solutions, however, less is known about the effect of such surfactants in oil-based solvents or in cavitating flow conditions. Small-angle neutron scattering (SANS) investigations show that in traditional DCA fuel compositions only reverse spherical micelles form, whereas reverse cylindrical micelles are detected by blending the fuel with the QAS additive. Moreover, experiments utilising X-ray micro computed tomography (micro-CT) in nozzle replicas, quantify that in cavitation regions the liquid fraction is increased in the presence of the QAS additive. Furthermore, high-flux X-ray phase contrast imaging (XPCI) measurements identify a flow stabilization effect in the region of vortex cavitation by the QAS additive. The effect of the formation of cylindrical micelles is reproduced with computational fluid dynamics (CFD) simulations by including viscoelastic characteristics for the flow. It is demonstrated that viscoelasticity can reduce turbulence and suppress cavitation, and subsequently increase the injector’s volumetric efficiency.

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Turbulence and Cavitation Suppression by Quaternary Ammonium Salt Additives.

Preparation of Nanostructures by Orthogonal Self‐Assembly of Hydrogelators and Surfactants

In this review, we report recent developments on the shear-induced transitions and instabilities found in surfactant wormlike micelles. The survey focuses on the nonlinear shear rheology and covers a broad range of surfactant concentrations, from the dilute to the liquid-crystalline states and including the semidilute and concentrated regimes. Based on a systematic analysis of many surfactant systems, the present approach aims to identify the essential features of the transitions. It is suggested that these features define classes of behaviors. The review describes three types of transitions and/or instabilities: the shear-thickening found in the dilute regime, the shear-banding which is linked in some systems to the isotropic-to-nematic transition, and the flow-aligning and tumbling instabilities characteristic of nematic structures. In these three classes of behaviors, the shear-induced transitions are the result of a coupling between the internal structure of the fluid and the flow, resulting in a new mesoscopic organization under shear. This survey finally highlights the potential use of wormlike micelles as model systems for complex fluids and for applications.

https://www.springer.com/cda/content/document/cda_downloaddocument/9783642135316-c1.pdf?SGWID=0-0-45-967041-p174010264

Shear-Induced Transitions and Instabilities in Surfactant Wormlike Micelles

We have studied the effect of the lipophilicity of the counterions X- on the rheological properties of aqueous solutions of cetyltrimethylammonium-based surfactants (CTAX): cetyltrimethylammonium n-hexane sulfonate (CTAC6SO3), cetyltrimethylammonium n-heptane sulfonate (CTAC7SO3), and mixtures of cetyltrimethylammonium 3-hydroxy-2-naphthalenecarboxylate (CTAHNC) and cetyltrimethylammonium bromide (CTAB) in the molar ratio 1:1 and 2:1. Both linear and nonlinear viscoelasticity properties have been investigated as a function of surfactant volume fraction and temperature. For the system CTAHNC + CTAB (1:1), the effect of added salt (NaBr) was also studied. In the concentration and temperature ranges used in this study, all the systems exhibit a viscoelastic behavior characteristic of wormlike micellar solutions. By changing the lipophilic character of the counterion, one varies the net electrical charge carried by the micelle, which allows us to present a general description of the effect of the electrostati...

Effect of the Lipophilicity of the Counterion on the Viscoelasticity of Micellar Solutions of Cationic Surfactants

Surfactants, Micelles and Fascinating Phenomena

The first part of the work presents an overview of the physical chemistry of surfactants which in aqueous solutions reduce the frictional loss in turbulent pipe flow. It is shown that these surfactants form rodlike micelles above a characteristic concentraionc
t. The experimental evidence for rodlike micelles are reviewed and the prerequisites that the surfactant system must fulfill in order to form rodlike micelles are given. It is demonstrated by electrical conductivity measurements that the critical concentration for the formation of spherical micelles shows little temperature dependence, whereasc
t increases very rapidly with temperature. The length of the rodlike micelles, as determined by electric birefringence, decreases with rising temperature and increases with rising surfactant concentration. The dynamic processes in these micellar systems at rest and the influence of additives such as electrolytes and short chain alcohols are discussed. In the second part, the rheological behaviour of these surfactant solutions under laminar and turbulent flow conditions are investigated. Viscosity measurements in laminar pipe and Couette flow show the build-up of a shear induced viscoelastic state, SIS, from normal Newtonian fluid flow. A complete alignment of the rodlike micelles in the flow direction in the SIS was verified by flow birefringence. In turbulent pipe flow, drag reduction occurs in these surfactant systems as soon as rodlike micelles are present in the solution. The extent and type of drag reduction, i.e. the shape of the friction factor versus Reynolds number curve, depends directly on the size, number and surface charge of the rodlike micelles. The friction factor curve of each surfactant investigated changes in the same characteristic way as a function of temperature. For each surfactant, independent of concentration, an upper absolute temperature limit,T
L, for drag reduction exists which is caused by the micellar dynamics.T
L is influenced by the hydrophobic chain length and the counter-ion of the surfactant system. A first attempt is made to explain the drag reduction of surfactants by combining the results of these rheological measurements with the physico-chemical properties of the micellar systems.

W. Interthal

Papers

Surfactant systems for drag reduction: physico-chemical properties and rheological behaviour