Experimental studies on drag reduction and rheology of mixed cationic surfactants with different alkyl chain lengths

Experimental studies of the effects of mixtures of cationic surfactants on their drag reduction and rheological behaviors are reported. Cationic alkyl trimethyl quaternary ammonium surfactants with alkyl chain lengths of C₁₂ and C₂₂ were mixed at different molar ratios (total surfactant concentrations were kept at 5 mM with 12.5 mM sodium salicylate (NaSal) as counterion). Drag reduction tests showed that by adding 10% (mol) of C₁₂, the effective drag reduction range expanded to 4–120 °C, compared with 80–130 °C with only the C₂₂ surfactant. Thus mixing cationic surfactants with different alkyl chain lengths is an effective way of tuning the drag reduction temperature range. Cryo-TEM micrographs revealed thread-like micellar networks for surfactant solutions in the drag reducing temperature range, while vesicles were the dominant microstructures at non-drag reducing temperatures. High extensional viscosity was the main rheological feature for all solutions except 50% C₁₂ (mol) solution, which also does not show strong viscoelasticity. It is not clear why this low extensional viscosity solution with relatively weak viscoelasticity is a good drag reducer. Received: 3 November 1999/Accepted: 5 January 2000     

Time-dependent drag reduction and ageing in aqueous solutions of a cationic surfactant

The turbulent pipe flow of a highly dilute aqueous cationic surfactant solution is investigated by means of a pulsed ultrasound Doppler method with special emphasis on the wall boundary layer. The velocity profiles are recorded for several Reynolds numbers at varying ages of the solution. The wall shear stress velocities u _τ used for the normalization of the velocity profiles are determined by fitting the measured profiles to the universal linear velocity profile in the viscous sublayer. The theoretical pressure loss is then calculated from the numerical values of u _τ and compared to the experimental values. Two different scaling methods are discussed for the velocity fluctuations concerning the correlation of the root-mean square values with the effect and the amount of drag reduction. It is shown that outer scaling with the mean velocity is appropriate for the detection of drag reduction in surfactant solutions, rather than inner scaling with the wall shear stress velocity, which is common practice in investigations of 'usual' turbulent flows.     

Formation of the shear-induced state in dilute cationic surfactant solutions

In cationic surfactant solutions a change of state occurs due to mechanical stresses. In the dilute regime of rodlike micelles the formation of a so-called Shear-Induced State (SIS) occurs above a critical shear rate. In this context dilute means that there is no sterical interaction between rodlike micelles, the solution is below the overlap concentration. Employing a mathematical model, it is shown that aggregation forces are weak compared to hydrodynamic forces. The mathematical formulation is based on a model of Israelachvili which describes the chemical potential of micelles. Hydrodynamic forces are calculated with a rigid-dumbbell model. SIS formation can be explained by the destruction of rodlike micelles.     

On the mechanism of drag reduction in dilute polymer solutions

Experimental evidence is given that drag reducing polymer molecules are preferentially collected by strained vortices. This can explain why extremely small amounts of additives can be so effective. They become concentrated in areas of a turbulent flow where they are most efficient.     

Effect of variations in counterion to surfactant ratio on rheology and microstructures of drag reducing cationic surfactant systems

Rheology, drag reduction and cryo-TEM experiments were performed on Arquad 16–50/NaSal and Ethoquad O/12/NaSal surfactant systems at different counterion-to-surfactant ratios and at constant low surfactant concentrations, 5 mM, appropriate for drag reduction. The molar ratio of counterion-to-surface was varied from 0.6 to 2.5. All the surfactant systems described here are viscoelastic and drag reducing. The viscoelasticity and drag reducing effectiveness increase with increase in counterion/surfactant ratio. Network are present in the solutions with high ratio, and they are viscoelastic. However, shear is needed to induce network formation for solutions at low ratio. Cryo-TEM images confirm the existence of thread-like micelles which form entanglement networks, and show that the micellar network becomes denser with increasing counterion/surfactant ratio in one surfactant series. Both increase in the counterion/surfactant ratio and increase in the shear rate result in shorter relaxation times. For some of these systems, abrupt increase in viscosity is observed at certain shear rates which are time effects affecting microstructure rearrangements rather than formation of shear induced structures.     

The velocity field of dilute cationic surfactant solutions in a Couette-viscometer

With the aid of particle image velocimetry (PIV) momentary two-dimensional velocity fields of dilute cationic surfactant solutions are measured in an annular gap between coaxial cylinders. In the shear-induced state the velocity profiles of the surfactant solutions differ from a Couette profile drastically: Time dependent flows with high shear rates near the walls are observed. The flow is almost one-dimensional, velocity components in radial or axial direction cannot be detected. The velocity profile can exhibit a local maximum. A macroscopic heterogeneous structure is observed, domains with high viscosity alternate with low viscosity regions. Elastic phenomena are observed in a relaxation experiment. Existing rheometric data of cationic surfactant solutions in the shear-induced state give only apparent viscosities.     

Degradation effects of dilute polymer solutions on turbulent drag reduction in pipe flows

An important practical problem in the application and study of drag reduction by polymer additives is the degradation of the polymer, for instance due to intense shearing, especially in recirculatory flow systems. Such degradation leads to a marked loss of the drag-reducing capability of the polymer.Three different polymer types were tested on degradation effects in a closed pipe flow system. The polymers used were Polyox WSR-301, Separan AP-273 and Superfloc A-110, dissolved in water in concentrations of 20 wppm each. The flow system consisted of a 16.3 mm pipe of 4.25 m length. Two different pumps were used: a centrifugal pump and a disc pump. Different solution-preparation procedures were tried and the experiments were performed at different flow rates.Superfloc A-110 proved to be both the most effective drag reducer and most resistant to degradation. Because of very fast degradation, Polyox WSR-301 was found to be unsuitable for being used as a drag reducer in re-circulatory systems. The disc pump proved to be much better suited for pumping the polymer solutions than the centrifugal pump. The degradation curve of the combination Superfloc/disc pump showed a plateau-like region with reasonable drag reduction, which makes it possible to perform (laser Doppler) measurements under nearly constant circumstances during a sufficient time.     

Kinetic theory and rheology of dilute polymer solutions

Summary A summary is given of some recent attempts to relate the results of the kinetic theory of rigid and flexible macromolecules to continuum mechanics results.With 1 table     

Extensional flow resistance of dilute polyacrylamide and surfactant solutions

The extensional flow behaviour of dilute aqueous solutions of a partiallyhy-drolyzed polyacrylamide and a surfactant were investigated in an extensional flow cell. The cell was designed such that fluids were subjected to steady shear before undergoing extensional motion in a converging channel. Extensional resistance was monitored by measuring the pressure drop through the channel. Such measurements were made over a range of extensional rates at fixed values of the upstream shear rate. Solutions of different concentrations were tested — up to 40 ppm of polyacrylamide and 450 ppm of surfactant — at various temperatures in the case of surfactant and for different types and amounts of salt in the case of polyacrylamide. Of the results, the more notable are that the extensional resistance of polyacrylamide solutions is affected much more by CaCl₂ than by NaCl and that surfactant solutions do not exhibit extensional resistance unless they are pre-sheared.     

Effects of chemical structures of para-halobenzoates on micelle nanostructure,drag reduction and rheological behaviors of dilute CTAC solutions

Counterion chemical structure and counterion to cationic surfactant molar ratio, ξ, control counterion binding, micelle nanostructures, drag reduction (DR) effectiveness and rheological behavior of quaternary ammonium surfactant systems. The effects of chemical structures of four sodium para-halobenzoate (F, Cl, Br, I) counterions with different ξ values on these properties were compared for dilute solutions of cetyltrimethylammonium chloride (CTAC). Counterion binding was determined by zeta-potential and ¹H NMR measurements. Nanostructures were determined by ¹H NMR and cryo-TEM imaging. Nanostructures, drag reduction effectiveness measured over a range of temperatures and Reynolds numbers, shear viscosities and first normal stress differences N1 were related to the chemical structures of the four counterions and their molar ratios to CTAC.     

Rheo-optics of cationic surfactant micellar solutions with mixed aromatic counterions

The effects of different ratios of mixed aromatic counterions of sodium salicylate (NaSal) and sodium 4-ethylbenzenesulfonate (NaEBS) at a total counterion concentration of 0.125 M on the rheo-optical behaviors of 0.05 M cetyltrimethylammonium chloride (CTAC) cationic surfactant solutions were studied. The introduction of NaEBS into CTAC–NaSal solution led to a significant deviation from the Cox–Merz rule. Mixing of the two aromatic counterions lowered the CTAC solutions’ zero shear viscosities, increased their plateau shear modulus, and decreased their relaxation time. In addition, the persistence lengths of the threadlike micelles and the mesh size of the micellar network were also significantly reduced. These effects are believed to be due to increased degree of branching and denser meshes of the micellar networks in the mixed counterion systems.     

Rheological and drag reduction characteristics of xanthan gum solutions

The rheological and turbulent drag reducing characteristics of commercial and purified xanthan gum solutions of concentrations 50–500 ppm have been studied with and without addition of 100 ppm NaCl. The purification by soxlet extraction of xanthan gum using 95% ethanol is effective in removing low-molecular-weight impurities from xanthan. The increased content of higher molecular-weight xanthan in purified xanthan is evident from rheological and drag reduction behavior. The addition of 100 ppm salt to dilute solutions introduces semi-flexibility in xanthan gum solution without occurrence of self-association. The change in molecular behavior in the presence of salt is evident from rheological normal-stress and turbulent drag reduction behaviors.     

Unusual temperature gap in turbulent drag reduction of cationic surfactants with mixed counterions

Unusual temperature gaps have previously been observed in the turbulent drag reduction effectiveness of Dobon and Habon cationic surfactant systems containing two anionic counterions of different binding strengths. Here, we report drag reduction data for a cationic surfactant with a mixture of dodecyl sulfate and tosylate counterions showing a temperature gap. Cryo-TEM images of nanostructures at different temperatures for this system support an explanation for this gap, based on the counterions’ relative binding strengths.     

Drag reduction and heat transfer in surfactant solutions with excess counterion

We present the results of a study of turbulent drag reduction in a small circulating loop using surfactant solutions with excess counterion. In addition, these solutions were used in measurements of heat transfer, both in pipe flow and in an impinging jet. Both frictional drag and heat transfer were reduced in the pipe flow experiments. Measurements of heat transfer in the impinging jet revealed a dependence on the molar concentration ratio of the counterion. When the counterion was added at a molar concentration 30 times higher than that of the surfactant, the resulting surfactant solution did not reduce the rate of heat transfer in the impinging jet. By using this surfactant system in an impinging jet, we show both a reduction in pipe friction and normal heat transfer potential in a circulating heat exchange system. In order to investigate this difference in heat transfer between pipe flows and impinging jet flows, a comparison was made of the wall shear stress between these two flow regimes. The estimated wall shear stress was of the same order in both flows, and thus was not considered to be the primary cause of the difference in heat transfer. It is instead suggested that the micellar structure of the surfactant is influenced by a compressive deformation of the impinging flow in a manner that is different from the shear deformation observed in pipe flow.     

Turbulence structure of dilute polymer and surfactant solutions in artificially roughened pipes

Pressure drop and velocity profile measurements are presented for turbulent flows of drag reducing fluids. The investigation was done in two rough pipes, known as k- and d-type rough pipes. The results are compared with those obtained in hydraulically smooth pipe of identical diameter. The spatial arrangement of the roughness elements in the pipe determines the parallel shift in the elastic sublayer and in the core region of the dimensionless turbulent velocity profile. The slopes of the velocity profiles in these regions remain unaffected by the arrangement which is an indication that the hydrodynamic influence of the roughness is restricted to the near-wall region. The drag reducing surfactant solution exhibited a drag reduction in the smooth as well as in the rough pipes which was higher than that given by Virk's maximum drag reduction asymptote. For this solution no influence of the roughness on the turbulence was detected when the dimensionless roughness height in viscous units was less than 12.     

Friction reduction and degradation in turbulent flow of dilute polymer solutions

An experimental study of friction reduction and polymer degradation in turbulent pipe flow is described for dilute water solutions of guar gum, CMC, Separan NP-10 and Polyox WSR-301. The tests are made in a turbulent-flow rheometer with a 2 mm I.D. pipe over a Reynolds number range from 8,000 to 25,000. The maximum attainable friction reduction for guar gum, Separan NP-10 and Polyox WSR-301 is found to be almost equal, but large differences in effectiveness occur. The most effective polymers (Polyox WSR-301 and Separan NP-10) are also the most liable to degradation. Mixing of polymers does not ameliorate the maximum friction reducing ability of the most effective component.     

Comparison of a new internal viscosity model with other constrained-connector theories of dilute polymer solution rheology

Complex viscosity ^* = -i predictions of the Dasbach-Manke-Williams (DMW) internal viscosity (IV) model for dilute polymer solutions, which employs a mathematically rigorous formulation of the IV forces, are examined in the limit of infinite IV over the full range of frequency number of submolecules N, and hydrodynamic interaction h ^*. Although the DMW model employs linear entropic spring forces, infinite IV makes the submolecules rigid by suppressing spring deformations, thereby emulating the dynamics of a freely jointed chain of rigid links. The DMW () and () predictions are in close agreement with results for true freely jointed chain models obtained by Hassager (1974) and Fixman and Kovac (1974 a, b) with far more complicated formalisms. The infinite-frequency dynamic viscosity predicted by the DMW infinite-IV model is also found to be in remarkable agreement with the calculations of Doi et al. (1975). In contrast to the other freely jointed chain models cited above, however, the DMW model yields a simple closed-form solution for complex viscosity expressed in terms of Rouse-Zimm relaxation times.     

Drag reduction by coupled systems: microbubble injection with homogeneous polymer and surfactant solutions

 The influence of homogeneous surfactant and homogeneous polymer solutions on the performance of microbubble skin friction reduction was investigated on an axisymmetric body. Carbon dioxide was injected into water, homogeneous surfactant (Aerosol OT) solutions, and homogeneous dilute polymer (Polyethylene oxide) solutions. Integrated skin friction measurements were obtained at two freestream velocities as a function of gas injection rate and polyethylene-oxide concentration. A moderate (50%) decrease in surface tension had little to no effect on the drag reducing characteristics of microbubble injection. At similar gas injection rates, microbubble injection exhibited more drag reduction in the polymer solutions than obtained with microbubble injection into water. However, the increased drag reduction obtained with polymer additives was no more than a multiplicative factor related to the baseline levels of drag reduction achieved by the individual methods, and suggests the mechanism for microbubble skin friction reduction acts independently of the polymer drag reduction. Received: 17 April 1998 / Accepted: 12 October 1998     

Headgroup effect on drag reduction and rheological properties of micellar solutions of quaternary ammonium surfactants

Two sets of cationic surfactants each with essentially the same alkyl chains but different headgroup structures were studied to investigate the effects of surfactant headgroup structure on micelle microstructures, drag reduction (DR) and rheological properties at certain counterion and surfactant concentrations. Cetyldimethylethylammonium bromide (CDMEAB) was compared with alkyltrimethyl ammonium bromide (CnTAB) and benzyldimethyl(hydrogenated tallow)ammonium chloride (DMHTB) was compared with alkyltrimethylammonium chloride (CmTAC), respectively. Surfactants with larger headgroups showed lower high temperature limits for DR. CDMEAB systems have better DR abilities than CnTAB below room temperature but the opposite is true at higher temperatures. DMHTB has stronger counterion binding ability than CmTAC, giving better DR properties than CmTAC at low counterion concentration, but has a lower upper temperature limit for DR. These results provide further understanding of the self-assembly nature of threadlike micelles of cationic surfactants and guidance for design of effective surfactant structures to meet particular DR requirements.