Viscoelastic Surfactant Solutions Research Articles

APPLICATION OF VISCOELASTIC SYSTEMS IN OIL PRODUCTION INTENSIFICATION PROCESSES

This paper provides an overview of viscoelastic behavior and the use of viscoelastic fluids (VBLs) in various oil production intensification processes. The first part provides basic models describing viscoelastic behavior and corresponding equations of state. Describes the method of testing under the action of harmonic vibrations (method of dynamic tests), possible options for its implementation and the determined rheological properties of the viscoelastic sample. To interpret the results of dynamic tests, a complex module is most often introduced, which consists of two components — the elastic modulus (G’) and the loss modulus (G’’). These dynamic modules and their relationship describe the behavior of the viscoelastic sample. It is shown that with respect to modules, it is possible to conclude on the type of sample - viscoelastic liquid or viscoelastic body. It is also noted that dynamic tests should be carried out in the area of low amplitudes of oscillating loads, in which the rheological properties do not depend on the applied stress or strain. The second part provides examples of viscoelastic systems used in operations such as water flow isolation, hydraulic fracturing, and modified acid treatments. The most common viscoelastic systems used in various technologies are solutions of surfactants and polymers of different structures. It is noted that in order to obtain a viscoelastic gel, polymer solutions are used with a crosslinker, while the rheological properties of surfactant solutions are controlled by external factors — pH, temperature, water mineralization. Viscoelastic surfactant solutions have an advantage over polymer solutions ‑ they are able to recover their properties after mechanical or thermal exposure due to the formation of supramolecular structures as a result of selforganization of surfactant molecules. The surfactant-based HLT property of reversibly collapsing and recovering is used, in particular, to reduce pressure losses to overcome friction. In flow-deflecting acid compositions, viscoelastic surfactants are mainly used, the most common of which are given in the second part of the article.

Stability of Viscoelastic Solutions: BrijL4 and Sodium Cholate Mixtures with Metal Ions Across a Broad pH and Temperature Range.

The impact of pH, temperature, and metal ions on the rheological and interfacial properties of aqueous mixed surfactant solutions composed of anionic NaC (sodium cholate) and nonionic BrijL4 [polyoxyethylene (4) lauryl ether] surfactants has been investigated. The various compound systems were analyzed, considering variations in each selected factor. The results highlight the unique characteristics of the BrijL4/NaC mixture, suggesting its potential as a viable alternative to other existing surfactants. The synergistic effect between BrijL4 and NaC significantly reduces the critical micelle concentration (CMC) and improves the wetting properties on hydrophobic surfaces, surpassing those of single-component solutions. Additionally, sodium, calcium, and magnesium ions enhance surface wetting and decrease the CMC. Besides, the BrijL4/NaC solutions exhibit viscoelastic fluid behavior at higher surfactant concentrations. These viscoelastic BrijL4/NaC solutions demonstrate stability over various pH and temperature variations, exhibiting lower flow activation and scission energy values than those of other viscoelastic surfactant solutions. Notably, the BrijL4/NaC mixture has potential applications in gel-based foliar fertilizers and drug delivery systems. Furthermore, the rheological studies examine the impact of humic acid on the rheological properties of BrijL4/NaC mixture solutions, revealing that incorporating additional humic acids can achieve stable rheological properties.

CO2-induced viscoelastic fluids based on a C22-tailed tertiary amine under different pressure and temperature

CO2-switchable viscoelastic surfactant solutions have gained much attention over the last decade; however, large-scale application and the impact of temperature and pressure on such soft colloidal materials have not been explored yet. Here in this work, the impact of environmental parameters including shear rate, pressure, temperature, and salinity on CO2-thickening behavior of aqueous dispersion of N-erucamidopropyl-N,N-dimethylamine (UC22AMPM) was assessed rheologically. Preliminary immiscible water-alternating-CO2 (CO2-WAG) flooding tests with and without UC22AMPM were conducted to highlight the potential of CO2-switchable surfactants in the process of improved oil recovery. The results demonstrated that the dispersion can be viscosified by CO2 15 times higher at 60 °C and 5 MPa, and such thickening power can be switched “on” and “off” upon bubbling and removal of CO2. A universal double-plateau viscosity profile was found as a function of CO2 bubbling time irrespective environmental condition, but the higher the shear rate or the pressure, the shorter the time is needed to reach the plateau viscosity; the higher the temperature, the longer the time is needed to get to the second plateau viscosity. UC22AMPM is first protonated into a long-chain cationic surfactant to form wormlike micellar solution, then the first viscosity plateau; expansion of the viscoelastic solution with CO2 attributes to the second viscosity plateau. Under the simulated oil reservoir of Daqing oilfield, immiscible CO2-WAG flood alone produced an additional 10.5% oil over the initial waterflood recovery, while the addition of 2.1 wt% UC22AMPM in the water slug could recover incremental 5.1% oil relative to pure CO2-WAG. These unique features suggest that coupling injection of CO2 and UC22AMPM dispersion holds great promise for recovering oil during the immiscible CO2-WAG process.

Anionic–zwitterionic viscoelastic surfactant strengthened air foams for heterogeneous reservoirs

Foam flooding technology can effectively enhance oil recovery (EOR) and has conformance control in heterogeneous reservoirs. However, the stability of foam migrating through porous media is still a challenge with individual surfactant. Herein, we developed viscoelastic surfactant (VES) via the synergisms of anionic (sodium dodecyl sulfate, SDS) and zwitterionic (erucamidopropyl hydroxysulfobetaine, EHSB) surfactants to strengthen the air foams stability. First, the VES formulation was optimized by investigating the initial foaming volume (V0), drainage half-life (td), and foam half-life (tf) for VES solutions. Then, the oil–water interfacial tension (IFT) and emulsification of the optimum VES system were systematically studied. Particularly, emphasizes were placed on the anti-gas channeling ability, EOR performance, and mechanisms using the core-flooding and microfluidic experiments. The results showed that the V0, td, tf of air forms could be greatly improved to 400 ml, 56.6 min, and 36 h, respectively, with 0.3 wt. % SDS:EHSB at mass ratio of 1:1.3. This was attributed to the synergisms of anionic and zwitterionic surfactants through electrostatic attraction, which promoted the formation of entangled wormlike micelles and increase in the solution viscosity. In addition, the oil–water IFT could be lowered to 0.1 mN/m and thus rapidly emulsifying the crude oil. Finally, the VES strengthened air foams exhibited excellent anti-gas channeling ability and great EOR efficiency of 20.44% after water-flooding process in heterogeneous reservoirs. The EOR mechanisms involving emulsification, stripping the residual oil, and jamming effect were identified from pore-scale during the microfluidics experiments.

Application of erucyl dimethyl betaine sulfonate viscoelastic surfactant for optimization of acidizing operations in heterogeneous carbonate reservoirs

AbstractReservoir heterogeneity is an important factor which should be considered for each operation in oilfields. Acidizing operation is performed to eliminate or reduce the damages of reservoirs. The dissolution of the minerals and rock texture in damaged areas is the main goal of acidizing. However, the existence of high permeable zones through flowing paths results in extra damages and fluid loss. Some additives could be added to the injected acid to avoid these undesirable phenomena. In this research, we added a Zwitterionic sulfonate surfactant to hydrochloric acid (HCl) solution and evaluated the rheological behavior of several compositions of acid‐based viscoelastic surfactant (VES) solutions under various temperature and shear rates. The capability of optimum solution to plug the high permeable zone and divert the acid flow to low permeable zone is examined. The optimum solution was injected into a parallel sandpacked system with different permeabilities at various flow rates (1, 3 and 5 cc/min). It was observed that the injection rate of 3 cc/min has the most efficiency to reduce the heterogeneity. Besides the tendency of VES based solutions for altering wettability of rock to more water‐wet condition is observed. The findings of this study can help the better understanding of practical application of VES in acidizing through heterogeneous reservoirs. Besides, effective parameters for such application are determined.

Synergistic Effects of Aqueous Phase Viscoelasticity and Reduced Interfacial Tension on Nonwetting Phase Displacement Efficiency: An In Situ Experimental Investigation.

This study uses in situ experimental investigation techniques to probe the synergistic effects of aqueous phase viscoelasticity and reduced interfacial tension on nonwetting phase displacement efficiency in natural porous media. Specifically, it examines the efficacy of viscoelastic surfactant (VES) solutions in enhancing oil recovery and investigates the pore-scale displacement mechanisms and VES-oil interactions. To this end, we first performed an extensive rheological characterization to select two VES solutions from multiple CTAB/NaNO3 (CTN) and Cocobetaine/SDS/NaCl (CSN) mixtures. The selected aqueous solutions were then used in unsteady-state imbibition experiments conducted on miniature, water-wet Prairie Shell carbonate core samples. The experiments were performed utilizing a high-pressure, high-temperature two-phase core-flooding apparatus integrated with a high-resolution X-ray imaging system to acquire pore-scale fluid occupancy maps during the displacements. The results indicate that the injection of the selected CSN and CTN solutions into the carbonate samples under capillary-dominated flow regimes boosts the oil recovery by 12 and 2%, respectively, compared to those of the base waterflood. The VES solutions lowered the oil-water interfacial tension and exerted significant shear forces at the entrance of pores. The shear forces could exceed the threshold surface free energy required to deform oil globules, and consequently, large oil clusters were fragmented into smaller blobs and eventually produced from the pore space. The higher oil recovery by CSN flooding was attributed to its stronger viscoelastic properties. The VES injection under the viscous-dominated flow regime intensified the frequency of oil globule fragmentation. As a result, residual oil saturation values were sharply reduced in the rock samples from 44 to 55% during the capillary-dominated flows to approximately 8%. The above-mentioned observations were also verified using morphological analysis of residual oil clusters. The impacts of VES flooding on the pore-scale oil configuration and the fragmentation of large oil clusters were particularly evident at higher VES flow rates.

Effect of the channel geometries on flow regimes of a viscoelastic surfactant solution in a cavity

• Characteristic flow behaviors of viscoelastic surfactant solutions in the cavity of a flow channel are visualized and analyzed. • The flow regimes were categorized in Barus effect, the bulge structure and the separation flow. • Viscoelastic Mach number and Deborah number determine the flow regimes. • Results implied that an interaction between the flow entering and leaving the cavity may occur. Viscoelastic surfactant solutions exhibit a characteristic flow behavior in the cavity of a flow channel. These flows are categorized as the flow regimes of the Barus effect, bulge structure, and separation flow. The former two flow regimes sweep into the cavity, which helps increase the heat transfer efficiency of a heat exchanger with a cavity to increase the heat transfer surface. Therefore, this study visualized and quantified these characteristic flow behaviors in channels with different geometries, based on inertia and elasticity, using Reynolds number, Re , and Weissenberg number, Wi . The flow regimes were affected by both inertia and elasticity; thus, viscoelastic Mach number, Ma , was also useful for characterizing the flow transition. The reattachment and separation lengths of the streamlines that sweep the bottom wall of the cavity were quantified using Re and Ma . These characteristic flow lengths were characterized and modeled by dimensionless numbers in the flow regime of the Barus effect but were not uniformly characterized in the flow regime of the bulge structure. The bulge structure was an entirely unique flow regime, but it significantly reduced the recirculating regions in the cavity, which is promising for increasing heat transfer efficiency.

Evaluation of the Dynamic Interfacial Tension between Viscoelastic Surfactant Solutions and Oil Using Porous Micromodels.

Interfacial tension (IFT) is a crucial parameter in many natural and industrial processes, such as enhanced oil recovery and subsurface energy storage. IFT determines how easy the fluids can pass through pore throats and hence will decide how much residual fluids will be left behind. Here, we use a porous glass micromodel to investigate the dynamic IFT between oil and Armovis viscoelastic surfactant (VES) solution based on the concept of drop deformation while passing through a pore throat. Three different concentrations of VES, that is, 0.5, 0.75, and 1.25% vol% prepared using 57 K ppm synthetic seawater, were used in this study. The rheology obtained using a rheometer at ambient temperature showed zero shear viscosity of 325, 1101, and 1953 cP for 0.5%, 0.75%, and 1.25% VES, respectively, with a power-law region between 2 and 50 1/s. The dynamic IFT increases with the shear rate and then reaches a plateau. The results of IFT were compared with those obtained from the spinning drop method, which shows 97% accuracy for 1.25% VES, whereas the accuracy decreased to 65% for 0.75 VES and 51% for 0.5% VES. The findings indicate that we can reliably estimate the IFT of VES at higher concentrations directly during multiphase flow in porous micromodels without the need to perform separate experiments and wait for a long time to reach equilibrium.

Turbulent drag reduction of viscoelastic wormlike micellar gels

Long-chained, viscoelastic surfactant solutions (VES) have been widely employed in the oil and gas industry, particularly in hydraulic fracturing and gravel-packing operations, where turbulence is commonly reached due to high pumping rates. With this motivation, we experimentally investigate the turbulent duct flow of an under-studied class of wormlike micellar solutions that forms a gel at room temperature. The fluid is characterized via rotational rheometry, and the turbulent velocity and Reynolds stress profiles are measured via Laser Doppler Anemometry (LDA). Three surfactant concentrations are investigated at increasing Reynolds numbers. The turbulent flow fields of water, and semi-dilute solutions of partially hydrolysed polyacrylamide (HPAM) and xanthan gum (XG) are used as comparisons. Our study reveals that the gel-like structure of the wormlike micellar gel is mostly broken down during turbulent flow, especially in the near-wall region where the results indicate the presence of a water layer. Turbulent flow at low concentrations of surfactant show a Newtonian-like flow field throughout most of the duct, where the energy spectra shows a − 5 / 3 power law scale with wavenumber, whereas higher concentrations lead to drag reduction and lower power spectral densities at large wavenumbers. A comparison of the flow of polymeric fluids and the wormlike micellar solutions at maximum drag reduction (MDR) shows comparable drag-reduction effects, with a large decrease in Reynolds shear stresses, and increased turbulence anisotropy in the buffer layer due to the large streamwise fluctuations and near-zero wall-normal fluctuations. At the centreline of the duct, the power spectral densities of polymers and VES are significantly reduced at all wavenumbers probed. Additionally, the MDR regime was bounded by Virk’s asymptote for both polymers and the micellar gel, which implies a similar mechanism of drag reduction at MDR. • Turbulent flows of wormlike micellar gels (VES) are experimentally studied. • Turbulent flows of VES and polymeric liquids are quantified via LDA measurements. • Steady flow curves of VES are not representative of its rheology in turbulent flow. • At MDR, the VES and polymer solutions show decreased energy spectra. • Our results show similar drag reduction mechanisms between VES and polymers at MDR.

Experimental Study on the Stability of a Novel Nanocomposite-Enhanced Viscoelastic Surfactant Solution as a Fracturing Fluid under Unconventional Reservoir Stimulation

Fe3O4@ZnO nanocomposites (NCs) were synthesized to improve the stability of the wormlike micelle (WLM) network structure of viscoelastic surfactant (VES) fracturing fluid and were characterized by Fourier transform infrared spectrometry (FT-IR), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), X-ray diffraction (XRD) and vibrating sample magnetometry (VSM). Then, an NC-enhanced viscoelastic surfactant solution as a fracturing fluid (NC-VES) was prepared, and its properties, including settlement stability, interactions between NCs and WLMs, proppant-transporting performance and gel-breaking properties, were systematically studied. More importantly, the influences of the NC concentration, shear rate, temperature and pH level on the stability of NC-VES were systematically investigated. The experimental results show that the NC-VES with a suitable content of NCs (0.1 wt.%) shows superior stability at 95 °C or at a high shear rate. Meanwhile, the NC-VES has an acceptable wide pH stability range of 6–9. In addition, the NC-VES possesses good sand-carrying performance and gel-breaking properties, while the NCs can be easily separated and recycled by applying a magnetic field. The temperature-resistant, stable and environmentally friendly fracturing fluid opens an opportunity for the future hydraulic fracturing of unconventional reservoirs.

Experimental Study on the Drag Reduction Performance of Clear Fracturing Fluid Using Wormlike Surfactant Micelles and Magnetic Nanoparticles under a Magnetic Field.

This paper examines a new study on the synergistic effect of magnetic nanoparticles and wormlike micelles (WLMs) on drag reduction. Fe3O4 magnetic nanoparticles (FE-NPs) are utilized to improve the performance of viscoelastic surfactant (VES) solutions used as fracturing fluids. The chemical composition and micromorphology of the FE-NPs were analyzed with FT-IR and an electron microscope. The stability and interaction of the WLM-particle system were studied by zeta potential and cryo-TEM measurements. More importantly, the influences of the temperature, FE-NP concentration, magnetic field intensity, and direction on the drag reduction rate of WLMs were systematically investigated in a circuit pipe flow system with an electromagnetic unit. The experimental results show that a suitable content of magnetic nanoparticles can enhance the settlement stability and temperature resistance of WLMs. A magnetic field along the flow direction of the fracturing fluid can improve the drag reduction performance of the magnetic WLM system. However, under a magnetic field perpendicular to the direction of fluid flow, an additional flow resistance is generated by the vertical chaining behavior of FE-NPs, which is unfavorable for the drag reduction performance of magnetic VES fracturing fluids. This study may shed light on the mechanism of the synergistic drag reduction effects of magnetic nanoparticles and wormlike micelles.

Effect of aromatic acid on the rheological behaviors and microstructural mechanism of wormlike micelles in betaine surfactant

The rheology and microstructural behaviors of viscoelastic surfactant solutions induced by different organic acids are fascinating. However, the viscosity and viscoelasticity behavior of some surfactants such as oleyl amidopropyl betaine (OAPB) surfactant is very poor even at high concentrations. To improve these properties at low concentrations, a system of OAPB and phthalic acid (OPA) at a mass ratio of 3:2 has been developed. The rheological behaviors and microstructural aggregations of the OAPB/OPA system were measured by rheometer, transmission electron microscopy (Cryo-TEM), and dynamic light scattering (DLS). The experimental results showed that the addition of OPA significantly increases the viscoelasticity and viscosity properties of the OAPB viscoelastic micelle solutions at 25°C. The improvements of these properties are attributed to the formation of a micellar network due to the addition of OPA. The rheology and micellar properties were found to transform a transparent viscous water like a fluid at pH 7 to a viscoelastic fluid at pH 2.9. The zero-shear viscosity and relaxation times declined exponentially with an increase of temperature indicating a reduction in the average micellar lengths. The properties changes were attributed to the strength of electrostatic attractions, hydrophobic effects, and hydrogen bonding between OAPB and OPA molecules. Since the current system can be easily tuned by pH for at least 3 circles without deterioration of its properties, the system unlocks the significant prospect for designing stimuli-responsive smart materials for various industrial applications.

Disruption of Cationic/Anionic Viscoelastic Surfactant Micellar Networks by Hydrocarbon as a Basis of Enhanced Fracturing Fluids Clean-Up.

Studies of the effects produced by the solubilization of hydrophobic substances by micellar aggregates in water medium are quite important for applications of viscoelastic surfactant solutions for enhanced oil recovery (EOR), especially in hydraulic fracturing technology. The present paper aims at the investigation of the structural transformations produced by the absorption of an aliphatic hydrocarbon (n-decane) by mixed wormlike micelles of cationic (n-octyltrimethylammonium bromide, C8TAB) and anionic (potassium oleate) surfactants enriched by C8TAB. As a result of contact with a small amount (0.5 wt%) of oil, a highly viscoelastic fluid is transformed to a water-like liquid. By small-angle neutron scattering (SANS) combined with cryo-TEM, it was shown that this is due to the transition of long wormlike micelles with elliptical cross-sections to ellipsoidal microemulsion droplets. The non-spherical shape was attributed to partial segregation of longer- and shorter-tail surfactant molecules inside the surfactant monolayer, providing an optimum curvature for both of them. As a result, the long-chain surfactant could preferably be located in the flatter part of the aggregates and the short-chain surfactant—at the ellipsoid edges with higher curvature. It is proven that the transition proceeds via a co-existence of microemulsion droplets and wormlike micelles, and upon the increase of hydrocarbon content, the size and volume fraction of ellipsoidal microemulsion droplets increase. The internal structure of the droplets was revealed by contrast variation SANS, and it was shown that, despite the excess of the cationic surfactant, the radius of surfactant shell is controlled by the anionic surfactant with longer tail. These findings open a way for optimizing the performance of viscoelastic surfactant fluids by regulating both the mechanical properties of the fluids and their clean-up from the fracture induced by contact with hydrocarbons.

Application of gemini viscoelastic surfactant with high salt in brine-based fracturing fluid

In this study, a viscoelastic surfactant (VES), named VES-Q, with erucyl amide tails and cyclodextrin was successfully synthesized, and the phase behavior of aqueous VES solutions at different salt concentrations was observed. In addition, the size of the molecules under different salt types was measured using dynamic light scattering. The viscosity of VES-Q in salt solution corresponds to the molecular size of the surfactant was found. Due to the high salinity tolerance of VES-Q to common formation salts, we used formation water from West Sichuan and seawater the Gulf of Mexico to allocate VES-Q to two clean fracturing fluids, namely, fluid 1 and fluid 2, with a concentration of 2%. Rheological properties under high-temperature reservoir conditions were simulated. The clean fracturing fluid was prepared by the VES-Q method, and its performance met the requirements of hydraulic fracturing operations.

Effects of moving contact line on filament pinch-off dynamics of viscoelastic surfactant fluids

Experiments illustrate that the moving fluid contact line can significantly affect the extensional rheology of viscoelastic surfactant solutions. A scaling analysis is provided to rationalize the effects of contact-line spreading on extensional rheology of viscoelastic fluids.

Effect of hydrocarbon structure on viscosity reduction of long chain viscoelastic surfactant

This paper studied the effect of different hydrocarbon structures on viscosity reduction of long-chain viscoelastic surfactant (VES) solution. The hydrocarbons studied were an alkane, cycloalkane, and aromatic hydrocarbon. The decrease of viscosity with the time of VES solution was determined at various temperatures. The study shows that aromatic hydrocarbon, benzene, and cycloalkane, cyclohexane, reduce the viscosity of VES solution faster than that of the alkane, hexane. The difference between small molecules of alkane and aromatic hydrocarbons in the viscosity reduction rate of VES solution is >100 times. The study also shows that higher temperature increases the viscosity reduction rate of VES solution. The rate of the viscosity reduction of VES solution decreases as the molecular weight of hydrocarbon increases.

Viscoelastic surfactant fluids filtration in porous media: A pore‐scale study

AbstractWe investigated the flow of viscoelastic surfactant (VES) solutions, an important type of fracturing fluids for unconventional hydrocarbon recovery, through a diverging–converging microfluidic channel that mimics realistic unit in porous media. Newtonian fluid and viscoelastic hydrolyzed polyacrylamide (HPAM) solution were used as control groups. We vary Deborah numbers (De) up to 61.2, and found that the flow patterns of HPAM and VES solutions become very different once De ≥ 6.12. This is attributed to different generation mechanisms of viscoelasticity, thus different responses to extensional rates at pore‐throats, for HPAM and VES solutions. It results in significantly smaller pressure drop of VES solutions through the microchannel compared to HPAM solution. It interprets higher filtration loss of VES solution than HPAM in core experiments and in field observations. The set‐up can be generalized as a prototype to effectively evaluate the filtration of fracturing fluids.

Reduction of clean fracturing fluid filtration loss by viscosity enhancement using nanoparticles: Is it feasible?

Fracturing fluid filtration loss is a significant research topic related to fracture propagation, formation damage and the cost of hydraulic fracturing. To reduce the loss, nanoparticles have been used to enhance the fracturing fluid properties such as viscosity and well-building ability. In this work, silica nanoparticles (NPs) with different concentrations (0, 0.01, 0.03 wt%) were added into the viscoelastic surfactant (VES) fluid composed of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal). A micro pore-throat structure was applied to investigate the flow characteristics of those three fracturing fluids with Deborah numbers (De) spanning from 1.52 to 73.2, and a Newtonian fluid was set as the control group. It was found that the flow patterns of VES solutions performed as the convergent flow similar to those of the Newtonian fluid at upstream, and turned into divergent flow with the gathering of streamlines at downstream. Surprisingly, the flow characteristics between VES solution with no nanoparticles and nanoparticle-enhanced the VES solution showed an insignificant difference with high flow rates (5 and 20 ml/h). The nanoparticle was unable to enhance or influence the properties of VES fluid when flowing through convergence–divergence and divergence–convergence structure. Neither the breakage nor the recombination of wormlike micellar were barely affected by the nanoparticle.

Linear rheology of nanoparticle-enhanced viscoelastic surfactants

Nanoparticles have been mixed with viscoelastic surfactants to improve the thermal sensitivity and elastic features of the original surfactant solution. The resulting mixtures have demonstrated improvements on rheology and thermal stability. However, very little attempt has been made to implement novel models that can accurately predict their rheological behavior in the linear regime. In this study, we present the capability of the Continuous Maxwell Model on representing the Maxwellian rheological behavior of nanoparticle-enhanced viscoelastic surfactant solutions at high frequencies and various temperatures. This capability is demonstrated by using rheological data from five different solutions: one data set generated in this work, and four from the literature. We, for the first time, provide evidence for the frequency-dependence of the relaxation time for these mixtures. We also found that for the prepared solution (Aromox APA-TW surfactant and aluminum oxide nanoparticle mixture) the Al2O3 nanoparticles increased the viscosity but only slightly affected the viscoelastic properties of the solution. A plausible hypothesis for the nanoparticle-micelle interactions that explains the results is proposed.

Evidence of Sticky Contacts between Wormlike Micelles in Viscoelastic Surfactant Solutions.

Many cationic surfactants form highly viscoelastic solutions at concentrations of only a few percent. These solutions contain wormlike micelles (WLM) which can be several micrometers long. The structural relaxation time, τs in these solutions can be as long as many seconds, and the zero-shear viscosity can be in the range of 106 Pas. Electric birefringence measurements on such solutions showed four different relaxation times with increasing concentration. The two shortest ones, τ1 and τ2 in the μs region were due to the alignment of small rodlike or ringlike micelles. The third one, τ3, was observed in the viscoelastic region in the ms region and finally, a fourth one, τ4, which was the same as the structural relaxation time, τs. In this article, it is shown that the τ3 process is due to the formation and opening of contacts between the WLM. The reason for the contacts is the attraction between the WLM which is due to the hydrophobic surfaces of the micelles. The contacts crosslink the WLM and thus form a three-dimensional network. This network is the reason for the high viscosity and viscoelastic properties of the samples. If the attraction between the WLM is reduced by adding glycerol to the solution, the viscosity of the solution breaks down. The same happens if the surface of the WLM is made hydrophilic by the addition of small amphiphilic molecules. The WLM are not destroyed by these procedures.