Viscoelastic Wormlike Micellar Solutions Research Articles

Flow of wormlike micellar solutions over concavities.

We present a comprehensive investigation combining numerical simulations with experimental validation, focusing on the creeping flow behavior of a shear-banding, viscoelastic wormlike micellar (WLM) solution over concavities with various depths (D) and lengths (L). The fluid is modeled using the diffusive Giesekus model, with model parameters set to quantitatively describe the shear rheology of a 100 : 60 mM cetylpyridinium chloride:sodium salicylate aqueous WLM solution used for the experimental validation. We observe a transition from "cavity flow" to "expansion-contraction flow" as the length L exceeds the sum of depth D and channel width W. This transition is manifested by a change of vortical structures within the concavity. For L ≤ D + W, "cavity flow" is characterized by large scale recirculations spanning the concavity length. For L > D + W, the recirculations observed in "expansion-contraction flow" are confined to the salient corners downstream of the expansion plane and upstream of the contraction plane. Using the numerical dataset, we construct phase diagrams in L-D at various fixed Weissenberg numbers Wi, characterizing the transitions and describing the evolution of vortical structures influenced by viscoelastic effects.

Effect of micelle breakage rate on flows of wormlike micellar solutions through pore throats

Many practical applications such as enhanced oil recovery or groundwater remediation encounter the flow of viscoelastic wormlike micellar solutions (WLMs) through porous media. A model porous media, consisting of a microchannel with pore throats present in it, is often used to understand the flow dynamics of these complex fluids. This study performs an extensive numerical investigation to understand the complex flow behaviour of these WLMs through such a model porous media based on the two-species Vasquez–Cook–McKinley (VCM) model for micelles. Within the present range of conditions encompassed in this study, we find the existence of an elastic instability once the Weissenberg number exceeds a critical value as it was seen in many prior experimental and numerical studies dealing with polymer solutions. However, for the present case of a WLM solution, we observe that the micelle breakage rate greatly influences these instabilities. In particular, we observe that the intensity of this instability (characterized by the fluctuating flow field) increases with the Weissenberg number up to a critical value of it. Beyond that, it starts to decrease on further increment in the Weissenberg number. This is in contrast to that seen in a polymer solution where the flow field gradually transits to a more chaotic and turbulent-like state (the so-called elastic turbulence state) as the Weissenberg number gradually increases. Furthermore, we notice that the flow dynamics of these WLM solutions strongly depend on the type (symmetric or asymmetric), number, and spacing between two consecutive pore throats. Additionally, an extensive discussion on the pressure drop and relative viscosity is presented in the present study. Although this study is carried out for a model porous system, we hope it will facilitate a better understanding of the flow behaviour of wormlike micellar solutions in an actual porous media. • An elastic instability emerges once the Weissenberg number exceeds a critical value. • As the micelles are become progressively easy to break, the tendency of appearing the elastic instability decreases. • A non-monotonic trend in the flow behaviour is observed as the Weissenberg number gradually increases. • The elastic instability strongly depends on the throat type, number of throats, and the spacing between them.

Non-Newtonian flows and instabilities in 3D glass microfluidic devices

<h2>Abstract</h2> In this virtual seminar we will discuss the use of selective laser-induced etching (SLE) for the fabrication of glass microfluidic devices, in particular for the study of non-Newtonian fluid dynamics problems [1]. SLE enables the precise monolithic fabrication of three-dimensional (3D) channels and structures embedded in a rigid and transparent fused-silica substrate, and thus provides opportunities for a wealth of novel microscale flow experiments. We will highlight various recent and ongoing microfluidic projects from our laboratory that have been made possible by SLE fabrication. Our focus will be on flows of viscoelastic wormlike micellar and polymer solutions in microscale geometries featuring, for instance, slender rigid or cantilevered posts [2,3]. 3D intersections [4,5], and axisymmetric contraction/expansions [6]. We make use of state-of-the-art quantitative techniques such as high-speed flow-induced birefringence imaging and volumetric 3-component micro-particle image velocimetry to characterize and understand purely-elastic and inertioelastic flow instabilities arising in these complex systems.

Effect of blockage ratio on flow of a viscoelastic wormlike micellar solution past a cylinder in a microchannel.

We present experiments on the flow of a viscoelastic wormlike micellar solution around cylinders (radius R) confined in straight microchannels (width W). Thirteen flow geometries are tested where the blockage ratio is varied over a wide range 0.055 ≤ BR = 2R/W ≤ 0.63. Experiments are performed at negligible Reynolds number, and for Weissenberg numbers Wi = λU/R up to 1000, where U is the average flow speed and λ is the relaxation time of the fluid. Micro-particle image velocimetry is used to characterise the flow state at each BR and Wi. In all of the geometries, a first critical Weissenberg number marks a transition from symmetric flow to an asymmetric but time-steady flow state, while a second higher critical Weissenberg number marks the onset of time-dependent flows. However, we report a clear shift in behaviour over a narrow intermediate range of 0.33 ≲ BR ≲ 0.41. Channels with BR ≤ 0.33 fall in a 'low' BR regime, with instabilities that originate from the downstream stagnation point, while those with BR ≥ 0.44 fall in a 'high' BR regime, with instabilities developing at the upstream stagnation point. Behaviour within the newly-identified intermediate BR regime is complex due to the competing influence of the two stagnation points. We summarise all our results in a flow state diagram covering Wi-BR parameter space, clearly defining the different regimes of blockage ratio for the first time. Our results contribute to the understanding of the complexities of viscoelastic flow in this benchmark geometry.

Asymmetric flow of polymer solutions around microfluidic cylinders: Interaction between shear-thinning and viscoelasticity

Viscoelastic flow around a cylinder is a model problem representing a wide range of relevant industrial processing and biological applications. Reducing the cylinder to microscopic dimensions conveniently enables the problem to be examined in the absence of inertia. Recently, we have developed glass microfluidic geometries containing long and slender, yet rigidly fixed, microfluidic cylinders, which present a low blockage ratio β=2r/W=0.1, where r=20μm is the cylinder radius and W=400μm is the channel width. Using a shear-banding viscoelastic wormlike micellar (WLM) solution, we showed how the flow around such a cylinder could destabilize beyond a critical Weissenberg number (Wi=λU/r, where λ is a characteristic time of the fluid and U is the average flow velocity), resulting in the asymmetric division of the fluid around either side of the cylinder [Haward et al, Soft Matter15:1927]. In the present work we investigate this flow instability in greater detail using a range of polymer solutions formulated from hydrolyzed poly(acrylamide) (HPAA) dissolved at different concentrations in deionized water. The test fluids present a range of shear-thinning responses under steady shear, and also a wide variety of characteristic times. At low HPAA concentrations, the flow around the cylinder remains essentially symmetric for all Wi, but as the concentration increases, so does the maximum degree of the flow asymmetry observed with increasing Wi. Interestingly, at intermediate concentrations, the flow can resymmetrize at very high Wi. We understand these effects by considering simultaneously both the degree of shear-thinning of the fluid and the imposed Wi, and our analysis shows that both strong shear-thinning and high elasticity are required for the formation of strongly asymmetric flows. Our results represent the first report of this highly asymmetric flow state in polymer solutions, showing that it is a general phenomenon and not only specific to WLM. Our analysis provides a clear insight into the origins of this unusual flow state and may also be relevant to understanding other instances of asymmetries arising in shear-thinning viscoelastic flows.

Inverse integral transformation method to derive local viscosity distribution measured by optical tweezers.

Complex fluids have a non-uniform local inner structure; this is enhanced under deformation, inducing a characteristic flow, such as an abrupt increase in extensional viscosity and drag reduction. However, it is challenging to derive and quantify the non-uniform local structure of a low-concentration solution. In this study, we attempted to characterize the non-uniformity of dilute and semi-dilute polymer and worm-like micellar solutions using the local viscosity at the micro scale. The power spectrum density (PSD) of the particle displacement, measured using optical tweezers, was analyzed to calculate the local viscosity, and two methods were compared. One is based on the PSD roll-off method, which yields a single representative viscosity of the solution. The other is based on our proposed method, called the inverse integral transformation method (IITM), for deriving the local viscosity distribution. The distribution obtained through the IITM reflects the non-uniformity of the solutions at the micro scale, i.e., the distribution widens above the entanglement concentrations of the polymer or viscoelastic worm-like micellar solutions.

Worm-like micelles and vesicles formed by alkyl-oligo(ethylene glycol)-glycoside carbohydrate surfactants: The effect of precisely tuned amphiphilicity on aggregate packing

Carbohydrates are appealing non-ionic surfactant head-groups as they are naturally abundant, generally biocompatible and biodegradable, and readily functionalized. Recent work has produced a promising molecular candidate for the formation of viscoelastic worm-like micellar solutions: a tri(ethylene glycol)-linked oleyl-β-D-glucoside surfactant (GlcC18:1) exhibited near ideal Maxwell behavior at low concentrations (2.9 wt%) without additives at room temperature. Here, fourteen surfactants have been synthesized with structural variations based around GlcC18:1. Each contain an oligo(ethylene glycol) linker of varying length (2, 3, 4, 6 EO units) between a carbohydrate head-group (glucose, galactose, mannose, maltose, lactose, cellobiose) and a cis-unsaturated alkyl tail-group (oleyl, linoleyl, erucyl). The aqueous adsorption kinetics and self-assembly of these surfactants was explored using tensiometry and small-angle neutron scattering (SANS), respectively. With SANS we observed the formation of worm-like micelles for four surfactants, and vesicles for two surfactants which exhibited behavior similar to insoluble lipids. We also observed temperature-induced micellar elongation due to dehydration of the oligo(ethylene glycol) linker, resulting in a further three surfactants forming worm-like micelles at 50 °C. Worm-like micellar fluids were further characterized using rheology to reveal two surfactants with vastly superior viscoelastic properties compared to GlcC18:1, with >2 orders of magnitude increase in viscosity and >3 orders of magnitude increase in stress relaxation time. These results provide insight into structure-function relationships for non-ionic surfactants and demonstrate a class of designed amphiphiles with a special propensity for forming viscoelastic worm-like micellar solutions at low concentrations.

Local flow around a tiny bubble under a pressure-oscillation field in a viscoelastic worm-like micellar solution

The motion of a tiny air bubble in an aqueous solution of cetyl trimethyl ammonium bromide and sodium salicylate (CTAB/NaSal), which forms wormlike micelles, was observed using a high-speed polarization camera. Complex local flow is observed around the bubble surface because the bubble shape deforms repeatedly at 100 Hz. The camera can be used to measure the retardation distribution, which is related to the stress field around the bubble. Different retardation and orientation angle distributions were observed during the contraction and expansion phases. In the contraction phase, a strong retardation distribution appears at the tail of the cusped bubble. The occurrence of uniaxial elongation deformation is considered to be due to the negative wake because they are closely correlated. In contrast, a weak retardation distribution spreads at the upper side of the bubble during the expansion phase, where biaxial extensional deformation occurs due to expansion of the bubble surface. These significant changes in strong elastic stress distributions, which correspond to the orientation angle profiles, can result in increase of bubble rising velocities.

Wormlike micelle formation of novel alkyl-tri(ethylene glycol)-glucoside carbohydrate surfactants: Structure–function relationships and rheology

Carbohydrates are appealing non-ionic surfactant head-groups as they are naturally abundant, generally biocompatible and biodegradable, and readily functionalized. Here, seven novel carbohydrate based surfactants (CBS) have been synthesized that contain a tri-ethylene glycol (TEG) linker between a glucose head-group and alkyl tail-group, with linear saturated (C8–18) and unsaturated (C18:1) alkyl chains. The aqueous adsorption and self-assembly of these surfactants was explored using tensiometry and small- and ultra-small-angle neutron scattering (SANS and USANS). With SANS we observed elongation from spherical to cylindrical micelles with increasing alkyl chain length. C16 and C18 chains exhibited pronounced Krafft points, yet formed worm-like micelles as single components upon heating to 43 and 48 °C respectively. The introduction of mono-unsaturation in the form of a C18:1 chain reduced the Krafft point and gave a surfactant that produced worm-like micelles in water without additives at room temperature. We also observed micellar elongation for C12 and C14 chains at 50 °C due to dehydration of the TEG linker. The room temperature worm-like micelles were further characterized using rheo-SANS and rheology, revealing the C18:1 surfactant to exhibit near ideal Maxwell behavior at low concentrations (2.9 wt.%). These results provide insight into structure-function relationships for CBS, and demonstrate a promising molecular candidate for the formation of viscoelastic worm-like micellar solutions.

Reversibly Switching Wormlike Micelles Formed by a Selenium-Containing Surfactant and Benzyl Tertiary Amine Using CO2/N2 and Redox Reaction.

Multiresponsive wormlike micelles (WLMs) remain a significant challenge in the construction of smart soft materials based on surfactants. Herein, we report the preparation of a viscoelastic wormlike micellar solution based on a new redox-responsive surfactant, sodium dodecylselanylpropyl sulfate (SDSePS), and commercially available benzyl tertiary amine (BTA) in the presence of CO2. In this system, SDSePS can be reversibly switched on (selenide) and off (selenoxide) by a redox reaction, akin to that previously reported for benzylselanyl or phenylselanyl surfactants. By alternately adding H2O2 and N2H4·H2O, WLMs can be reversibly broken and formed because of the transformation of the hydrophilic headgroup of SDSePS, originating from the reversible formation of selenoxide. Moreover, WLMs can also be switched on and off by cyclically bubbling CO2 and N2 because of the variation of the binding interaction between SDSePS and BTA, resulting from the reversible protonation of BTA. This interesting and unique multiresponsive behavior makes the current WLMs a potential candidate for smart control of the "sol-gel" transition or substantial thickening of solutions.

Phase behaviour and temperature-responsive properties of a gemini surfactant/Brij-30/water system.

The phase behaviour of a ternary system composed of cationic gemini surfactant 2-hydroxypropyl-1,3-bis(myristyldimethylammonium chloride) (14-3(OH)-14(2Cl)), nonionic surfactant polyoxyethylene laurel ether (C12H25(OCH2CH2)10OH, Brij-30) and H2O was studied and its phase diagram was determined. In several different phase regions identified in the phase diagram, the viscoelastic worm-like micellar solution region was investigated. The focus of the investigation was the increase in the viscosity of the micellar solution with the temperature and the Brij-30 content, as suggested by a partial region of the triangular phase diagram. With an increase in the Brij-30 content or temperature, one-dimensional micellar growth occurred, and a maximum appeared in the zero-shear viscosity, η0, versus Brij-30 content or temperature curves; this was true for mixed solutions containing 14-3(OH)-14(2Cl) and Brij-30 in different mass ratios (WBrij-30/W14-3(OH)-14(2Cl)). After the maximum point, the decrease in viscosity is the result of the micellar shortening in the size with Brij-30 content and of the micellar branching in the network structure with temperature. This rheological analysis was performed to investigate the structural changes in different solutions as well as the effects of various factors on micellar growth. Various techniques were used to study the phase-transition processes occurring in this system, including cryogenic transmission electron microscopy, small-angle X-ray scattering measurements, and differential scanning calorimetry. The transition mechanisms of the aggregates were analysed on the basis of the obtained results.

Synthesis of oligomer betaine surfactant (DDTPA) and rheological properties of wormlike micellar solution system

Novel oligomer betaine surfactant penta sodium N, N′, N″- dodecyl diethylene triamine pentaacetic acid (DDTPA) with three dodecyls, five carboxylic acid groups and three quaternary ammonium were synthesized through two steps in this paper. Rheological properties of mixed oligomer betaine surfactant DDTPA/anionic surfactant sodium dodecylsulfonate (SDS) solution systems were studied as functions of SDS concentration, inorganic salt KCl dosage and temperature. Stable and dynamic viscoelastic measurements were carried out to investigate the characters of wormlike micelles formed in certain condition. In the 2.5wt% DDTPA, 4wt% KCl and SDS system, the optimal SDS concentration is 1.2wt%, at which the viscosity reaches a maximum. Interestingly, with adding inorganic salt KCl to mixed surfactant solution(2.5wt% DDTPA, 1.5wt% SDS), the water-like fluid turns into gel-like viscoelastic wormlike micellar solution immediately owing to the screening electrostatic repulsion among the surfactant charged groups by counterions. When KCl concentration is 3wt%, the micellar solution obtains a peak value in viscosity. For the system with 2.5wt% DDTPA, 1.5wt% SDS, 4wt% KCl, the micellar contour length decreases with increasing temperature, leading to an exponential decrease in the values of η0 and τR. This work provides a clear understanding of the profound relationship between surfactant wormlike micellar characteristic and the rheological properties. The dynamic viscoelastic behavior of wormlike micelles usually fits for the Maxwell mode in low oscillation frequency region, but in medium and high frequency region it fits for t the modified Maxwell model.

Effects of Spacer Chain Length of Amino Acid-Based Gemini Surfactants on Wormlike Micelle Formation

We studied the effects of the spacer chain length of amino acid-based gemini surfactants on the formation of wormlike micelles in aqueous solutions. The surfactants used were synthesized by reacting dodecanoylglutamic acid anhydride with diamine compounds (ethylenediamine, pentanediamine, and octanediamine), and were abbreviated as 12-GsG-12 (s: the spacer chain length of 2, 5, and 8 methylene units). These surfactants yielded viscoelastic wormlike micellar solutions at pH 9 upon mixing with a cationic monomeric surfactant, hexadecyltrimethylammonium bromide (HTAB). We found that the rheological behavior was strongly dependent on the spacer chain length and HTAB concentration. When the shortest spacer chain analogue (12-G2G-12) was used, an increased HTAB concentration resulted in the following structural transformations of the micelles: (i) spherical or rodlike micelles; (ii) anionic wormlike micelles exhibiting a transient network structure; (iii) anionic wormlike micelles with a micellar branching or interconnected structure; and (iv) cationic wormlike micelles. Similarly, when the middle spacer chain analogue (12-G5G-12) was used, a structural transformation from anionic to cationic wormlike micelles occurs; however, molecular aggregates with a lower positive curvature were also formed in this transition region. When the longest spacer analogue (12-G8G-12) was used, the formation of cation-rich molecular aggregates was not observed. These transition behaviors were attributed to the packing geometry of the gemini surfactants with HTAB. Additionally, as the spacer chain length increased, the zero-shear viscosity in the anionic wormlike micellar region decreased, suggesting limited one-dimensional micellar growth of spherical, rodlike, or anionic wormlike micelles.

Construction of a highly viscoelastic anionic wormlike micellar solution by carboxylate gemini surfactant with a p-dibenzenediol spacer

The viscoelastic wormlike micellar solutions formed by the carboxylate gemini surfactant, O,O′-bis(sodium 2-dodecylcarboxylate)-p-dibenzenediol (referred to as C12ϕ2C12), have been investigated by steady-state and dynamic frequency-sweep rheological techniques. Another system using its homologue, O,O′-bis(sodium 2-dodecylcarboxylate)-p-benzenediol (C12ϕC12), was also examined for comparison. Both C12ϕ2C12 and C12ϕC12 formed wormlike micelles without any additives. The C* corresponding to the onset of the semi-dilute regime was 48 mmol L−1 for C12ϕ2C12 and 160 mmol L−1 for C12ϕC12, respectively, showing the considerably high ability and efficiency of C12ϕ2C12 in forming wormlike micelles. The wormlike micelles started to entangle with each other at 60 mmol L−1 for C12ϕ2C12 and 300 mmol L−1 for C12ϕC12. The average length of the C12ϕ2C12 wormlike micelles was estimated to be about 1.0–1.9 μm, significantly longer than the C12ϕC12 ones. The solution viscosity of C12ϕ2C12 at 200 mmol L−1 reached 613 Pa s, much higher than 24.2 Pa s for C12ϕC12 at 500 mmol L−1. The mechanisms of forming wormlike micelles in both cases were revealed by dynamic light scattering measurements. Differently from the normal micellar growth that C12ϕC12 displayed, C12ϕ2C12 was found to form large network-like aggregates at very low concentrations. These aggregates could more conveniently transform into threadlike (wormlike) micelles by continuously increasing the concentration of C12ϕ2C12, from which a new approach was realised for highly viscoelastic anionic wormlike micellar solutions.

Wormlike Micelle Formation by Acylglutamic Acid with Alkylamines

Rheological properties of alkyl dicarboxylic acid-alkylamine complex systems have been characterized. The complex materials employed in this study consist of an amino acid-based surfactant (dodecanoylglutamic acid, C12Glu) and a tertiary alkylamine (dodecyldimethylamine, C12DMA) or a secondary alkylamine (dodecylmethylamine, C12MA). (1)H NMR and mass spectroscopic data have suggested that C12Glu forms a stoichiometric 1:1 complex with C12DMA and C12MA. Rheological measurements have suggested that the complex systems yield viscoelastic wormlike micellar solutions and the rheological behavior is strongly dependent on the aqueous solution pH. This pH-dependent behavior results from the structural transformation of the wormlike micelles to occur in the narrow pH range 5.5-6.2 (in the case of C12Glu-C12DMA system); i.e., positive curved aggregates such as spherical or rodlike micelles tend to be formed at high pH values. Our current study offers a unique way to obtain viscoelastic wormlike micellar solutions by means of alkyl dicarboxylic acid-alkylamine complex as gemini-like amphiphiles.

Formation of wormlike micelles with natural-sourced ingredients (sucrose fatty acid ester and fatty acid) and a viscosity-boosting effect induced by fatty acid soap

We have investigated the formation of a viscoelastic wormlike micellar solution with natural-sourced materials such as sucrose monopalmitate (C16SE) and fatty acid (FA). Zero-shear viscosity of 10wt% aqueous solutions of C16SE and lauric acid (LA) increased with the increase in the C16SE fraction to a certain level but it decreased after a maximum value. Viscous solutions having almost maximum zero-shear viscosity showed non-Newtonian behavior and had Maxwell-type mechanical properties. When a small fraction of LA of the water/C16SE/LA system was substituted with sodium laurate (SL), the maximum zero-shear viscosity increased by nearly one order of magnitude from the original system due to a decrease in the diffusion constant of the wormlike micelles. With further substitutions of LA with SL, the zero-shear viscosity decreased because the highly hydrophilic fatty acid soap makes elongated micelles shorter.

Comparative study of the viscoelastic wormlike micellar solutions of hexanediyl-α,ω-bis(dimethylcetylammonium bromide) and cetyltrimethylammonium bromide in the presence of sodium salicylate

A comparative study of the viscoelastic properties of the wormlike micellar solutions of hexanediyl-α,ω-bis(dimethylcetylammonium bromide), referred to as 16-6-16, and cetyltrimethylammonium bromide (CTAB) in the presence of sodium salicylate (NaSal) has been carried out. In terms of Cates model on Maxwell fluid, the characteristic parameters of both wormlike micelles were revealed and compared. The results were discussed on the basis of their different molecular structures, which leaded to different response of 16-6-16 and CTAB to salt.

Viscoelastic solutions formed by worm-like micelles of amine oxide surfactant

Formation and properties of viscoelastic wormlike aqueous micellar solutions of the zwitterionic surfactant p-dodecyloxybenzyldimethylamine oxide ( pDoAO) were studied. Semi-dilute aqueous solutions of pDoAO show a sharp increase in viscosity, which exceeds 160 cST for concentrations >50 mM, leading to viscoelastic solutions. Viscoelasticity relates to the surfactant charge type. In fact this viscoelastic system reverses to fluid when acid is added (pH < 2), which changes the system to cationic. Under acidic conditions the system resembles solutions of the similar cationic surfactant p-dodecyloxybenzyltrimethylammonium bromide, ( pDoTABr) in terms of viscosity. Properties of aqueous solutions of pDoAO were investigated by dynamic light scattering (DLS), rheology and small angle neutron scattering (SANS). Data support the idea that small micelles grow in length (wormlike or threadlike micelles) as surfactant concentration increases and viscoelastic solutions form as micelles become entangled. The micellar diameter as calculated by different techniques is about 5 nm.

Viscoelastic wormlike micellar solutions in mixed environmentally friendly nonionic surfactant systems

A study of the phase and rheological behavior of polyoxyethylene(20)sorbitan monooleate (Tween 80)/monolaurin (ML)/water is presented at 25 °C. In water/surfactant binary system, Tween 80 forms an isotropic micellar solution over wide concentration range (∼42 wt%). With successive addition of ML to the aqueous Tween 80 solution, viscosity increases rapidly and a viscoelastic solution is formed. The viscoelastic micellar solution formed follows the Maxwell model typical of wormlike micellar systems at low-frequency region. With further addition of ML, the viscosity drops after a maximum and phase separation occurs. On increasing the concentration of Tween 80, micellar growth starts at a slightly lower concentration of ML. The micellar growth can be simply explained by reducing the effective area of Tween 80 head group upon addition of ML.

Structural investigation of viscoelastic micellar water/CTAB/NaNO3 solutions

A highly viscoelastic worm-like micellar solution is formed in hexa- decyltrimethylammonium bromide (CTAB) in the presence of sodium nitrate (NaNO3). A gradual increase in micellar length with increasing NaNO3 was assumed from the rhe- ological measurements where the zero-shear viscosity (·0) versus NaNO3 concentration curve exhibits a maximum. However, upon increase in temperature, the viscosity de- creases. Changes in the structural parameters of the micelles with addition of NaNO3 were inferred from small angle neutron scattering measurements (SANS). The intensity of scattered neutrons in the low q region was found to increase with increasing NaNO3 concentration. This suggests an increase in the size of the micelles and/or decrease of intermicellar interaction with increasing salt concentration. Analysis of the SANS data using prolate ellipsoidal structure and Yukawa form of interaction potential between mi- celles indicate that addition of NaNO3 leads to a decrease in the surface charge of the ellipsoidal micelles which induces micellar growth. Cryo-TEM measurements support the presence of thread-like micelles in CTAB and NaNO3.