Wormlike Micellar Solutions Research Articles

Aging mechanism of unsaturated long-chain amidosulfobetaine worm fluids at high temperature

Both fundamental and technological research interests have highlighted in recent years a remarkable growing focus on surfactant wormlike micelles. Nevertheless, long-term thermal stability, as well as the corresponding aging mechanism of viscoelastic worm fluids was rarely examined. We report for the first time the high-temperature aging effect on wormlike micellar solutions formed by an unsaturated C22-tailed amidosulfobetaine surfactant in the absence and presence of a reducer, by means of rheological methods, cryo-TEM observations, IR and 1H NMR measurements. It was found that the aging effect mainly resulted from the degradation of the carbon–carbon double bond in the surfactant tail rather than its amido head group; and the addition of a small amount of reducing agent could effectively improve thermal stability of the worm fluids.

Rheology of wormlike micelles from non-equilibrium molecular dynamics

In this work, the rheology of complex fluids, i.e., surfactants of varying concentration in a Lennard–Jones fluid, is analyzed with non-equilibrium molecular dynamics simulations. The molecular model considers that the surfactant molecule is composed of a hydrophilic head, affine to solvent, and a hydrophobic tail made of four monomers. The solvent is modeled by a Lennard–Jones fluid, which shows mostly a Newtonian behavior, but at relatively high shear rates, a slight shear-thinning followed by a slight shear thickening are exhibited. The intermolecular potential produces an equilibrium configuration, in which the surfactant molecules self-assemble in a wormlike micelle. With the aim to analyze the system behavior with various stress fields, two flows are simulated under non-equilibrium conditions: (1) simple shear and (2) Poiseuille's flow. In simple shear, by keeping the velocity of the upper plate of the flow cell constant, a monotonic flow curve is predicted within a range of shear rates. At low shear rates, a concentration-dependent Newtonian region of viscosity η0 corresponds to an isotropic condition in which the wormlike micelle preserves its equilibrium conformation. At intermediate shear rates, the solution exhibits a slight shear thinning, generating bands placed normal to the gradient direction (gradient banding). At high shear rates the solution exhibits shear-thickening, with bands now generated normal to the vorticity direction. These predictions by molecular models explain, to our knowledge for the first time, experiments in shear-thickening wormlike micellar solutions, where shear-thickening appears simultaneously with bands generated perpendicular to the vorticity axis. In Poiseuille's flow, we also find agreement between predictions of the model with theoretical developments and experiments performed by other authors.

Pressure-driven flow of wormlike micellar solutions in rectilinear microchannels

AbstractIn this paper the inhomogeneous response of the (two species) VCM model (Vasquez et al., A network scission model for wormlike micellar solutions. I. Model formulation and homogeneous flow predictions, J. Non-Newtonian Fluid Mech. 144 (2007) 122–139) is examined in steady rectilinear pressure-driven flow through a planar channel. This microstructural network model incorporates elastically active network connections that break and reform mimicking the behavior of concentrated wormlike micellar solutions. The constitutive model, which includes non-local effects arising from Brownian motion and from the coupling between the stress and the microstructure (finite length worms), consists of a set of coupled nonlinear partial differential equations describing the two micellar species (a long species ‘A’ and a shorter species ‘B’) which relax due to reptative and Rouse-like mechanisms as well as rupture of the long micellar chains. In pressure-driven flow, the velocity profile predicted by the VCM model deviates from the regular parabolic profile expected for a Newtonian fluid and exhibits a complex spatial structure. An apparent slip layer develops near the wall as a consequence of the microstructural boundary conditions and the shear-induced diffusion and rupture of the micellar species. Above a critical pressure drop, the flow exhibits shear banding with a high shear rate band located near the channel walls. This pressure-driven shear banding transition or ‘spurt’ has been observed experimentally in macroscopic and microscopic channel flow experiments. The detailed structure of the shear banding profiles and the resulting flow curves predicted by the model depend on the magnitude of the dimensionless diffusion parameter. For small channel dimensions, the solutions exhibit ‘non-local’ effects that are consistent with very recent experiments in microfluidic geometries (Masselon et al., Influence of boundary conditions and confinement on non local effects in flows of wormlike micellar systems, Phys. Rev. E 81 (2010) 021502).

Highly viscous wormlike micellar phases formed from the mixed AOT/C14DMAO/H2O system

The phase behavior and rheological properties of an anionic surfactant, bis(2-ethylhexyl) sulfosuccinate (AOT), mixed with a zwitterionic tetradecyldimethylamine oxide (C(14)DMAO) in aqueous solutions, were studied at different ratios, R=w(AOT)/(w(C(14)DMAO + w(AOT)). When R=1, the 6.0 wt% AOT solution is two-phase with dense vesicles as the lower phase. With an increase of C(14)DMAO fraction (decreasing R) at a total concentration of 6.0 wt%, the lower vesicle-phase (L(αv)-phase) extends to generate a single L(αv)-phase. Then the L(αv)-phase turns into a viscoelastic wormlike micellar phase and finally rod-like or spherical C(14)DMAO micelles. The wormlike micellar solutions (from R=0.3 to 0.2) are highly viscoelastic, indicating the formation of rigid network structures. The rheological properties of the viscoelastic solutions exhibit a typical Maxwell characteristic at low and intermediate oscillatory frequencies. A pronounced temperature effect on the wormlike micellar structures can be observed by rheological studies. With an increase in temperature, the samples become less structured due to shortening of the micelles. After introducing certain additives, e.g., octanol and divalent metal ions, a transition from wormlike micellar phases to birefringent L(αv)-phases was observed.

Anomalous shear banding: multidimensional dynamics under fluctuating slip conditions

The strain-controlled flow of a wormlike micellar solution in cylindrical Couette geometries with smooth and rough glass inner walls is investigated using 2D 1H NMR velocimetry. We find anomalous shear banding in which fluctuating slip dynamics in combination with surfactant properties lead to a non-lever rule behaviour where the interface position remains constant while the high and low shear rates change. Velocities in the flow direction are imaged in the flow-gradient/vorticity plane. The spatiotemporal resolution achieved reveals fluctuations in flow structure along the vorticity axis and instability of the high shear band.

Probing shear-banding transitions of the VCM model for entangled wormlike micellar solutions using large amplitude oscillatory shear (LAOS) deformations

We explore the use of large amplitude oscillatory shear (LAOS) deformation to probe the dynamics of shear-banding in soft entangled materials, primarily wormlike micellar solutions which are prone to breakage and disentanglement under strong deformations. The state of stress in these complex fluids is described by a class of viscoelastic constitutive models which capture the key linear and nonlinear rheological features of wormlike micellar solutions, including the breakage and reforming of an entangled network. At a frequency-dependent critical strain, the imposed deformation field localizes to form a shear band, with a phase response that depends on the frequency and amplitude of the forcing. The different material responses are compactly represented in the form of Lissajous (phase plane) orbits and a corresponding strain-rate and frequency-dependent Pipkin diagram. Comparisons between the full network model predictions and those of a simpler, limiting case are presented.

Wormlike micellar solutions: II. Comparison between experimental data and scission model predictions

Although many constitutive models for wormlike micellar solutions have been proposed, few quantitative comparisons have been made with detailed rheological measurements. The majority of comparative studies focus on the linear viscoelastic properties of micellar solutions, which are well described by monoexponential Maxwell-like behavior. In the present work we compare the predictions of a prototypical two-species reptation-reaction model [developed in Part 1, Vasquez et al., “A network scission model for wormlike micellar solutions: I. Model formulation and viscometric flow predictions,” J. Non-Newtonian Fluid Mech. 144(2–3), 122–139 (2007)] with rheological measurements performed using a concentrated cetyl pyridinium chloride/sodium salicylate (CPyCl/NaSal) solution in a range of steady and transient shear flows. The model captures the continuous rupture and reformation of the long entangled chains that form a physically entangled viscoelastic network and the enhanced breakage rates that occur during imp...

The Wormlike Micellar Solution made of a Zwitterionic Surfactant (TDPS), an Anionic Surfactant (SDS), and Brine in the Semidilute Regime

Structural and dynamical properties of a micellar solution are studied mainly through examining its rheological behavior in the semidilute regime. The micellar solution is made of a zwitterionic surfactant N-tetradecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, sodium dodecyl sulfate, and salty water. In particular, we are interested in how the system is affected when the ionic strength of the media is modified by adding salt. Until recently, it was known that this solution forms wormlike micelles. In a range of chemical composition, the solution behaves as a viscoelastic Maxwellian fluid at low frequencies. We present measurements of the elastic (storage) modulus and the viscous (loss) modulus varying the surfactant ratio (R = [SDS]/[TDPS]), and how the Maxwellian relaxation time abruptly increases when the NaCl concentration is also varied. Reptation and breaking/recombination times were estimated. The effect of temperature in the viscoelastic solution is also studied. Shear stress versus shear rate flow curves were measured under shear and stress control, for different micellar solutions with different composition, brine concentration, and temperature, showing a nonlinear behavior. Flow curves present two branches, one corresponding to high viscous fluid and another to low viscous fluid, separated by a stress plateau. We were able to develop a master dynamic phase diagram, which summarizes the nonlinear behavior by appropriately reducing the rheological variables. In the stress plateau, the micellar solution presents gradient shear banding, which was observed with the scattered light of a sheet of light perpendicular to the fluid flow velocity in the gap of a transparent Couette rheometer.

Strip waves in vibrated shear-thickening wormlike micellar solutions

We present an instability in vertically vibrated dilute wormlike micellar solutions. Above a critical driving acceleration the fluid forms elongated solitary domains of high amplitude waves. We model this instability using a Mathieu equation modified to account for the non-Newtonian character of the fluid. We find that our model successfully reproduces the observed transitions.

Interfacially Driven Instability in the Microchannel Flow of a Shear-Banding Fluid

Using microparticle image velocimetry, we resolve the spatial structure of the shear-banding flow of a wormlike micellar surfactant solution in a straight microchannel. We reveal an instability of the interface between the shear bands, associated with velocity modulations along the vorticity direction. We compare our results with a detailed theoretical study of the diffusive Johnson-Segalman model. The quantitative agreement obtained favors an instability scenario previously predicted theoretically but hitherto unobserved experimentally, driven by a normal stress jump across the interface between the bands.

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.

Effect of carbon chain length of cosurfactant on the rheological properties of nonionic wormlike micellar solutions formed by a sugar surfactant and monohydroxy alcohols

The effect of carbon chain length of monohydroxy alcohols on the rheological properties of micellar solutions formed in water/sucrose monohexadecanoate (C 16SE)/monohydroxy alcohol systems (C 3–C 9) was studied. By adding alcohols in the aqueous C 16SE solution, the zero-shear viscosity increases, and then decreases after a maximum for all the systems except for the 1-propanol system. The maximum η 0 increases with the chain length of the alcohol. The oscillatory-shear behavior of the viscous samples can be described by the Maxwell model which is typically observed for wormlike micellar solutions. The plateau modulus ( G 0) related to the number of entanglements between wormlike micelles increases monotonously and G 0 is higher for longer-chain alcohol systems except for the nonanol system, indicating more dense network is formed. The relaxation time ( τ R ) related to the micellar length increases and then decreases with weight fraction of alcohol. It indicates a faster stress relaxation model other than Maxwell model at high alcohol content due to the formation of branched micelles. Longer carbon chain length of alcohol gives higher maximum τ R . The weight fraction of alcohol at which the maximum τ R appears almost corresponds to that for η 0.

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.

Directed Self-Assembly of Spheres into a Two-Dimensional Colloidal Crystal by Viscoelastic Stresses

Ordering induced by shear flow can be used to direct the assembly of particles in suspensions. Flow-induced ordering is determined by the balance between a range of forces, such as direct interparticle, Brownian, and hydrodynamic forces. The latter are modified when dealing with viscoelastic rather than Newtonian matrices. In particular, 1D stringlike structures of spherical particles have been observed to form along the flow direction in shear thinning viscoelastic fluids, a phenomenon not observed in Newtonian fluids at similar particle volume fractions. Here we report on the formation of freestanding crystalline patches in planes parallel to the shearing surfaces. The novel microstructure is formed when particles are suspended in viscoelastic, wormlike micellar solutions and only when the applied shear rate exceeds a critical value. In spite of the very low volume fraction (less than 0.01), particles arrange themselves in 2D crystalline patches along the flow direction. This is a bulk phenomenon because 2D crystals form throughout the whole gap between plates, with the gap thickness being much larger than the particle size. Shear flow may hence be an easy method to drive particles into crystalline order in suspensions with viscoelastic properties. The crystalline structure reported here could be used to design new materials with special mechanical, optical, thermal, or electric properties.

Mesoscopic tube model of fluids composed of worm-like micelles

Rheological model of fluids involving Brownian relaxation, reptation, diffusion, and scission–recombination processes as relaxation mechanisms is formulated. Numerical solution of a particular example of the model displays the S-shape form of the shear rate versus shear stress curves observed in worm-like micellar solutions.

Composition-Insensitive Highly Viscous Wormlike Micellar Solutions Formed in Anionic and Cationic Surfactant Systems

We investigated phase behavior and rheological properties of aqueous micellar phase formed in water/cocoyl glutamate neutralized with triethanol amine (CGT-n)/hexadecyl trimethylammonium salt (CTAB or CTAC) systems, where n is a degree of neutralization. Micellar phase appears in wide composition range with respect to the surfactant mixing fraction in ternary phase diagrams at 25 degrees C. At high mixing fraction of cationic surfactant in the water/CGT-n/CTAB systems, one can observe a highly viscous micellar phase in which worm-like micelles are expected to form. Contrary to conventional systems in which worm-like micelles are formed, the zero-shear viscosity of the micellar solution in the water/CGT-n/CTAB system with n=1.2 increases with the addition of cationic cosurfactant and once decreases after a maximum, then increases again and decreases after the second maximum. At n=1.5 and 2, highly viscous solution is observed in the relatively wide range of surfactant mixing fraction instead of two maxima of the viscosity curve observed at n=1.2. In the case of CTAC instead of CTAB we can observe narrow composition range for the maximum viscosity. Frequency sweep measurements were performed on the highly viscous samples in the water/CGT-1.5/CTAB system. Typical viscoelastic behavior of worm-like micellar solutions is observed; i.e. the curves of storage (G') and loss (G") moduli make a crossover and the data points of G' and G" can be fitted to the Maxwell model. Relaxation time against the mixing fraction of two surfactants behaves similarly to the zero-shear viscosity change, whereas the plateau modulus continuously increases in the plateau region for the zero-shear viscosity curve.

Study on the pulsating flow of a worm-like micellar solution

In this work, the rectilinear flow of a complex liquid under a pulsating, time-dependent pressure gradient is analyzed. The fluctuating component of the pressure gradient is assumed to be of small amplitude and can be adequately represented by a weakly stochastic process, for which a quasi-static perturbation solution scheme is suggested. The pulsating pressure-gradient flow is analyzed with the Bautista–Manero–Puig model (BMP) constitutive equation, consisting in the upper convected Maxwell equation coupled to a kinetic equation to account for the breakdown and reformation of the fluid structure. According to the BMP model, thixotropy was found to have a negative effect on the energy associated to the maximum flow enhancement and reflects the relationship among the kinetic, viscous and structural mechanisms in the system. The flow enhancement is a function of the square of the amplitude of the oscillations, the Reynolds and Weissenberg numbers, and it is also dependent on the dimensionless numbers representing the viscoelastic, kinetic and structural mechanisms. Finally, flow enhancement is predicted in an aqueous worm-like micellar solution of cetyltrimethyl ammonium tosilate (CTAT) for various concentrations.

Velocity measurements for shear flows of CTAB/NaSal aqueous solutions between parallel plates

Shear flows of wormlike micellar solutions between parallel plates were experimentally investigated using aqueous solutions of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal) as test fluids. The solution having a molar concentration ratio of NaSal to CTAB of 2.0 was primarily considered. The velocity between the plates was measured by particle tracking velocimetry. In addition, flow-induced birefringence and shear stress were measured using a parallel-plate rheometer. The measurements were carried out for shear rates at which shear thinning or shear thickening was observed. Banded velocity profiles were found at every shear rate considered in the present experiments, and the profile was stable at relatively low shear rates but fluctuated with time at high shear rates. Furthermore, multi-band velocity profiles were observed in the shear-thinning region. The results of velocity measurements in the shear-thickening region suggest that the velocity profile largely varies with time, accompanying the movement of the position of a band interface. Moreover, both birefringence and shear stress fluctuated with time at high shear rates. These results suggest that the fluctuation of the velocity profile is related to the structural change in the micellar network that leads to the change in the thickness fraction of oriented and isotropic layers or to three-dimensional flows induced by material instabilities.

Flow Velocity Profiles and Shear Banding Onset in a Semidilute Wormlike Micellar System under Couette Flow

Velocity profiles in Couette flow are measured in a wormlike micellar solution made of cetyltrimethylammonium bromide (CTAB), sodium salicylate (NaSal), and water, at R (= [NaSal]/[CTAB]) = 2 and at R = 4; [CTAB] = 100 mM. Velocity profiles were obtained by using a two-incident beam laser Doppler technique. Profiles reveal that one of the micellar solutions (R = 2) becomes heterogeneous a long time after flow inception, even at very low imposed shear rates. However, profiles do not correspond to what is expected for gradient shear banding, because the fluid splits in one section close to the moving cylinder where the local mean velocity depends linearly on the gap position and in a second section presenting important velocity fluctuations. Close to the static cylinder, there is a third section where the fluid does not flow; it behaves like a slipping block. On the other hand, at high imposed shear rates, the former slipping block flows and presents a linear profile. Here, velocity profiles are consistent with gradient shear banding. The onset of shear banding was observed. The picture of two stable shear bands separated by a thin steady interface is not always valid. Inhomogeneous flow could be observed, although it cannot be classified as shear banding. In addition, conditions can be found where, as shear rate is increased and before shear banding appears, instead of a thin interface, a fluctuating intermediate band can be observed. On the contrary, for the R = 4 solution, the flow never becomes heterogeneous even at high shear rates. Flow curves were measured in a Couette cell under shear rate control in two cases, when stress is sensed with the moving cylinder and when it is sensed with the static cylinder of the cell. Differences between the flow curves can be explained by using the velocity profiles for both solutions.

Viscosity boosting effect of added ionic surfactant in nonionic wormlike micellar aqueous solutions

We have investigated the viscosity boosting effect of ionic surfactant on nonionic wormlike micellar solutions formed in the water/sucrose oleate (C 18:1SE) system. The zero-shear viscosity increases one order of magnitude from that of a binary water/C 18:1SE system by substituting the nonionic surfactant with ionic surfactant. The viscosity boosting effect depends on hydrophobic chain length and the ionic species of ionic surfactant. On increasing ionic surfactant composition, relaxation time increases; however, the plateau modulus is approximately constant. Zero-shear viscosity is a product of the relaxation time and the plateau modulus; hence we consider that the increase in the relaxation time is responsible for the viscosity boosting. Ionic surfactant added to nonionic wormlike micelles gives an electric double layer and it causes an electrostatic excluded volume effect. Therefore the diffusion constant of wormlike micelles, which can relax by reptation, may decrease due to the growth of the electric double layer and thus the relaxation time increases. However, excess addition of ionic surfactant causes reduction of viscosity. This may be attributed to the geometry change of wormlike micelles to spherical micelles due to the large repulsion between the hydrophilic groups of ionic surfactant.