Wormlike Micellar Solutions Research Articles

Sphere sedimentation in wormlike micelles: Effect of micellar relaxation spectrum and gradients in micellar extensions

We report experiments on the flow of wormlike micellar solutions based on cetylpyridinium chloride/sodium salicylate past a falling sphere via a combination of particle tracking velocimetry, particle image velocimetry, rheology, and flow induced birefringence (FIB). Recent studies have shown that beyond a critical extensional Deborah number, a falling sphere in wormlike micelles never reaches a constant terminal velocity; instead, it settles with an unsteady velocity. This behavior is linked to the wormlike micellar chain scission in the wake of the sphere. Similar instabilities in viscoelastic polymer solutions, where polymer chain scission is highly unlikely, are thought to be the results of a single-mode relaxation spectrum of the polymer chains or the asymmetry in the polymer chain extensions on the flanks of the falling sphere. In this paper, we examine the effect of micellar relaxation spectrum and gradients in micellar extensions on sphere instability in wormlike micelles over a wide range of flow parameters (10−2 < DeE < 40.7 and 10−6 < Re < 10). In wormlike micelles with a single-mode relaxation spectrum, the sphere instability occurs for DeE ≥ 2.6. However, for similar conditions (2.5 ≤ DeE ≤ 40.7 and 10−2 < Re < 10), spheres never exhibit unsteady motion in the wormlike micelles with a broad spectrum of relaxation times. This indicates the importance of the micellar relaxation spectrum on dynamics of sphere sedimentation in wormlike micelles. We show that a criterion based on the ratio of dissipated energy to the stored elastic energy of micelles can successfully describe the effect of micellar relaxation spectrum on sphere sedimentation dynamics. In addition, for conditions that give rise to sphere instability, FIB indicates that micellar extensions on sphere sideways are perfectly symmetric.

Colloidal fibers as structurant for worm-like micellar solutions

We investigate the rheological properties of a simplified version of a liquid detergent composed of an aqueous solution of the linear alkylbenzene sulphonate (LAS) surfactant, in which a small amount of fibers made of hydrogenated castor oil (HCO) is dispersed. At the concentration typically used in detergents, LAS is in a worm-like micellar phase exhibiting a Maxwellian behavior. The presence of HCO fibers provides elastic properties, such that the system behaves as a simple Zener body, mechanically characterized by a parallel connection of a spring and a Maxwell element. Despite this apparent independence of the contributions of the fibers and the surfactant medium to the mechanical characteristics of the system, we find that the low frequency modulus increases with increasing LAS concentration. This indicates that LAS induces attractive interactions among the HCO fibers, resulting in the formation of a stress-bearing structure that withstands shear at HCO concentrations, where the HCO fibers in the absence of attractive interactions would not sufficiently overlap to provide stress-bearing properties to the system.

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.

Elastic instability and secondary flow in cross-slot flow of wormlike micellar solutions

In this work, we focus on modeling the flow of wormlike micellar solutions in a cross-slot device. In particular, we investigate the role that the breakage of wormlike micelles plays in the formation of lip vortices and the development of an asymmetric elastic instability. The cross-slot induces a planar elongational flow field around the stagnation point in the center of the geometry, and a shear-dominated flow in the inlet and outlet arms. The extensional flow can induce the emergence of a bifurcation, in which the symmetric flow undergoes an instability and becomes asymmetric at sufficiently large flow rates. We further show that this instability can be completely suppressed by creating micelles that are easier to break under imposed stretching. Furthermore, at larger flow rates, unlike comparable simulations of polymer solutions, the asymmetric flow remains steady in agreement with experiments. In addition, the shear flow can induce the formation of a recirculation region in the inlet arm along the channel wall directly upstream of the corner. In particular, we reveal the possibility of controlling the vortex regions via flow-induced breakage; as the chains become easier to break, the degree of shear thinning increases and the size of the vortices decreases. Ultimately, our predictions show that chain scission plays an important role in altering fluid flow.

A constitutive rheological model for agglomerating blood derived from nonequilibrium thermodynamics

Red blood cells tend to aggregate in the presence of plasma proteins, forming structures known as rouleaux. Here, we derive a constitutive rheological model for human blood which accounts for the formation and dissociation of rouleaux using the generalized bracket formulation of nonequilibrium thermodynamics. Similar to the model derived by Owens and co-workers [“A non-homogeneous constitutive model for human blood. Part 1. Model derivation and steady flow,” J. Fluid Mech. 617, 327–354 (2008)] through polymer network theory, each rouleau in our model is represented as a dumbbell; the corresponding structural variable is the conformation tensor of the dumbbell. The kinetics of rouleau formation and dissociation is treated as in the work of Germann et al. [“Nonequilibrium thermodynamic modeling of the structure and rheology of concentrated wormlike micellar solutions,” J. Non-Newton. Fluid Mech. 196, 51–57 (2013)] by assuming a set of reversible reactions, each characterized by a forward and a reverse rate constant. The final set of evolution equations for the microstructure of each rouleau and the expression for the stress tensor turn out to be very similar to those of Owens and co-workers. However, by explicitly considering a mechanism for the formation and breakage of rouleaux, our model further provides expressions for the aggregation and disaggregation rates appearing in the final transport equations, which in the kinetic theory-based network model of Owens were absent and had to be specified separately. Despite this, the two models are found to provide similar descriptions of experimental data on the size distribution of rouleaux.

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.

Mechanistic constitutive model for wormlike micelle solutions with flow-induced structure formation

We present a tensor constitutive model for predicting stress and flow-induced structure formation in dilute wormlike micellar solutions. The micellar solution is treated as a dilute suspension of rigid Brownian rods whose length varies dynamically. Consistent with the mechanism presented by Turner and Cates [J. Phys.: Condens. Matter 4, 3719 (1992)], flow-induced alignment of the rods is assumed to promote increase of rod length that corresponds to the formation of flow-induced structures observed in experiments. At very high deformation rate, hydrodynamic stresses causes the rod length to decrease. These mechanisms are implemented in a phenomenological equation governing the evolution of rod length, with the number density of rods appropriately modified to ensure conservation of surfactant mass. The model leads first to an increase in both shear and extensional viscosity as deformation rate increases and then to a decrease at higher rates. If the rate constant for flow-induced rod growth is sufficiently large, the model predicts a multivalued relation between stress and deformation rate in both shear and uniaxial extension. Predictions for shear and extensional flow at steady state are in reasonable agreement with experimental results. By design, the model is simple enough to serve as a tractable constitutive relation for computational fluid dynamics studies.

Distinguishing shear banding from shear thinning in flows with a shear stress gradient

Shear banding occurs in complex fluids that exhibit a non-monotonic constitutive instability, such as wormlike micelles, and potentially also in polymeric fluids with presumably monotonic constitutive behavior. However, velocity profiles for shear thinning fluids in geometries possessing a stress gradient, such as Taylor-Couette flow, could be misidentified as shear banding. To address this, we present a model-free experimental procedure to distinguish shear banding from strong shear thinning using high-resolution velocimetry. The approach is developed and validated using simulations using the d-Giesekus model and is based upon the behavior of the width of the apparent interface between the high and low shear rate regions. It is then tested using experimental data for model wormlike micellar solutions. The method allows shear banding to be distinguished from shear thinning in cases where this difference is otherwise indistinguishable. As a by-product, it also provides an estimate of the stress diffusivities for shear banding fluids.

Wormlike micelles constructed by a highly water-soluble carboxylate surfactant containing a phenoxy and nonionic surfactant

Limited wormlike micelles were formed from anionic surfactants, especially for carboxylate surfactants due to their poor solubility. As we all know sodium hexadecanoate (C16Na) has a higher Krafft point (60°C). Introducing a phenoxy at α-position of C16Na, a new carboxylate surfactant, sodium 2-phenoxyhexadecanoate (C16phNa), was synthesized. And the aqueous solution of C16phNa could keep water-like even at a concentration of 2M at 25°C. Wormlike micelles were successfully constructed by adding tri(oxyethylene) monododecyl ether (C12EO3) into the dilute solution of C16phNa. Steady state and dynamic rheological measurements were used to investigate the viscoelasticity of C16phNa/C12EO3 wormlike micellar solution. The results show zero shear viscosity (η0) and relaxation time (τRmax) increase at first and then decrease with the increasing concentration of C16phNa or ratio of C12EO3 and C16phNa. The value of η0 in the system could reach 2194.4Pa·s. In addition, η0 is extremely dependent upon the concentration of C16phNa. And the value of exponents between the η0 and concentration is 13.1, 20.2 and 18.3 for a 1:1, 1:1.5 and 1:1.8 system of C16phNa and C12EO3, respectively, which is rather large. Cyro-TEM gave a direct image of wormlike micelles of the system. The present systems enrich the contents of anionic wormlike micelles systems.

The interface migration in shear-banded micellar solutions

In this work, we study the diffusion of the interface between bands in wormlike micellar solutions that exhibit shear banding flow regimes, namely, systems undergoing coexistence of states of different shear rates along a constant stress plateau. The migration of the interface between bands possessing different birefringence levels is predicted by the BMP (Bautista-Manero-Puig) model in which a structural parameter (the fluidity) presents two states with differing order separated by an interface. The mechanical potential derived from the constitutive equations and a diffusion term for the structure evolution equation predict various time scales of interface migration at the inception of shear flow and under shear-rate changes along the plateau stress. It is shown that the extremes of the plateau (binodals) correspond to the minima in the mechanical potential as a function of fluidity or shear rate. We also predict the dependence of the diffusive length scale on the applied shear rate.

Role of chain scission in cross-slot flow of wormlike micellar solutions

The role of chain scission is investigated numerically in the symmetry breaking of the flow of wormlike micellar solutions in a cross-slot geometry.

An RNA Aptamer Capable of Forming a Hydrogel by Self-Assembly.

Hydrogels are supramolecular assemblies with both solute transport properties like liquids and mechanical properties like elastomers. To date, every type of biomolecules except ribonucleic acid (RNA), is capable of forming a hydrogel. Here, we report an RNA that forms a hydrogel by self-assembly. This RNA is originally identified by systematic evolution of ligands by exponential enrichment (SELEX) to enhance the activity of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors as a potential RNA drug for the treatment of cognitive disorders. The RNA hydrogel exhibits an elastic modulus plateau on the order of 102 Pa and shows dynamic RNA chain interactions with relaxation behaviors similar to living wormlike micellar solutions. Small-angle X-ray scattering and cryogenic electron microscopy characterization support the RNA network structures. By sequence mutation and rheological measurements, we reveal two key sequence motifs in the RNA responsible for intermolecular recognition and the formation of a polymer network by self-assembly.

Accurate and fast creep test for viscoelastic fluids using disk-probe-type and quadrupole-arrangement-type electromagnetically spinning systems

Viscoelasticity is a unique characteristic of soft materials and describes its dynamic response to mechanical stimulations. A creep test is an experimental method for measuring the strain ratio/rate against an applied stress, thereby assessing the viscoelasticity of the materials. We propose two advanced experimental systems suitable for the creep test, adopting our original electromagnetically spinning (EMS) technique. This technique can apply a constant torque by a noncontact mechanism, thereby allowing more sensitive and rapid measurements. The viscosity and elasticity of a semidilute wormlike micellar solution were determined using two setups, and the consistency between the results was assessed.

Inertio-elastic instability in Taylor-Couette flow of a model wormlike micellar system

In this work, we use flow visualization and rheometry techniques to study the dynamics and evolution of secondary flows in a model wormlike micellar solution sheared between concentric cylinders, i.e., in a Taylor-Couette cell. The wormlike micellar solution studied in this work contains cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal). This system can be shear banding and highly elastic, nonshear banding and moderately elastic, or nearly Newtonian as the temperature is varied over a narrow range. The effect of elasticity on transitions and instabilities is probed by changing the temperature over a wide range of elasticity (El ≪ 1, El ≈ 1, and El ≫ 1). Elasticity is defined as the ratio of the Weissenberg number to the Reynolds number. For shear banding wormlike micelle solutions where El ≫ 1, a primary transition from the base Couette flow to stationary vortices that are evenly spaced in the axial direction of the shear cell and are characterized by an asymptotic wave-length is observed. The dimensionless wave-length at the onset of this shear banding transition for CTAB/NaSal system turns out to be much larger than those reported for other shear banding wormlike micelle systems. For the same fluid at a temperature where it shear-thins but does not display shear banding, El ≈ 1, and for slow ramp speeds, the primary transition is to distinct structures that are not stationary but rather travel in the axial direction. At low elasticity (El ≪ 1), where the fluid behaves as a nearly Newtonian fluid, several transitions from purely azimuthal Couette flow to modified Taylor vortex flows and finally chaotic regimes are documented. The behavior in the shear-banding and nonshear-banding regimes are discussed and compared with results in related systems. The possibility of hysteresis in the flow transitions as well as the effects of co-rotation and counter-rotation of the cylinders on transitions and instabilities are also examined for a wide range of elasticity.

π-π stacking interaction in mixed surfactant solutions assembled by cationic surfactant and organic salt with a naphthalene nucleus

Wormlike micellar solutions (WLMs) on the basis of ionic surfactants customarily show only one peak in curves of viscosity versus concentration of organic salt. Here, we report the highly viscoelastic states of wormlike micelles obtained in the solution, which contain a cationic surfactant N-hexadecane-N-methylpyrrolidinium bromide (C16MPB) and an organic salt, Sodium 2-naphthalenesulfonate (SNphS). It manifests an unusual double-hump with increasing concentration of SNphS while surfactant concentration remains constant. A possible mechanism for the formation of wormlike micellar gel, which does not flow when inverted due to having higher viscoelasticity, is proposed. The π-π aromatic interactions from hydrotrope with naphthalene nucleus induce the wormlike micelles to become more closely packed. Thus, this work shows that π-π stacking interactions represent an effective means of enhancing viscosity of WLMs.

Importance of Interfacial Interactions to Access Shear Elasticity of Liquids and Understand Flow Induced Birefringence from Liquid Crystals to Worm-Like Micellar Solutions

This work points out the importance of the substrate boundary conditions to lower the dissipation in the dynamic measurement and access the closest dynamic characteristics of liquids, in particular to access the low frequency shear elasticity. The liquid/surface interface is a source of dissipation that enters and impacts the measurement. Examples of steady-state shear flows or flow birefringence are presented to highlight the non-universality of the behavior with respect to the nature of the substrate or the sheared thickness. Additionally the present development completes and extends the identification of low frequency shear elasticity made at sub-millimeter gaps in various one-component liquids to salt-free aqueous solutions (CTAB-water (Hexadecyl-TrimethylAmmonium Bromide)).

ひも状ミセル水溶液の大振幅正弦的せん断流に生じる粘度の回転方向依存性に関する研究

ひも状ミセル水溶液の大振幅正弦的せん断流に生じる粘度の回転方向依存性に関する研究

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.

Impact of Salt Co- and Counterions on Rheological Properties and Structure of Wormlike Micellar Solutions.

Rheological properties of aqueous solutions of long-tailed cationic surfactant erucyl bis-(hydroxyethyl)methylammonium chloride (EHAC) were examined as a function of concentration Cs of different inorganic salts (KCl, CaCl2, and LaCl3) at a fixed surfactant concentration of 0.6 wt %. The structural evolution of micelles was followed by small-angle neutron scattering and cryogenic transmission electron microscopy. It was observed that, upon addition of salt, the zero-shear viscosity η0 of semidilute surfactant solutions goes through a maximum by passing the following three regimes: η0 ∼ Cs10 (regime I), η0 ∼ Cs3.5 (regime II), and η0 ∼ Cs-2 (regime III). In regime I, the micelles grow in length; in regime II, the linear growth of micelles proceeds simultaneously with their branching; and in regime III, the branching becomes dominating. With increase in the salt valence, the viscosity curves shift to a lower salt content, indicating that these salts are more effective in inducing micellar elongation and branching, as they contain a larger amount of anionic species Cl- screening the repulsion between cationic surfactant heads. Diverse roles of salt co- and counterions (i.e., salt ions that are similar and oppositely charged with respect to surfactant head groups) at different salt concentrations were demonstrated. It was shown that at low salt concentrations corresponding to the rising branch of the viscosity curve (regimes I and II), salt counterions (Cl-) fully determine the rheological behavior of the system. At high salt concentrations, when the electrostatic repulsions between micelles and salt co-ions are essentially screened, the co-ions start affecting the rheological properties. Under these conditions, monovalent co-ions (K+) provide much lower viscosity of surfactant solutions than the multivalent ones (Ca2+, La3+), which is consistent with theoretical predictions that suggest the penetration of K+ inside the micellar corona increasing the charge of the micelles and therefore hindering their growth.

Validation of constitutive modeling of shear banding, threadlike wormlike micellar fluids

In this work, we assess the capability of two thermodynamically consistent, microstructure-based models to predict the rheology and microstructure of a model wormlike micellar surfactant solution known to exhibit shear banding undergoing transient simple shear and large amplitude oscillatory shear deformations in a cylindrical Couette rheometer. The microstructure of the surfactant solution was simultaneously measured during the rheometric tests using small angle neutron scattering and this data is used along with the bulk rheometry to critically evaluate the models. The first model is a new, two-species model specifically developed for wormlike micelles, whereas the second model is the well-known Giesekus model with solvent and diffusion. The new model accounts for the microstructural dynamics including flow-induced micellar breakage and recombination, which is shown to be necessary to predict some of the measured rheology and microstructure. More species should be included to improve the quantitative co...