Rheology and shear induced structures in surfactant solutions

Effects of shear-induced crystallization on the complex viscosity of lamellar-structured concentrated surfactant solutions

Material relationships at low temperatures were determined for concentrated surfactant solutions using a combination of rheological experiments, cross-polarized microscopy, calorimetry, and small angle X-ray scattering. A lamellar structured 70 wt% solution of sodium laureth sulfate in water was used as a model system. At cold temperatures (5 °C and 10 °C), the formation of surfactant crystals resulted in extremely high viscosity. The bulk flow behavior of multi-lamellar vesicles (20 °C) and focal conic defects (90 °C) in the lamellar phase was similar. Shear-induced crystallization at temperatures higher than the equilibrium crystallization temperature range resulted in an unusual complex viscosity peak. The effects of processing-relevant parameters including temperature, cooling time, and applied shear were investigated. Knowledge of key low-temperature structure-property-processing relationships for concentrated feedstocks is essential for the sustainable design and manufacturing of surfactant-based consumer products for applications such as cold-water laundry.

Applicability Assessment of Viscoelastic Surfactants and Synthetic Polymers as a Base of Hydraulic Fracturing Fluids

Hydraulic fracturing (HF) is currently the most widespread and effective method of oil production stimulation. The most commonly used fracturing fluid is crosslinked guar gels. However, when using these systems, problems such as clogging of the pore space, cracking, and proppant packing with the remains of the undestroyed polymer arise. Therefore, the efficiency of the hydraulic fracturing process decreases. In this work, compositions based on viscoelastic surfactants (VES) and synthetic polymers (SP) were considered as alternatives capable of minimizing these disadvantages. Most often, the possibility of using a composition as a fracturing fluid is evaluated using rotational viscometry. However, rotational viscometry is not capable of fully assessing the structural and mechanical properties of fracturing fluid. This leads to a reduced spread of systems based on VES and SP. This paper proposes an integrated approach to assessing the effectiveness of a water-based fracturing fluid. The proposed comprehensive approach includes an assessment of the main characteristics of water-based fracturing fluids, including an analysis of their structural and mechanical properties, which is based on a combination of rotational and oscillatory rheology and a comparative analysis of methods for studying the influence of fluids on the reservoir rock. The use of the developed approach to assess the technological properties of fracturing fluids makes it possible to demonstrate the potential applicability of new, unconventional fracturing fluids such as systems based on VES and SP.

Formulation of More Efficacious Curcumin Delivery Systems Using Colloid Science: Enhanced Solubility, Stability, and Bioavailability

Curcumin is a bioactive constituent isolated from turmeric that has historically been used as a seasoning, pigment, and herbal medicine in food. Recently, it has become one of the most commonly studied nutraceuticals in the pharmaceutical, supplement, and food areas because of its myriad of potential health benefits. For instance, it is claimed to exhibit antioxidant, anti-inflammatory, antimicrobial, antiparasite, and anticancer activities when ingested as a drug, supplement, or food. Toxicity studies suggest that it is safe to consume, even at relatively high levels. Its broad-spectrum biological activities and low toxicity have meant that it has been widely explored as a nutraceutical ingredient for application in functional foods. However, there are several hurdles that formulators must overcome when incorporating curcumin into commercial products, such as its low water solubility (especially under acidic and neutral conditions), chemical instability (especially under neutral and alkaline conditions), rapid metabolism by enzymes in the human body, and limited bioavailability. As a result, only a small fraction of ingested curcumin is actually absorbed into the bloodstream. These hurdles can be at least partially overcome by using encapsulation technologies, which involve trapping the curcumin within small particles. Some of the most commonly used edible microparticles or nanoparticles utilized for this purpose are micelles, liposomes, emulsions, solid lipid particles, and biopolymer particles. Each of these encapsulation technologies has its own benefits and limitations for particular product applications and it is important to select the most appropriate one.

Vesicle formation mechanisms: an overview

Vesicle structures primarily embody spherical capsules composed of a single or multiple bilayers, entrapping a pool of aqueous solution in their interior. The bilayers can be synthesised by phospholipids or other amphiphiles (surfactants, block copolymers, etc.). Vesicles with broad-spectrum applications in numerous scientific disciplines, including biochemistry, biophysics, biology, and various pharmaceutical industries, have attracted widespread attention. Consequently, a multitude of protocols have been devised and proposed for their fabrication. In this review, with a motivation to derive the basic conditions for the formation of vesicles, the associated thermodynamic and kinetic aspects are comprehensively appraised. Contextually, an all-purpose overview of the underlying thermodynamics of bilayer/membrane generation and deformation, including the chemical potential of aggregates, geometric packing and the concept of elastic properties, is presented. Additionally, the current review highlights the probable, inherent mechanisms of vesicle formation under distinct modes of manufacturing. We lay focus on vesicle formation from pre-existing bilayers, as well as from bilayers, which form when lipids from an organic solvent are transferred into an aqueous medium. Furthermore, we outline the kinetic effects on vesicle formation from the lamellar phase, with and without the presence of shearing force. Wherever required, the experimental and/or theoretical outcomes, the driving forces for vesicle size selection, and various scaling laws are also reviewed, all of which facilitate an overall improved understanding of the vesicle formation mechanisms.

An experimental rheological phase diagram of a tri-block co-polymer in water validated against dissipative particle dynamics simulations

Aqueous solutions of tri-block co-polymer surfactants are able to aggregate into a rich variety of microstructures, which can exhibit different rheological behaviors. In this work, we study the diversity of structures detected in aqueous solutions of Pluronic L64 at various concentrations and temperatures by experimental rheometry and dissipative particle dynamics (DPD) simulations. Mixtures of Pluronic L64 in water (ranging from 0 to 90 wt% Pluronic L64) have been studied in both linear and non-linear regimes by oscillatory and steady shear flow. The measurements allowed for the determination of a complete rheological phase diagram of the Pluronic L64-water system. The linear and non-linear regimes have been compared to equilibrium and non-equilibrium DPD bulk simulations of similar systems obtained by using the software LAMMPS. The molecular results are capable of reproducing the equilibrium structures, which are in complete agreement with the ones predicted through experimental linear rheology. The simulations also depict micellar microstructures after long time periods when a strong flow is applied. These structures are directly compared, from a qualitative point of view, with the corresponding experimental results and differences between the equilibrium and non-equilibrium phase diagrams are highlighted, proving the capability of detecting morphological changes caused by deformation in both experiments and DPD simulations. The effect of temperature on the rheology of the systems has been eventually investigated and compared with the already existing non-rheological phase diagram.

CO2-responsive aqueous foams stabilized by pseudogemini surfactants

HYPOTHESIS

To obtain surfactants with superior surface activity and responsive behavior, "pseudogemini" surfactants (short for D-LCFA) are synthesized by mixing long chain fatty acids (LCFA) and polyetheramine D 230 at fixed molar ratio (2:1). Non-covalently bonded building blocks indicate that CO₂-responsive aqueous foams can be obtained by utilizing such pseudogemini surfactants.

EXPERIMENTS

¹H NMR and FT-IR characterizations prove that the building blocks of these surfactants are associated by electrostatic interaction. The synthesis (Brønsted acid-base reaction) is simple and eco-friendly. "Pseudogemini" structure enables D-LCFA to reduce surface tension of aqueous solution effectively, thus facilitating foam generation. Rheograms, FF-TEM and Cryo-TEM results prove that different aggregates in D-LCFA aqueous solutions lead to different foam properties.

FINDINGS

Bubbling of CO₂ for about 30 s leads to the rupture of aqueous foams generated by D-LCFA, while removing CO₂ by bubbling of N₂ at 65 °C for 10 min enables re-generation of foams. The CO₂-responsive foaming properties can be attributed to dissociation of D-LCFA upon bubbling of CO₂ and re-association upon removal of CO₂. The effective CO₂-responsive foams can be applied to many areas, such as foam fracturing, foam enhanced oil recovery or recovering of radioactive materials.

Multilamellar Vesicle Formation Probed by Rheo-NMR and Rheo-SALS under Large Amplitude Oscillatory Shear

The formation of multilamellar vesicles (MLVs) in the lyotropic lamellar phase of the system triethylene glycol mono n-decyl ether (C₁₀E₃)/water is investigated under large amplitude oscillatory shear (LAOS) using spatially resolved rheo-NMR spectroscopy and a combination of rheo-small angle light scattering (rheo-SALS) and conventional rheology. Recent advances in rheo-NMR hardware development facilitated the application of LAOS deformations in high-field NMR magnets. For the range of investigated strain amplitudes (10-50) and frequencies (1 and 2 rad s^-1), MLV formation is observed in all NMR and most SALS experiments. It is found that the MLV size depends on the applied frequency in contrast to previous steady shear experiments where the shear rate is the controlling parameter. The onset of MLV formation, however, is found to vary with the shear amplitude. The LAOS measurements bear no indication of the intermediate structures resembling aligned multilamellar cylinders observed in steady shear experiments. Lissajous curves of stress vs strain reveal a transition from a viscoelastic solid material to a pseudoplastic material.

Turbulence and Cavitation Suppression by Quaternary Ammonium Salt Additives

We identify the physical mechanism through which newly developed quaternary ammonium salt (QAS) deposit control additives (DCAs) affect the rheological properties of cavitating turbulent flows, resulting in an increase in the volumetric efficiency of clean injectors fuelled with diesel or biodiesel fuels. Quaternary ammonium surfactants with appropriate counterions can be very effective in reducing the turbulent drag in aqueous solutions, however, less is known about the effect of such surfactants in oil-based solvents or in cavitating flow conditions. Small-angle neutron scattering (SANS) investigations show that in traditional DCA fuel compositions only reverse spherical micelles form, whereas reverse cylindrical micelles are detected by blending the fuel with the QAS additive. Moreover, experiments utilising X-ray micro computed tomography (micro-CT) in nozzle replicas, quantify that in cavitation regions the liquid fraction is increased in the presence of the QAS additive. Furthermore, high-flux X-ray phase contrast imaging (XPCI) measurements identify a flow stabilization effect in the region of vortex cavitation by the QAS additive. The effect of the formation of cylindrical micelles is reproduced with computational fluid dynamics (CFD) simulations by including viscoelastic characteristics for the flow. It is demonstrated that viscoelasticity can reduce turbulence and suppress cavitation, and subsequently increase the injector's volumetric efficiency.

Effect of interlamellar interactions on shear induced multilamellar vesicle formation

Shear-induced multilamellar vesicle (MLV) formation has been studied by coupling the small-angle neutron scattering (SANS) technique with neutron spin echo (NSE) spectroscopy. A 10% mass fraction of the nonionic surfactant pentaethylene glycol dodecyl ether (C₁₂E₅) in water was selected as a model system for studying weak inter-lamellar interactions. These interactions are controlled either by adding an anionic surfactant, sodium dodecyl sulfate, or an antagonistic salt, rubidium tetraphenylborate. Increasing the charge density in the bilayer induces an enhanced ordering of the lamellar structure. The charge density dependence of the membrane bending modulus was determined by NSE and showed an increasing trend with charge. This behavior is well explained by a classical theoretical model. By considering the Caillé parameters calculated from the SANS data, the layer compressibility modulus B¯ is estimated and the nature of the dominant inter-lamellar interaction is determined. Shear flow induces MLV formation around a shear rate of 10 s^-1, when a small amount of charge is included in the membrane. The flow-induced layer undulations are in-phase between neighboring layers when the inter-lamellar interaction is sufficiently strong. Under these conditions, MLV formation can occur without significantly changing the inter-lamellar spacing. On the other hand, in the case of weak inter-lamellar interactions, the flow-induced undulations are not in-phase, and greater steric repulsion leads to an increase in the inter-lamellar spacing with shear rate. In this case, MLV formation occurs as the amplitude of the undulations gets larger and the steric interaction leads to in-phase undulations between neighboring membranes.

Effect of the salt-induced micellar microstructure on the nonlinear shear flow behavior of ionic cetylpyridinium chloride surfactant solutions

The shear flow dynamics of linear and branched wormlike micellar systems based on cetylpyridinium chloride and sodium salicylate in brine solution is investigated through rheometric and scattering techniques. In particular, the flow and the structural flow response are explored via velocimetry measurements and rheological and rheometric small-angle neutron scattering (SANS) experiments, respectively. Although all micellar solutions display a similar shear thinning behavior in the nonlinear regime, the experimental results show that shear banding sets in only when the micelle contour length L[over ¯] is sufficiently long, independent of the nature of the micellar connections (either linear or branched micelles). Using rheometric SANS, we observe that the shear banding systems both show very similar orientational ordering as a function of Weissenberg number, while the short branched micelles manifest an unexpected increase of ordering at very low Weissenberg numbers. This suggests the presence of an additional flow-induced relaxation process that is peculiar for branched systems.

Effects of Oscillatory Shear on the Orientation of the Inverse Bicontinuous Cubic Phase in a Nonionic Surfactant/Water System

The bicontinuous inverse cubic phase (V2 phase) formed in amphiphilic systems consists of bilayer networks with a long-range order. We have investigated effects of oscillatory shear on the orientation of the V2 phase with space group Ia3d formed in a nonionic surfactant (C12E2)/water system by using simultaneous measurements of rheology/small-angle X-ray scattering. It is shown that grain refining occurs by applying the large amplitude oscillatory shear (LAOS) with a strain amplitude (γ0) of ∼20, which gives the ratio of the loss modulus (G″) to the storage modulus (G') (G″/G' = tan δ) of ∼100. On the other hand, orientation of the cubic lattice occurs when the small amplitude (γ0 ≈ 0.0004) oscillatory shear (SAOS) in the linear regime is applied to the sample just after the LAOS. Interestingly, the orientation is strongly enhanced by the "medium amplitude" (γ0 ≈ 0.05) oscillatory shear ("MAOS") after the SAOS. When the MAOS is applied before applying the LAOS, orientation to a particular direction is not observed, indicating that the grain refining process by the LAOS is necessary for the orientation during the MAOS. The results of additional experiments show that the shear sequence "LAOS-MAOS" is effective for the orientation of the cubic lattice. When the LAOS and MAOS are applied to the sample alternatively, grain refining and orientation occur during the LAOS and MAOS, respectively, indicating reversibility of the orientation. It is shown that (i) the degree of the orientation is dependent on γ0 and the frequency (ω) of the MAOS and (ii) relatively higher orientation can be obtained for the combination of γ0 and ω, which gives tan δ = 2-3. The lattice constant does not change throughout all the shearing processes and is equal to that before shearing within the experimental errors, indicating that the shear melting does not occur. These results suggest a possibility to control the orientation of the cubic lattice only by changing the conditions of oscillatory shear without using the epitaxial transition from other anisotropic phases, such as the hexagonal and lamellar phases.

Microfluidic processing of concentrated surfactant mixtures: online SAXS, microscopy and rheology

We investigate the effect of microfluidic flow on the microstructure and dynamics of a model surfactant mixture, combining synchrotron Small Angle X-ray Scattering (SAXS), microscopy and rheology. A system comprising a single-chain cationic surfactant, hexadecyl trimethyl ammonium chloride (C16TAC), a short-chain alcohol (1-pentanol) and water was selected for the study due to its flow responsiveness and industrial relevance. Model flow fields, including sequential contraction-expansion (extensional) and rotational flows, were investigated and the fluid response in terms of the lamellar d-spacing, orientation and birefringence was monitored in situ, as well as the recovery processes after cessation of flow. Extensional flows are found to result in considerable d-spacing increase (from approx 59 Å to 65 Å). However, under continuous flow, swelling decreases with increasing flow velocity, eventually approaching the equilibrium values at velocities ≃2 cm s(-1). Through individual constrictions we observe the alignment of lamellae along the flow velocity, accompanied by increasing birefringence, followed by an orientation flip whereby lamellae exit perpendicularly to the flow direction. The resulting microstructures are mapped quantitatively onto the flow field in 2D with 200 μm spatial resolution. Rotational flows alone do not result in appreciable changes in lamellar spacing and flow type and magnitude evidently impact the fluid microstructure under flow, as well as upon relaxation. The findings are correlated with rheological properties measured ex situ to provide a mechanistic understanding of the effect of flow imposed by tubular processing units in the phase behavior and performance of a model surfactant system with ubiquitous applications in personal care and coating industries.

Structure formation of surfactant membranes under shear flow

Shear-flow-induced structure formation in surfactant-water mixtures is investigated numerically using a meshless-membrane model in combination with a particle-based hydrodynamics simulation approach for the solvent. At low shear rates, uni-lamellar vesicles and planar lamellae structures are formed at small and large membrane volume fractions, respectively. At high shear rates, lamellar states exhibit an undulation instability, leading to rolled or cylindrical membrane shapes oriented in the flow direction. The spatial symmetry and structure factor of this rolled state agree with those of intermediate states during lamellar-to-onion transition measured by time-resolved scatting experiments. Structural evolution in time exhibits a moderate dependence on the initial condition.

Anisotropic particles align perpendicular to the flow direction in narrow microchannels

The flow orientation of anisotropic particles through narrow channels is of importance in many fields, ranging from the spinning and molding of fibers to the flow of cells and proteins through thin capillaries. It is commonly assumed that anisotropic particles align parallel to the flow direction. When flowing through narrowed channel sections, one expects the increased flow rate to improve the parallel alignment. Here, we show by microfocus synchrotron X-ray scattering and polarized optical microscopy that anisotropic colloidal particles align perpendicular to the flow direction after passing a narrow channel section. We find this to be a general behavior of anisotropic colloids, which is also observed for disk-like particles. This perpendicular particle alignment is stable, extending downstream throughout the remaining part of the channel. We show by microparticle image velocimetry that the particle reorientation in the expansion zone after a narrow channel section occurs in a region with considerable extensional flow. This extensional flow is promoted by shear thinning, a typical property of complex fluids. Our discovery has important consequences when considering the flow orientation of polymers, micelles, fibers, proteins, or cells through narrow channels, pipes, or capillary sections. An immediate consequence for the production of fibers is the necessity for realignment by extension in the flow direction. For fibrous proteins, reorientation and stable plug flow are likely mechanisms for protein coagulation.

The Micelle-to-Vesicle Phase Transition in Dilute Aqueous Solution from Undecylamine Induced by Metal(II) ion (Cu2+)

A metal(II) ion (Cu2+)-induced vesicle phase was prepared from a mixture of n-undecylamine and CuCl2 in aqueous solution. Cu2+-ligand coordination with n-undecylamine results in the formation of molecular bilayers because Cu2+ can firmly bind to the amine groups of n-undecylamine which reduced the area of the head-group. In this system, no counterions in aqueous solution exist because of the Cu2+-ligand coordination, and the bilayer membranes are not shielded by salts. The vesicles were determined by transmission electron microscopy (TEM) images and dynamic light scattering (DLS) measurements.

Re-entrant lamellar/onion transition with varying temperature under shear flow

We have found for the first time the reentrant lamellar/onion (lamellar-onion-lamellar) transition with varying temperature under constant shear rate by using simultaneous measurements of shear stress and small-angle X-ray scattering (Rheo-SAXS) for a nonionic surfactant (C(14)E(5))/water system, which exhibits the lamellar phase in a wide temperature range from 15-75 °C. The onion state exists in a closed region in the temperature-concentration diagram at a constant shear rate. Temperature dependence of the lamellar repeat distance (d) at rest has also been measured at several concentrations. It is shown that the increase of d with increasing temperature is necessary for the existence of the lower transition. We have investigated the change in the lamellar orientation in the lamellar-to-onion and onion-to-lamellar transition processes near the upper and lower transition temperatures. For all four kinds of transition processes, the following change in the lamellar orientation is observed; lamellar state (oriented to the velocity gradient direction) ↔ further enhancement of the orientation to the velocity gradient direction ↔ enhancement of the orientation to the neutral direction ↔ onion state.

Sizes of multilamellar vesicles in shear

The dynamics of the multilamellar vesicle (MLV) is analyzed theoretically, where membrane interaction squeezes the solvent to flow between the neighboring membranes. With the applied affine shear, the dynamic free energy density of the MLV develops a minima, which selects the MLV size. The model predicts a terminal shear rate, below which the metastable MLV exists. The scaling relations for the MLV size and the terminal shear are both consistent with the experiments.

Structural transitions induced by shear flow and temperature variation in a nonionic surfactant/water system

In this study, we investigate structural transitions of tetraethylene glycol monohexadecyl ether (C(16)E(4)) in D(2)O as a function of shear flow and temperature. Via a combination of rheology, rheo-small-angle neutron scattering and rheo-small-angle light scattering, we probe the structural evolution of the system with respect to shear and temperature. Multi-lamellar vesicles, planar lamellae, and a sponge phase were found to compete as a function of shear rate and temperature, with the sponge phase involving the formation of a new transient lamellar phase with a larger spacing, coexisting with the preceding lamellar phase within a narrow temperature-time range. The shear flow behavior of C(16)E(4) is also found to deviate from other nonionic surfactants with shorter alkyl chains (C(10)E(3) and C(12)E(4)), resembling to the C(16)E(7) case, of longer chain.

Rheo-small-angle neutron scattering at the National Institute of Standards and Technology Center for Neutron Research

We describe the design and operation of a modified commercial rheometer to simultaneously perform rheological measurements and structural studies by small angle neutron scattering (SANS). The apparatus uses a Couette geometry shear cell allowing two of the three scattering planes to be observed by performing experiments in either the radial or tangential geometries. The device enables small angle neutron scattering patterns to be obtained simultaneously with a wide variety of rheological measurements such as stress/strain flow curves, oscillatory deformations, and creep, recovery and relaxation tests, from -20 °C to 150 °C, for samples with viscosities varying by several orders of magnitude. We give a brief report of recent experiments performed on a dispersion of acicular nanoparticles and biopolymer network under stress demonstrating the utility of such measurements. This device has been developed at the National Institute of Standards and Technology's Center for Neutron Research (NCNR) and made available to the complex fluids community as part of the standard sample environment equipment.

Transition processes from the lamellar to the onion state with increasing temperature under shear flow in a nonionic surfactant/water system studied by Rheo-SAXS

In a previous paper, we reported for the first time the lamellar-to-onion transition with increasing temperature at around 67 °C under a constant shear rate (0.3-10 s(-1)) in a nonionic surfactant C(16)E(7)/water system. In this study, the first temperature-shear rate diagram has been constructed in a wider range of shear rate (0.05-30 s(-1)) than in our previous study based on the temperature dependence of the shear stress at constant shear rate. The results suggest that the critical temperature above which the transition begins does not depend on the shear rate very much, although it takes a very shallow minimum. Then we have performed simultaneous measurements of small-angle X-ray scattering/shear stress (rheo-SAXS) with a stepwise increase in temperature of 0.1 K per 15 min at a constant shear rate of 3 s(-1) near the transition temperature. When the temperature exceeds 67 °C, just before the increase in the shear stress, the intensity of the Bragg peak for the velocity gradient direction (approximately proportional to the number of lamellae with their normal along this direction) is suddenly increased. As the temperature increases by 0.2 K, the shear stress begins to increase. At the same time, the peak intensity in the velocity gradient direction rapidly decreases and instead the intensity in the neutral direction increases. As the temperature increases further, the intensities in both the neutral and gradient directions decrease whereas the intensity in the flow direction increases, corresponding to the formation of onions. We have also performed rheo-SAXS experiments with a stepwise increase in shear rate at 72 °C. The sequence of the change in the intensity in each direction is almost the same in the temperature scan experiments at constant shear rate, suggesting that the transition mechanisms along these two paths are similar. The abrupt enhancement of the lamellar orientation with the layer normal along the velocity gradient direction just before the transition is the first finding and strongly supports the coherent buckling mechanism in the lamellar-to-onion transition proposed by Zilman and Granek (Zilman, A. G.; Granek, R. Eur. Phys. J. B 1999, 11, 593).

Homogeneous length scale of shear-induced multilamellar vesicles studied by diffusion NMR

A recently developed protocol for pulsed gradient spin echo (PGSE) NMR is applied for the size determination of multilamellar vesicles (MLVs). By monitoring the self-diffusion behavior of water, the technique yields an estimate of the homogeneous length scale λ(hom), i.e. the maximum length scale at which there is local structural heterogeneity in a globally homogeneous material. A cross-over between local non-Gaussian to global Gaussian diffusion is observed by varying the experimentally defined length- and time-scales. Occasional observation of a weak Bragg peak in the PGSE signal attenuation curves permits the direct estimation of the MLV radius in favorable cases, thus yielding the constant of proportionality between λ(hom) and radius. The microstructural origin of the Bragg peak is verified through Brownian dynamics simulations and a theoretical analysis based on the center-of-mass diffusion propagator. λ(hom) is decreasing with increasing shear rate in agreement with theoretical expectations and results from (2)H NMR lineshape analysis.

Effect of shear rates on the MLV formation and MLV stability region in the C12E5/D2O system: rheology and rheo-NMR and rheo-SANS experiments

At high temperatures, pentaethylene glycol monododecyl ether (C12E5) in D2O forms a swollen lamellar phase. This letter reports the shear-induced multilamellar vesicle (MLV) formation in a sample that contains 40 wt % C12E5 dissolved in D2O at 55 °C. This transition has been investigated by time-resolved rheo-nuclear magnetic resonance, rheo small-angle neutron scattering, and rheometry. The typical transient viscosity behavior of MLV formation has been discovered at 1 s(-1). For the first time, it has been found that MLVs are not stable over time when subjected to high shear rates. Our results show that the MLV stability is confined in a narrow region in the range 1-10 s(-1) shear rates. This is not observed for other CnEm surfactants.

Thermodynamics and kinetics of vesicles formation processes

Vesicles are hollow aggregates, composed of bilayers of amphiphilic molecules, dispersed into and filled with a liquid solvent. These aggregates can be formed either as equilibrium or as out of equilibrium meta-stable structures and they exhibit a rich variety of different morphologies. The surprising richness of structures, the vast range of industrial applications and the presence of vesicles in a number of biological systems have attracted the interest of numerous researchers and scientists. In this article, we review both the thermodynamics and the kinetics aspects of the phenomena of formation of vesicles. We start presenting the thermodynamics of bilayer membranes formation and deformation, with the aim of deriving the conditions for the existence of equilibrium vesicles. Specifically, we use the results from continuum thermodynamics to discuss the possibility of formation of stable equilibrium vesicles, from both mixed amphiphiles and single component systems. We also link the bilayer membrane properties to the molecular structure of the starting amphiphiles. In the second part of this article, we focus on the dynamics and kinetics of vesiculation. We review the process of vesicles formation both from planar lamellar phase under shear and from isotropic micelles. In order to clarify the physical mechanisms of vesicles formation, we continuously draw a parallel between emulsification and vesiculation processes. Specifically, we compare the experimental results, the driving forces and the relative scaling laws identified for the two processes. Describing the dynamics of vesicles formation, we also discuss why non equilibrium vesicles can be formed by kinetics control and why they are meta-stable. Understanding how to control the properties, the stability and the formation process of vesicles is of fundamental importance for a vast number of industrial applications.

Reversible photorheology in solutions of cetyltrimethylammonium bromide, salicylic acid, and trans-2,4,4'-trihydroxychalcone

We show photorheology in aqueous solutions of weakly entangled wormlike micelles prepared with cetyltrimethylammonium bromide (CTAB), salicylic acid (HSal), and dilute amounts of the photochromic multistate compound trans-2,4,4'-trihydroxychalcone (Ct). Different chemical species of Ct are associated with different colorations and propensities to reside within or outside CTAB micelles. A light-induced transfer between the intra- and intermicellar space is used to alter the mean length of wormlike micelles and hence the rheological properties of the fluid, studied in steady-state shear flow and in dynamic rheological measurements. Light-induced changes of fluid rheology are reversible by a thermal relaxation process, at relaxation rates which depend on pH and which are consistent with photochromic reversion rates measured by UV-vis absorption spectroscopy. Parameterizing viscoelastic rheological states by their effective relaxation time τ(c) and corresponding response modulus G(c), we find the light and dark states of the system to fall onto a characteristic state curve defined by comparable experiments conducted without photosensitive components. These reference experiments were prepared with the same concentration of CTAB, but different concentrations of HSal or sodium salicylate (NaSal), and tested at different temperatures.

Shear-induced defect formation in a nonionic lamellar phase

(2)H NMR experiments on a nonionic oriented lamellar phase demonstrate that shear flow induces structural defects in the lamellar structure. These substantial structural changes give rise to a transition from a viscous to a solidlike behavior; the elastic modulus of presheared samples was found to increase, reversibly, with the applied preshear rate. A similar behavior was found when step-cycling the temperature toward the layer-to-multilamellar-vesicle transition and back at constant shear rate. However, while shear rate controls the defect density, the temperature is found to control the defect rigidity. The lamellar phase exhibits a shear-thinning behavior under steady shear conditions, following the power law eta approximately gamma(n), with n approximately -0.4. Both the shear thinning and the elastic behavior are in agreement with the available theoretical models. The observed shear-induced structural defects are reversible and can be regarded as a pretransition prior to the shear-induced formation of multilamellar vesicles.

Lamellar-to-onion transition with increasing temperature under shear flow in a nonionic surfactant/water system

Simultaneous measurements of small-angle light scattering/shear stress (Rheo-SALS) and small-angle X-ray scattering/shear stress (Rheo-SAXS) have been performed in the lamellar phase of the C(16)E(7)/D(2)O system. As the temperature is increased and exceeds 67 degrees C at constant shear rates (at 1 and 3 s(-1)), the shear stress increases abruptly and a four-lobe pattern is observed in the depolarized SALS. These results suggest that the lamellar-to-onion transition occurs with increasing temperature, which has not yet been reported. The diameter of onions obtained from the depolarized SALS pattern at 3 s(-1) increases with increasing temperature. The transition is reversible against the change in temperature. The Rheo-SALS measurements have also been made with a stepwise increase in shear rate at constant temperature. The results are consistent with the above temperature-scan experiments at constant shear rate, suggesting that the transition does not depend on the path. The variation of the SAXS pattern at 3 s(-1) indicates that the orientation of lamellae becomes isotropic as the temperature is increased from 67 to 69 degrees C, which also supports the lamellar-to-onion transition. The transition temperature at constant shear rate (at 3 s(-1)) increases rapidly with a slight increase in surfactant concentration. From this, together with the SAXS results at rest in our previous study, we deduce that an increase in the water-layer thickness is necessary for the lamellar-to-onion transition with increasing temperature.

Worm-like micelles of CTAB and sodium salicylate under turbulent flow

Polymers with high molecular weight and worm-like micelles are drag-reducing agents under turbulent flow. However, in contrast to the polymeric systems, the worm-like micelles do not undergo mechanical degradation due to the turbulence, because their macromolecular structure can be spontaneously restored. This very favorable property, together with their drag-reduction capability, offer the possibility to use such worm-like micelles in heating and cooling systems to recirculate water while expending less energy. The formation, growth, and stability of worm-like micelles formed by cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal) were investigated using the self-fluorescence of salicylate ions and the ability of the giant micelles to promote hydrodynamic drag reduction under turbulent flow. The turbulence in solutions of CTAB-Sal was produced within the double-gap cell of a rotational rheometer. Detailed diagrams were obtained for different ratios of Sal and CTAB, which revealed transitions associated with the thermal stability of giant micelles under turbulent flow.

Shear-induced transitions between a planar lamellar phase and multilamellar vesicles: continuous versus discontinuous transformation

The shear-induced transitions between an oriented lamellar phase and shear-induced multilamellar vesicles (MLVs) in a nonionic surfactant system were studied by deuterium rheo-NMR spectroscopy as a function of time in start-up experiments at several temperatures and shear rates. By starting from an initial state of oriented lamellae and observing the transformation to the final steady state of MLVs and vice-versa, two different mechanisms were found, depending on the direction of the transition and the initial state. The transition is continuous when MLVs are formed, starting from the oriented lamellar phase. On the other hand, a discontinuous nucleation-and-growth process with a coexistence region is observed when transforming MLVs into an oriented lamellar phase.

Highly ordered patterns of parabolic focal conics in lamellar lyotropic systems

In this paper, we report the experimental observation of the formation of highly ordered parabolic focal conical patterns in lamellar surfactant solutions. Predominantly, we investigated mixtures of sodium dodecyl sulfate, water, hexanol, and decane, located in the immediate vicinity of the region of the L3 and Lalpha phase coexistence. The experimental studies of the formation of the patterns and of their temporal development are described. We give a simple model picture for the underlying structure, corroborated by the experimental results. There appears to be only one independent length scale that controls the appearance of the whole pattern.

Wormlike micelles: where do we stand? Recent developments, linear rheology and scattering techniques

Wormlike micelles are elongated flexible self-assembly structures formed by the aggregation of amphiphiles. Above a threshold concentration, they entangle into a dynamic network, reminiscent of polymer solutions, and display remarkable visco-elastic properties, which have been exploited in numerous industrial and technological fields. Relating the microstructure of these intricate structures with their bulk properties is still an ongoing quest. In this review, we present a classification of wormlike micelles, with a focus on novel systems and applications. We describe the current state of understanding of their linear rheology and give a detailed account of recent progress in small-angle neutron scattering, a particularly powerful technique to elucidate their microstructure on a wide range of length-scales.

Viscoelasticity of a nonionic lamellar phase

The linear viscoelastic properties of a nonionic lamellar phase in C-orientation were studied as a function of temperature by small-amplitude oscillatory measurements in the frequency range 0.5-5 Hz. An almost solid-like elastic response was observed at all studied temperatures, from 42 to 20 degrees C. In this range, the elastic modulus was found to increase strongly with decreasing temperature. The elasticity is attributed to screw dislocations connecting layers in the stack, and the data thus suggest that the density of screw dislocations decreases with increasing temperature. The lamellar phase forms an "onion" texture when continuously sheared at lower temperatures. It is argued that a possible origin for the shear-induced "onion" texture is the instability of the screw dislocations in shear flow. By 2H NMR experimentation, we also find the formation of a random mesh phase at lower temperatures. The presence of equilibrium bilayer perforations, however, does not correlate with the "onion" stability.

Properties, main applications and perspectives of worm micelles

This tutorial review deals with one of the most remarkable forms of surfactant aggregates, described as having a flexible, elongated cylindrical shape. Three structural scale lengths are pertinent to the flexibility and mobility of worm micelles: the cross-sectional radius, r(cs), the overall (contour) length, L, and the persistence length, l(p). The diversity of l(p) values in amphiphilic systems is demonstrated as well as the relation between L and l(p). The review also discusses the viscoelasticity of worm micelles and the relaxation mechanisms underlying this dominant property. Many aspects of viscoelasticity--such as non-linearity, shear banding, flow-induced phase transition, rheochaos--are only shortly described. The prevailing application of worm micelles, namely as fracture fluids and drag reducing agents are discussed in detail, stressing the effect of variations in the surfactant molecular structure on the efficacy of worm micelles. The vague possibility of using "smart" worm micelles in the foreseeable future is tersely outlined.