Light scattering investigations of the behavior of semidilute aqueous micellar solutions of cetyltrimethylammonium bromide: Analogy with semidilute polymer solutions

Worm blobs as entangled living polymers: from topological active matter to flexible soft robot collectives

Recently, the study of long, slender living worms has gained attention due to their unique ability to form highly entangled physical structures, exhibiting emergent behaviors. These organisms can assemble into an active three-dimensional soft entity referred to as the "blob", which exhibits both solid-like and liquid-like properties. This blob can respond to external stimuli such as light, to move or change shape. In this perspective article, we acknowledge the extensive and rich history of polymer physics, while illustrating how these living worms provide a fascinating experimental platform for investigating the physics of active, polymer-like entities. The combination of activity, long aspect ratio, and entanglement in these worms gives rise to a diverse range of emergent behaviors. By understanding the intricate dynamics of the worm blob, we could potentially stimulate further research into the behavior of entangled active polymers, and guide the advancement of synthetic topological active matter and bioinspired tangling soft robot collectives.

In situ formation and dispersion of lanthanide complexes in wormlike micelles

Lanthanide-containing, water-based fluids normally suffer from low photoluminescent (PL) and/or colloidal stability, which greatly hinders their applications. Herein, we report the preparation of PL fluids which contain in situ formed europium complexes in aqueous solution. The strategy first relies on the construction of wormlike micelles by mixing a zwitterionic surfactant (tetradecyldimethylaminoxide, C₁₄DMAO) and a tridentate ligand for a lanthanide cation (2,6-dipicolinic acid, DPA) in water. The addition of the dual-functionalized DPA to an aqueous solution of C₁₄DMAO (100 mol L^-1) induced non-monotonic rheological changes, with the expected formation of a pseudogemini surfactant at a DPA-to-C₁₄DMAO molar ratio of approximately 1 : 2. When a third component of EuCl₃ is introduced to this system, complexes formed in situ between Eu³⁺ and DPA, resulting in bright red-emission. Besides DPA, C₁₄DMAO is also involved in the complexation, which squeezes out water molecules and greatly improves the PL stability of the fluid. The synergetic effect among Eu³⁺, DPA and C₁₄DMAO leads to the high colloidal stability of the fluid, opening the door for a wide range of potential applications. Further tests indicate that this strategy can be easily expanded to other lanthanide cations such as Tb³⁺.

Physicochemical characterization of green sodium oleate-based formulations. Part 1. Structure and rheology

HYPOTHESIS

The structure, rheology and other physicochemical properties of dilute aqueous dispersions of sodium oleate (NaOL) are well known. This paper is the first report in which a moderately concentrated (13% w/w) dispersion of NaOL in water is investigated. In fact, at this concentration the phase and rheology behavior of the surfactant remarkably deviates from those of its dilute solutions in water and a significant effect is imparted by the addition of potassium chloride.

EXPERIMENTAL

The structural, thermal and rheological properties of a 13% w/w dispersion of NaOL in water were investigated by cryo-TEM, rheology, and DSC experiments with and without the addition of potassium chloride. The system is comprised of elongated wormlike micelles that turn into a gel-like more disordered viscous material upon addition of small amounts of KCl (4% w/w).

FINDINGS

This paper illustrates the multifaceted behavior of sodium oleate dispersions at intermediate concentrations that depends on the presence of other cosolutes (such as KCl). The results show that viscoelastic aqueous dispersions of NaOL are excellent candidates for the preparation of stimuli-responsive green materials to be used in a number of different applications. We also discuss the genesis of wormlike micelles (WLMs) in terms of the general theory of self-assembly.

Effect of residual chemicals on wormlike micelles assembled from a C22-tailed cationic surfactant

HYPOTHESIS

Ultra-long-chain surfactants, particularly C₂₂-tailed ones, have attracted considerable attention because of their ease of self-assembly into wormlike micelles (WLMs). Commercial C₂₂-tailed surfactants often contain non-negligible amounts of chemical residues introduced during their production. Since the noncovalent driving force of wormlike self-assembly can be greatly affected by the composition, we hypothesized that the residual chemicals could play a significant role in tuning the micelle microstructure and macroscopic properties of the surfactants.

EXPERIMENTS

To confirm this hypothesis, a highly pure (>99%) C₂₂-tailed cationic surfactant, N-erucylamidopropyl-N,N,N-trimethylammonium iodide (EDAI) was synthesized, and various amounts of corresponding reactants (iodomethane or N-erucamidopropyl-N,N-dimethylamine) or solvents (acetone) commonly used in surfactant synthesis were introduced as residues. The impact of each individual residue on the macroscopic appearances, rheological properties, and micelle morphology of the surfactant solution were investigated.

FINDINGS

Increasing the residue fraction in the EDAI solution resulted in an initial increase, followed by a dramatic drop in solution viscosity. This behavior was described in terms of micellar structural transformations based on analysis of cryo-TEM observations and surface tension measurements. These findings are of crucial importance in understanding the sophisticated behaviors of WLMs and will benefit the industrial preparation of ultra-long-chain surfactants for commercial use.

Structural properties of the evolution of CTAB/NaSal micelles investigated by SANS and rheometry

Surfactants are amphiphilic molecules that spontaneously self-assemble in aqueous solution into various ordered and disordered phases. Under certain conditions, one-dimensional structures in the form of long, flexible wormlike micelles can develop. Cetyltrimethylammonium bromide (CTAB) is one of the most widely studied surfactants, and in the presence of sodium salicylate (NaSal), wormlike micelles can form at very dilute concentrations of surfactant. We carry out a systematic study of the structures of CTAB/NaSal over a surfactant concentration range of 2.5-15 mM and at salt-to-surfactant molar ratios of 0.5-10. Using small-angle neutron scattering (SANS), we qualitatively and quantitatively characterize the equilibrium structures of CTAB/NaSal, mapping the phase behavior of CTAB/NaSal at low concentrations within the region of phase space where nascent wormlike micelles transition into long and entangled structures. Complementary rheological assessments not only demonstrate the significant influence of the inter-micellar Coulombic interaction on the micellar structure but also suggest the potential existence of a hierarchical structure which is beyond the accessibility of the SANS technique.

Wormlike micelles versus water-soluble polymers as rheology-modifiers: similarities and differences

Rheological behaviors and aqueous solution microstructures of wormlike micelles and a water-soluble polymer are compared.

Customizing wormlike mesoscale structures via self-assembly of amphiphilic star polymers

We examine the phase behavior of end-functionalized diblock copolymer stars by means of Grand Canonical Monte Carlo simulations. We focus on solutions of diblock copolymer stars with a solvophobic outer block shorter than the solvophilic inner block, which are expected to nucleate microphase aggregates. By tuning the temperature and rigidity of the molecules, we target specific mesoscale structures, which can act as powerful rheology modifiers. In particular, we control the hierarchical self-assembly of single micelles in a "pearl-necklace" fashion, which eventually merge into elongated, wormlike supermicelles.

Formation of worm-like micelles in mixed N-hexadecyl-N-methylpyrrolidinium bromide-based cationic surfactant and anionic surfactant systems

Through the descriptive and rheological characterization of worm-like micelles formed by N-hexadecyl-N-methylpyrrolidinium bromide and sodium laurate, the formation and properties of the worm-like micelles were affected by the concentrations of sodium laurate and temperature. Additionally, cryogenic transmission electron microscopy images further validated the formation of worm-like micelles.

Effects of Electrolytes on Interfacial and Micelle Properties of C.I. Reactive Orange 16 – Dodecylpyridinium Chloride Binary System

The effect of electrolytes on the interaction between the anionic dye C.I. Reactive Orange 16 and the cationic surfactant dodecylpyridinium chloride was investigated using surface tension measurement in a certain concentration range. The influence of the concentration of electrolyte on critical micelle concentration (CMC) values was observed in the following order; 0.1 M NaCl > 0.5 M NaCl > 1.0 M NaCl. Also, the influence of the electrolyte cations was observed as Na+ > Mg2+ > K+. An increase on CMC values of dye-surfactant solution with increasing electrolyte concentration is explained as charge screening and also the decrease in these values for higher concentration of electrolyte is attributed to the change of micelle shape. Furthermore this change is due to ionic polarizability, valency and hydrated radius. Using Rubingh's regular solution theory, the values of micellar interaction parameters (β), were found as negative in all studied mixtures.

Instabilities in wormlike micelle systems. From shear-banding to elastic turbulence

Shear-banding is ubiquitous in complex fluids. It is related to the organization of the flow into macroscopic bands bearing different viscosities and local shear rates and stacked along the velocity gradient direction. This flow-induced transition towards a heterogeneous flow state has been reported in a variety of systems, including wormlike micellar solutions, telechelic polymers, emulsions, clay suspensions, colloidal gels, star polymers, granular materials, or foams. In the past twenty years, shear-banding flows have been probed by various techniques, such as rheometry, velocimetry and flow birefringence. In wormlike micelle solutions, many of the data collected exhibit unexplained spatio-temporal fluctuations. Different candidates have been identified, the main ones being wall slip, interfacial instability between bands or bulk instability of one of the bands. In this review, we present experimental evidence for a purely elastic instability of the high shear rate band as the main origin for fluctuating shear-banding flows.

A dually effective inorganic salt at inducing obvious viscoelastic behavior of both cationic and anionic surfactant solutions

Hydrazine nitrate (HN), an inorganic salt, was first found to have dual effects on inducing obvious viscoelasticity of both cationic and anionic surfactant solutions. It was interesting that the surfactant solutions exhibited characteristic wormlike micelle features with strong viscoelastic properties upon the addition of this inorganic salt. The rheological properties of the surfactant solutions have been measured and discussed. The apparent viscosity of the solutions showed a volcano change with an increase of the HN concentration. Correspondingly, the microstructures of the micelles in the solutions changed with the apparent viscosity. First, wormlike micelles began to form and grew with an increase of the HN concentration. Subsequently, the systems exhibited linear viscoelasticity with characteristics of a Maxwell fluid in the intermediate mass fraction range, which originated from a 3D entangled network of wormlike micelles. Finally, a transition from linear micelles to branched ones probably took place at higher HN contents. In addition, the origin of the dual effects brought by HN addition on inducing viscoelasticity in both cationic and anionic surfactant solutions was investigated.

Lipophilic tail architecture and molecular structure of neutralizing agent for the controlled rheology of viscoelastic fluid in amino acid-based anionic surfactant system

The amino acid-based anionic surfactant, N-dodecanoylglutamic acid (designated as LAD), when neutralized with 2-aminoethanol (MEA), 2,2'-iminodiethanol (DEA), or 2,2',2''-nitrilotriethanol (TEA) forms globular type of micelles in aqueous system at 25 °C, and the viscosity of each solution is very close to that of pure water. Addition of cationic surfactants, e.g., tetradecyltrimethylammonium bromide (TTAB) or hexadecyltrimethylammonium bromide (CTAB), induced a sphere-to-rod transition, and the micelles grew axially, resulting in formation of viscoelastic wormlike micelles that could be described by a Maxwell model. The viscosity was ca. 5 orders of magnitude as large as that of water. The viscosity maximum was observed in zero-shear viscosity (η0) curves, and there was a phase separation to liquid crystal phase at higher cosurfactants concentration. The η0 curve shifted toward lower cosurfactant concentration, and the value of maximum viscosity (η0(max)) decreased by reducing lipophilic moiety of cosurfactant. Furthermore, the value of η0(max) decreased with decreasing the number of ethanol content per molecule of neutralizing agent. The η0 decays exponentially with the rise of temperature, following the Arrhenius type of behavior. However, addition of dodecyltrimethylammonium bromide (DTAB) increases viscosity only slightly; viscoelastic wormlike micelles are not formed.

Charge-free reverse wormlike micelles in nonaqueous media

We report a facile method for the formation of charge-free reverse wormlike micelles in a nonionic surfactant/oil system without addition of water under ambient conditions. This route involves the addition of sucrose dioleate (SDO) to semidilute solutions of sucrose trioleate (STO) in hexadecane. A reverse wormlike micelle was possible to achieve only with ionic surfactants in which water and/or salts are fundamentally required to induce micellar growth so far. In this contribution, we have shown that less lipophilic nonionic surfactant SDO promotes one-dimensional growth to STO reverse micelles and leads to the formation of transient networks of viscoelastic reverse wormlike micelles. The zero-shear viscosity increases by ∼4 orders of magnitude, and it is the mixing fraction of SDO to STO that determines the viscosity growth. The structure and dynamics of the reverse micelles are confirmed by small-angle X-ray scattering (SAXS) and rheometry measurements.

Rheological and Kinetic Properties of Semi-Dilute Solutions of Elongated Micelles

Measurements of the breaking time of cylindrical micelles of cetyltrimethylammonium bromide in aqueous solutions have been performed by means of T-Jump experiments with light scattering detection. The results provide an unambiguous test of a recent theoretical model for stress relaxation.

CO2: a wild solvent, tamed

This article reviews approaches for modification of solvent properties of supercritical carbon dioxide (scCO(2)), with particular reference to self-assembly of oligomeric and polymeric solute additives. Of special interest are viscosity modifiers for scCO(2) based on molecular self-assembly. Background on polymers and surfactants with CO(2)-compatible functionalities is covered, leading on to the attempts made so far to increase the scCO(2) viscosity, which are described in detail. The significance of this field, and the implications a breakthrough could bring environmentally and economically will be addressed.

Self-organization in the flow of complex fluids (colloid and polymer systems): part 1: experimental evidence

Different types of regular and irregular self-organized structures observed in deformation of colloid and polymer substances ("complex fluids") are discussed and classified. This review is focused on experimental evidence of structure formation and self-organization in shear flows, which have many similar features in systems of different types. For single-phase (uniform) polymer systems regular periodic surface structures are observed. Two main types of these structures are possible: small-scale regular screw-like periodic structures along the whole stream (usually called "shark-skin") and long-period smooth and distorted parts of a stream attributed as a "stick-slip" effect. The origin of surface irregularities of both types is elasticity of a liquid. In the limiting case of high enough Weissenberg numbers, medium loses fluidity and should be treated as a rubbery matter. The liquid-to-rubbery transition at high Weissenberg numbers is considered as the dominating mechanism of instability, leading in particular to the wall slip and rupture of a stream. Secondary flows ("vorticity") in deformation polymeric substances and complex fluids are also obliged to their elasticity and the observed Couette-Taylor-like cells, though being similar to well-known inertial secondary flows, are completely determined by elasticity of colloid and polymeric systems. In deformation of colloidal systems, suspensions and other dense concentrated heterophase materials, structure formation takes place at rest and the destroying of the structure happens as the yield stress. In opposite to this case, strong deformations can lead to the shear-induced structure formation and jamming. These effects are of general meaning for any complex fluids as well as for dense suspensions and granular media. Strong deformations also lead to separation of a stream into different parts (several "bands") with various properties of liquids in these parts. So, two principal effects common for any polymers and complex fluids can be pointed at as the physical origin of self-organization in shearing. This is elasticity of a liquid and a possibility of its existence in different phases or relaxation states, while in many cases elasticity of a fluid is considered as the most important provoking factor for transitions between different types of rheological behavior, e.g. the fluid-to-rubbery-like behavior at high deformation rates and the transition from the real laminar flow to wall slip.

Surfactant-based gels

This article reviews known approaches to generating viscoelastic and gel-like surfactant systems focusing on how the formation of these viscous phases are often sensitive to a variety of chemical and physio-chemical factors. An understanding of this sensitivity is essential for generating high viscosity surfactant phases in more challenging solvent environments. The initial focus is on the generation of worm-like and reverse worm-like micelles. In addition, other approaches for using surfactant self-assembly for viscosity enhancement have been examined, namely gelatin microemulsion based organogels and the addition of substituted phenols to AOT reverse micelles.

Wormlike micelles in mixed amino acid-based anionic/nonionic surfactant systems

We present the formation of viscoelastic wormlike micelles in mixed amino acid-based anionic and nonionic surfactants in aqueous systems in the absence of salt. N-Dodecylglutamic acid (designated as LAD) has a higher Krafft temperature; however, on neutralization with alkaline amino acid l-lysine, it forms micelles and the solution behaves like a Newtonian fluid at 25 degrees C. Addition of tri(oxyethylene) monododecyl ether (C(12)EO(3)) and tri(oxyethylene) monotetradecyl ether (C(14)EO(3)) to the dilute aqueous solution of the LAD-lysine induces one-dimensional micellar growth. With increasing C(12)EO(3) or C(14)EO(3) concentration, the solution viscosity increases gradually, but after a certain concentration, the elongated micelles entangle forming a rigid network of wormlike micelles and the solution viscosity increases tremendously. Thus formed wormlike micelles show a viscoelastic character and follow the Maxwell model. Tri(oxyethylene) monohexadecyl ether (C(16)EO(3)), on the other hand, could not form wormlike micelles, although the solution viscosity increases too. The micelles become elongated; however, they do not appear to form a rigid network of wormlike micelles in the case of C(16)EO(3). Rheological measurements have shown that zero shear viscosity (eta(0)) increases with the C(12)EO(3) concentration gradually at first and then sharply, and finally decreases before phase separation. However, no such maximum in the eta(0) plot is observed with the C(14)EO(3). The eta(0) increases monotonously with the C(14)EO(3) concentration till phase separation. In studies of the effect of temperature on the wormlike micellar behavior it has been found that the eta(0) decays exponentially with temperature, following an Arrehenius behavior and at sufficiently higher temperatures the solutions follow a Newtonian behavior. The flow activation energy calculated from the slope of log eta(0) versus 1/T plot is very close to the value reported for typical wormlike micelles. Finally, we also present the effect of neutralization degree of lysine on the rheology and phase behavior. The formation of wormlike micelles is confirmed by the Maxwell model fit to the experimental rheological data and by Cole-Cole plots.

Dynamic light scattering from non-entangled wormlike micellar solutions

The fuzzy cylinder theory, originally proposed for conventional polymer solutions, was applied to wormlike micellar solutions to take into account effects of the intermicellar collision and hydrodynamic interaction on the self-diffusion of wormlike micelles in solution at finite concentrations. Previously reported apparent hydrodynamic radius data obtained by dynamic light scattering for non-entangled wormlike micelles formed in aqueous solution by non-ionic surfactants, polyoxyethylene monoalkyl ethers C(i)E(j), were analyzed by this theory to estimate the persistence length q of the wormlike micelles. The results of q estimated were consistent with those obtained from radius of gyration data obtained by static light scattering.

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.

Kinetics of the shear banding instability in startup flows

Motivated by recent light scattering experiments on semidilute wormlike micelles, we study the early stages of the shear banding instability using the nonlocal Johnson-Segalman model with a "two-fluid" coupling of flow to micellar concentration. We perform a linear stability analysis for coupled fluctuations in shear rate gamma;, micellar strain W, and concentration phi about an initially homogeneous state. This resembles the Cahn-Hilliard (CH) analysis of fluid-fluid demixing (although we discuss important differences). First, assuming the initial state to lie on the intrinsic constitutive curve, we calculate the "spinodal" onset of instability in sweeps along this curve. We then consider start-up "quenches" into the unstable region. Here the instability in general occurs before the intrinsic constitutive curve can be attained, so we analyze the fluctuations with respect to the time-dependent start-up flow. We calculate the selected length and time scales at which inhomogeneity first emerges. When the coupling between flow and concentration is switched off, fluctuations in the "mechanical variables" gamma; and W are independent of those in phi, and are unstable when the intrinsic constitutive curve has negative slope; but no length scale is selected. Coupling to the concentration enhances this instability at short length scales, thereby selecting a length scale, consistent with the recent light scattering experiments. The spinodal region is then broadened by an extent that increases with proximity to an underlying (zero-shear) CH fluid-fluid (phi) demixing instability. Far from demixing, the broadening is slight and the instability is still mechanically dominated (by deltagamma; and deltaW) with only small deltaphi. Close to demixing, instability sets in at a very low shear rate, where it is dominated instead by deltaphi. In this way, the model captures a smooth crossover from shear banding instabilities that are perturbed by concentration coupling to demixing instabilities that are induced by shear.

Early stage kinetics in a unified model of shear-induced demixing and mechanical shear banding instabilities

We present a unified model of shear-induced demixing and "mechanical" shear banding instabilities in polymeric and surfactant solutions, by combining a simple flow instability with a two-fluid approach to concentration fluctuations. Within this model, we calculate the "spinodal" limit of stability of initially homogeneous shear states to demixing/banding, and predict the selected length and time scales at which inhomogeneity first emerges after a shear start-up "quench" into the unstable region, finding qualitative agreement with experiment. Our analysis is the counterpart, for this driven phase transition, of the Cahn-Hilliard calculation for unsheared fluid-fluid demixing.

Shear banding and the isotropic-to-nematic transition in wormlike micelles

Using deuterium NMR spectroscopy in a Couette cell, we observe shear-induced nematic ordering in the concentrated wormlike-micelle system CTAB/D(2)O, and our results are qualitatively consistent with birefringence studies, and in exact quantitative agreement with the degree of order measured in neutron-diffraction measurements. The width of the nematic region depends on shear rate, as well as on the temperature proximity to the equilibrium isotropic-nematic transition. Comparison of the nematic order profiles with velocity profiles obtained under identical conditions shows quite clearly that the nematic state is not identifiable with a highly sheared, low viscosity layer, and we conclude that the process of shearing induces a nematic state of high viscosity, possibly associated with mesoscale ordering. We present a simple model in which transition from the high shear branch to the viscous nematic branch is counterbalanced by subsequent relaxation of nematic order.