Formation Of Wormlike Micelles Research Articles

Molecular origin for pH-responsive wormlike micelles of zwitterionic surfactants triggered by aromatic acid derivatives

Introduction of aromatic acid derivatives (AADs) into zwitterionic surfactants is an efficient method to prepare wormlike micelles with pH-controllable viscosity; however, the coincident molecular origin of AAD/zwitterionic surfactant binary mixtures remains unclear. Herein, the self-assembly of hydroxyl derivatives of benzoic acid (BA) and cetyldimethyl betaine (BS-16) mixtures in water was systematically assessed, and various factors, such as the molecular structure, molar ratio of AAD and BS-16, and solution pH, were investigated. The structure–property relationship of AAD/BS-16 binary mixtures was established, which provided the molecular origin for the effect of AAD on micellar microstructures and the pH-induced morphological transitions. The ortho-substituted hydroxyl moiety in the BA molecule facilitated the formation of larger wormlike micelles, whereas the effect of the meta-substituted moiety was less significant. The para-substituted hydroxyl moiety in BA did not favor micellar growth. This moiety exhibited similar characteristics to the increasing hydroxyl moiety number in the AAD molecules or solution pH where the negative effects of steric hindrance and electrostatic interactions of molecules in micelles are dominant.

A novel pH-responsive wormlike micelles combinated sodium dodecyl sulfate (SDS) and diethylenetriamine (DETA) based on noncovalent electrostatic interaction

pH-Responsive wormlike micelles (WLMs) have attracted lots of attention due to their abundant advantages, for example, ease of adjustment and strong reversibility. We have learned that the degree of protonation of diethylenetriamine (DETA) varies with pH, and the anionic sodium dodecyl sulfate (SDS) can connect with cationic groups. Inspired by this, a novel pH-responsive WLMs was prepared based on noncovalent electrostatic interaction with sodium dodecyl sulfate (SDS) and DETA at a molar ratio of 3:1. Properties of the SDS/DETA system were assessed from the aspects of rheology, macro-morphology, aggregates diameter, and micro-morphology. When the pH decreased from 9.98 to 6.80, the SDS/DETA system represented as a weak gel with the viscosity increasing from 7.6 mPa s to 1.4 × 103 mPa·s and the average hydrodynamic radius increasing from 5.6 to 91.3 nm. The phenomenon is attributed to the protonation of DETA when reducing the pH and the formation of WLMs based on electrostatic interaction between DETA and SDS. However, when pH was further reduced by adding HCl, the SDS/DETA system would reach its isoelectric point, thus the solution became turbid with low viscosity. The SDS/DETA solution represents excellent reversibility (at least three cycles) in viscoelasticity by adjusting the pH value, and such pH-responsive WLM holds significant potential for broader applications in biomedical, oilfield chemistry, and other fields.

Increasing the aqueous solubility of the anesthetic propofol through wormlike micelle formation

Propofol, a phenol derivative, is commonly employed as an intravenous anesthetic during clinical procedures, formulated as an oil/water emulsion due to its poor solubility in water. The stability limitations associated with emulsions have prompted research efforts towards developing aqueous formulations of propofol. In this work, we investigate the solubility enhancement of propofol in anionic and cationic surfactants. Our findings reveal that the solubility of propofol can increase significantly, up to 100-fold, depending on the nature of the micellar aggregate, as observed for alkylammonium halogenates CnTAB (for n = 12, 14 and 16), contrasting with the lower solubility with SDS. Interestingly, C14TAB and C16TAB demonstrate significantly higher solubility than C12TAB. This was attributed to the formation of wormlike micelles, in which the propofol molecules are positioned between the cationic heads of the surfactant molecules, changing the micellar curvature and the morphology of the aggregate. Therefore, the aromatic molecules in the micellar environment can be partitioned into the micellar cores and their palisades. Regarding C12TAB, the alkyl chain is too short to form wormlike micelles, thus, concentrating propofol molecules mainly into the micellar core, and consequently, leading to their aggregation. Solubility diagrams of propofol were constructed in conjunction with different surfactants. The systems exhibiting viscoelastic behavior, indicative of wormlike micelle formation, were further investigated using rheology. Additionally, the fluorescent properties of propofol enabled the examination of the anesthetic molecule within diverse micellar environments.

Can wormlike micelle stiffness be estimated from zero‐shear viscosity? Experimental investigation with a model system and specific salt interactions

AbstractThe microstructure of wormlike micelles (WLM) directly affects the rheological properties of their solutions. Investigating the structure–property relationships of WLM has long been a popular topic for researchers who have developed theoretical and empirical models to describe their viscoelastic behavior. All these models rely on a collection of characteristic “length” parameters that are often difficult to estimate using relatively simple rheological tests. In particular, the micelle stiffness, as described by the persistence length, can be difficult to measure experimentally, while being very impactful on the bulk rheology of WLM solutions. Here, an array of inorganic salts (NaCl, LiCl, MgCl2, NaBr, NaI, and Na2SO4) have been used to induce wormlike micelle formation in an aqueous solution of the surfactant sodium lauryl ether sulfate. Ion dissociation/association with the surfactant head groups and the hydrogen bond network of the water alters the effective stiffness of the micelles, allowing for an estimation of the stiffness using three different methods: (1) small angle neutron scattering (SANS) measurements, (2) oscillatory rheological measurements, and (3) steady rheological measurements with a thermodynamic packing parameter model. Each of these methods are then compared and shown to be consistent with each other for the micelle solutions tested. The consistency of the results across all the measurements suggests that the approach used in this study, which estimates micelle parameters using steady shear rheology and a thermodynamic model, could provide a simpler and more accessible method for estimating micelle parameters in a wide range of surfactant systems.

Fabrication and enzymatic dissociation of long-chain alkanoylcholine-based worm-like micelles

In order to elucidate the possibility of the formation and enzymatic dissociation of worm-like micelles (WLMs) from long-chain alkanoylcholines (LCACs), strategies of fabrication and butyrylcholine esterase (BChE)-catalyzed dissociation of LCAC-based WLMs have been developed. Three LCACs with different hydrophobic tail lengths, namely dodecanoylcholine bromide (DCB), myristoylcholine bromide (MCB), and palmitoylcholine bromide (PCB), have been used as model cationic surfactants. It was demonstrated that the hydrophobic tail length of an LCAC played an important role in the formation of WLMs. With the assistance of sodium salicylate (NaSal), DCB proved to be incapable of efficiently forming WLMs, whereas MCB and PCB performed well. PCB-based WLMs have also been fabricated with the aid of NaNO3. To fabricate WLMs at a surfactant concentration of 0.1 mol L−1, the optimal molar ratio of the additive (NaSal and NaNO3) to the surfactant (MCB and PCB) was around 0.83 (NaSal/MCB), 0.63 (NaSal/PCB) and 20 (NaNO3/PCB), respectively. The respective average diameters of the WLMs were about 4.11 nm (MCB-NaSal), 4.82 nm (PCB-NaSal), and 4.51 nm (PCB-NaNO3). Meanwhile, the respective zero-shear viscosities of the WLMs were about 5.38 × 102 mPa s (MCB-NaSal), 1.81 × 105 mPa s (PCB-NaSal), and 7.35 × 104 mPa s (PCB-NaNO3). Jelly-like PCB-NaSal and PCB-NaNO3 WLMs underwent efficient dissociation through a BChE-catalyzing strategy under mild conditions of pH 7.4 and 37 °C. Its chemical principle was a hydrolysis reaction of PCB catalyzed by BChE.

PH and light dual-stimuli responsive wormlike micelles based on azophenyl surfactant and Sodium oleate

Herein, pH and light dual-stimuli responsive wormlike micelles (WLMs) were prepared with (4-butylphenyl)-2-(4-trimethylammoniumhexyloxyphen-yl) diazene (C4AzoC6N) and sodium oleate (NaOA). The viscoelasticity of the system was tested by steady state and dynamic rheology. In the alkaline condition, the binary system is a viscoelastic gel-like liquid with a viscosity of up to 230 Pa·s as the presence of the carboxylate group in NaOA, which promotes the formation of WLMs. After the pH of the solution was changed from alkaline to acidic, NaOA was gradually converted into oleic acid, which precipitated out of the system and the viscoelastic system was turned into a cloudy, low-viscosity flowing liquid. On the other hand, the viscosity of the solution could be repeatedly and reversibly switched more than three times without any deterioration upon light trigger due to the cis-trans isomerization of C4AzoC6N. Compared with other surfactants, NaOA is one of cheap and commercially available compounds, which avoids complex organic synthesis or purification processes. Furthermore, the WLMs can be precisely controlled by pH and UV light dual-stimuli, which will have a wide range of applications in large-scale processes, such as in functional materials and fluid drag reducers.

Phase behavior, micellization and rheology of erucic acid-based ionic liquid surfactant in deep eutectic solvents

Fatty acid-based ionic liquid surfactants have attracted significant attention due to their environmental friendliness, showing significant potential in some specific applications (e.g., personal & house care products). Deep eutectic solvents (DES) are a kind of new green and promising solvent. However, the self-assembly of ultra-long-chain fatty acid-based ionic liquid surfactants in DES has rarely been reported to date. In this paper, erucic choline (ErCho), as a representative of ultra-long-chain ionic liquid surfactants, was synthesized and characterized, and then its micellization behavior in three different DES (ChGly, ChEG, and ChPh, which were obtained by mixing glycerol, ethylene glycol, phenol, and choline chloride in a 2:1 M ratio) was investigated. ErCho can self-assemble into micelles in DES, as well as in water, but a relatively larger critical micelle concentration (cmc) was obtained in DES due to a small hydrophobic association. The hydrophobic association of ErCho in ChGly was the smallest, and the standard free energy of micellization per mole of monomer unit was the largest (-37.67 kJ·mol−1). However, at high shear rates, typical shear-thinning was observed in the ChGly solution of ErCho, showing pseudoplastic fluids behavior, which was not found in ChEG, ChPh, and H2O. This reflects the presence of large aggregates, and cryo-TEM observation confirmed the formation of wormlike micelles in the ChGly solution of ErCho with a small flow activation energy (∼43 kJ·mol−1). In addition, ErCho in ChGly solution can further evolve into a hexagonal phase, and then to lamellar crystalline phases upon increasing concentration.

Using Solution History to Control Hydrogel Properties of a Perylene Bisimide.

pH dependence on water soluble aggregates is well-known in the field of low molecular weight gelators (LMWGs), with different aggregates sometimes having very different properties depending on their final pH. This aggregation determines their applications and performance. Here, we investigate the pH dependence of perylene bisimide gels; initially solutions are formed at a high pH and gels form as the pH is decreased. We find it is not only the final pH but also the starting pH that can impact the resulting gel. We use small angle neutron scattering (SANS), rheology, 1 H NMR spectroscopy and absorption spectroscopy to examine the effect of starting pH on gelation kinetics and final gel properties. Adjusting the solution from pH 9 (where there are few or no aggregates) to pH 6 results in the formation of different worm-like micelles than the ones directly formed at pH 6, leading to again gels with different mechanical properties. This work highlights the importance of controlling the pH of solutions before gelation, but also opens up more possible morphologies and therefore more properties from the same molecule.

Wormlike Micelles with Photo and pH Dual-Stimuli-Responsive Behaviors Formed by Aqueous Mixture of Cationic and Anionic Surfactants

Responsive wormlike micelles, innovative soft materials with stimuli-tunable rheological properties, have recently attracted significant attention. Herein, the construction of dual responsive wormlike micelles exhibiting photo- and pH-stimulated viscoelastic behaviors is demonstrated from a cationic surfactant 4,4′-bis(trimethylammonium-6-hexyloxy) azobenzene bromide (BTHA), and anionic surfactant sodium oleate (NaOA). Rheological measurement, dynamic light scattering, and cryogenic transmission electron microscopy were used to explore the dual-responsive behaviors of wormlike micelles system systematically. The results show that the self-assembly structures and viscoelastic behaviors of the mixed system can be regulated by light irradiation and pH. With BTHA concentration ≥ 20 mM, the mixed solutions show a shear-thinning behavior, implying the formation of wormlike micelles. Under UV light irradiation at 365 nm, the photoisomerization of BTHA from trans to cis affects the molecular packing at micellar interface, that leads to the aggregate structures transformation from wormlike micelles into spherical micelles, accompanied by an apparent decrease in zero-shear viscosity of the mixed solution by three orders of magnitude. The transformation is reversible in the presence of Vis light (420 nm). Moreover, the pH-responsive behavior could also be reversibly switched by adding HCl or NaOH, due to the protonation of NaOA molecules. The interconversion of gel-like solution and water-like fluid could occur by adjusting pH values between 10.26 and 8.95, which reflect variations in microstructures and was ascertained the transformation between wormlike micelles and spherical micelles by cryo-TEM. The relationship of temperature and the structural relaxation time (τR) follows the Arrhenius law, resulting from the decreasing micellar contour length. This work will enhance our understanding of the multi-responsiveness of intelligent materials and hold great promising applications in biomedicine, oil industry, smart molecular device, and sensors.

Gradual transformation of anionic/zwitterionic wormlike micelles from viscous to elastic domains: Unravelling the effect of anionic surfactant chain length

HypothesisUltra-long tailed zwitterionic surfactants often form aqueous wormlike elastic micelles, whereas the shorter ones mainly exhibit spherical viscous micelles. Anionic surfactants are widely used to tune the micellar morphology from spherical into wormlike. Systematic investigations in the molecular level are insightful to understand the viscoelasticity regulative mechanism. ExperimentsAnionic/zwitterionic hybrid wormlike micelles are composed of sodium alkylsulfate (SAS) homologues and dodecyl dimethyl amidopropyl hydroxyl sulfobetaine (DHSB). The formation of wormlike micelles was studied by employing rheometer, cryogenic transmission electron microscopy (cryo-TEM) and small angle X-ray scattering (SAXS) techniques. The effects of surfactant concentration, molar ratio, anionic surfactant chain length and temperature were investigated systematically. FindingsSAS promoted the formation of SAS/DHSB hybrid wormlike micelles. The increase in both chain length and molar ratio (xSAS) of SAS are advantageous in the enhancement of viscosity. Interestingly, sodium hexadecylsulfate (SHS) endowed elastic wormlike micelles with thermally insensitive viscosity below its Krafft temperature (Tk), which was distinguished from the viscous ones formed by sodium octylsulfate (SOS). SAXS results showed that the size of SAS/DHSB wormlike micelles was primarily determinate by surfactants with longer hydrophobic tails.

Gas channeling control with an in-situ smart surfactant gel during water-alternating-CO2 enhanced oil recovery

Undesirable gas channeling always occurs along the high-permeability layers in heterogeneous oil reservoirs during water-alternating-CO2 (WAG) flooding, and conventional polymer gels used for blocking the “channeling” paths usually suffer from either low injectivity or poor gelation control. Herein, we for the first time developed an in-situ high-pressure CO2-triggered gel system based on a smart surfactant, N-erucamidopropyl-N,N-dimethylamine (UC22AMPM), which was introduced into the aqueous slugs to control gas channeling in WAG processes. The water-like, low-viscosity UC22AMPM brine solution can be thickened by high-pressure CO2 owing to the formation of wormlike micelles (WLMs), as well as their growth and shear-induced structure buildup under shear flow. The thickening power can be further potentiated by the generation of denser WLMs resulting from either surfactant concentration augmentation or a certain range of heating, and can be impaired via pressurization above the critical pressure of CO2 because of its soaring solvent power. Core flooding tests using heterogeneous cores demonstrated that gas channeling was alleviated by plugging of high-capacity channels due to the in-situ gelation of UC22AMPM slugs upon their reaction with the pre- or post-injected CO2 slugs under shear flow, thereupon driving chase fluids into unrecovered low-permeability areas and producing an 8.0% higher oil recovery factor than the conventional WAG mode. This smart surfactant enabled high injectivity and satisfactory gelation control, attributable to low initial viscosity and the combined properties of one component and CO2-triggered gelation, respectively. This work could provide a guide towards designing gels for reducing CO2 spillover and reinforcing the CO2 sequestration effect during CO2 enhanced oil recovery processes.

Synergistic interaction of α-olefin sodium sulfonate/cocamidopropyl betaine surfactant mixtures and preparation of wormlike micelles

The unique structure and macro-rheological properties make worm-like micelles an important basic system for surfactant applications, which are widely used in many fields and have received extensive attention from researchers. In this paper, α-olefin sodium sulfonate (AOS) and cocamidopropyl betaine (CAPB) were mixed in aqueous solution to prepare viscoelastic worm-like micelles, which can be observed by cryogenic-transmission electron microscopy (cryo-TEM). It was found that the concentration of AOS and temperature have a greater impact on the formation and rheological behavior of worm-like micelles. The typical characteristics of wormlike micelles were shown in the range of AOS concentration from 0.7 wt% to 1.7 wt%; when the temperature was too high, the worm-like micelles were destroyed. In addition, the surface tension, foaming property and wettability of AOS/CAPB mixed solutions with different ratios were studied. The two show excellent synergistic effect, in which electrostatic interaction is the decisive factor, which can provide basic data for practical applications. This work further enriches the basic theoretical research on the preparation of worm-like micelles from anionic-zwitterionic surfactant systems.

Research on viscoelastic properties of SLG-LHSB system: Effects of pH and concentration on micelles in the system

The pH and the state of SLG in aqueous solution are strongly connected. When the pH was below 5.5, it would be transformed into N-lauroyl glutamic acid (LGA) with limited solubility; when the pH was between 5.5 and 6.5, SLG was the major component in the system; and when the pH was above 6.5, disodium lauroyl glutamate predominated (DLG). Because SLG and lauramidopropyl hydroxy sulfobetaine (LHSB, also known as lauramidopropyl hydroxy Sultaine) can combine to generate mixed micelles that can solubilize insoluble materials, the compound system outperformed the single system in terms of dissolving. The viscosity of the SLG and LHSB compound system changed dramatically with pH, corresponding to the change in SLG components with pH. The viscoelastic properties of the composite system were investigated by using steady-state and oscillatory shear sweep rheological measurements. The zero-shear viscosity (η0) had a strong dependence on the concentration of the compound system. The compound system began to form worm-like micelles at 0.05 mol/L. When the system concentration hit 0.35 mol/L, the worm-like micelles began to break, and their length reduced while their quantity rose. (Cryogenic-transmission electron microscopy (Cryo-TEM) confirmed the formation of worm-like micelles. The system demonstrated benign features and outstanding foaming performance in the foam performance and irritation tests.

Effect of Sodium Salicylate on Improving the Lubricating Properties of [C18imC9]Br Wormlike Micelles in Glycerol/Propylene Glycol at Low Temperatures

The formation of long cylindrical wormlike micelles at low temperatures (near 0 °C) is relatively rare. In this work, the effect of different salts on the viscoelasticity of the formed wormlike micelles by the ionic liquid surfactant (1-octadecyl-3-nonyl imidazolium bromide, [C18imC9]Br) and glycerol/1,2-propanediol mixture at low temperature was studied. The rheological measurements revealed that only sodium salicylate (NaSal) can increase the viscoelasticity of [C18imC9]Br wormlike micelles by up to 15 times and the existence of temperature range. When the [C18imC9]Br concentration is fixed, the viscoelasticity of the [C18imC9]Br/NaSal wormlike micelles increases and then decreases with the increase of NaSal concentration. Tribological results show that the [C18imC9]Br/NaSal wormlike micelles have excellent friction reduction and antiwear properties for steel/steel contact at 6.00 °C. The friction coefficient and wear volume of [C18imC9]Br/NaSal wormlike micelles first decrease and then increase with the increase of the NaSal concentration. In addition, their tribological properties are significantly better than those of pure [C18imC9]Br wormlike micelles under the same conditions. The lubrication effect of [C18imC9]Br/NaSal wormlike micelles at low temperatures is improved due to the adhesion of [C18imC9]Br/NaSal wormlike micelles and the protective film formed by the tribochemical reaction.

Interfacial Composition of Surfactant Aggregates in the Presence of Fragrance: A Chemical Trapping Study.

In recent years, there has been increasing interest in daily-use chemical products providing a pleasant scent. The added fragrance molecules may induce microstructural transitions of surfactant aggregates, which further affect the physical and chemical properties of the products. Here, the effects of four types of aromatic alcohols (cinnamyl alcohol, phenyl ethanol, phenyl methanol and anisyl alcohol) on cetyltrimethylammonium bromide (CTAB)/KBr aggregates were studied. The combined results from rheology, dynamic light scattering, and transmission electron microscopy measurements showed that cinnamyl alcohol induced significant micellar growth, while increases in micellar growth were less obvious for the other aromatic alcohols. The changes in the interfacial molarities of water, aromatic alcohol, and bromide ions during such transitions were studied using the chemical trapping method. Transitions resulting from added cinnamyl alcohol were accompanied by significant declines in interfacial water and bromide ion molarities, and a rise in interfacial alcohol molarity. The marked decrease in interfacial water molarity was not observed in previous studies of the octanol induced formation of wormlike micelles and vesicles, indicating that a different mechanism was presented in the current system. Nuclear magnetic resonance investigation showed that π–π stacking between cinnamyl alcohols, but not cation–π interactions between alcohols and CTAB headgroups, facilitated the tight packing of alcohol molecules in CTAB aggregates and the repulsion of water from the interfacial region. The current study may provide a theoretical basis for the morphological regulation of surfactant aggregates in the presence of additives.

Effect of counter-ion on rheological properties of mixed surfactant solutions

The effects of the charge, size and concentration of counter-ions (Na+, NH4+, K+, Mg2+, Ca2+ and Al3+) on the rheological properties of 300 mM SLES+CAPB surfactant solutions are studied and compared with results available in the literature for the effects of Na+ and K+ on SLES solution. The results show that all counter-ions studied are able to induce formation of wormlike micelles in a certain concentration range – as a consequence, the solution viscosity passes through a high maximum as a function of salt concentration. The salt concentration leading to maximum in the solution viscosity decreases with increasing the charge density of the counter-ion, defined as the ratio of the charge of the counterion and its hydrated radius, Z/R. The maximum viscosity in the salt curve decreases with increasing the hydrated radius of the counter-ion. The addition of CAPB to SLES leads to about 10-fold higher viscosity and to higher maximum viscosity in the presence of K+ as compared to Na+, whereas the opposite trend was observed for SLES alone. These effects are attributed to the interplay between the anionic and zwitterionic surfactants, and the electrolyte in the mixed SLES+CAPB system.

The Role of the Presence of Aliphatic Alcohols on the Formation of Wormlike Micelle of Anionic-Cationic Surfactants Mixture

Thermodynamically controlled supramolecular gel or aggregate is commonly resulted from the formation of one-dimensional micelles through specific self-assembling of amphiphilic molecules. This phenomenon is commonly referred to as the formation of worm- or thread-like micelle which is still considered as quite complicated and obscure. This, in spite of the recent physical insight for interpreting this phenomenon, is referred to as the critical intermolecular forces (CIF) of which still needs further support. Therefore, attention by scientists in this field should be given in order to make this transition phenomenon clear. In this work, investigations concerning the effect of addition of aliphatic alcohols with different chain lengths of even carbon numbers (ethanol, 1-butanol, 1-hexanol, 1-octanol and 1-decanol), at different concentrations (0.002-0.040 M), and temperatures on the formation of wormlike micelle for the mixture of anionic sodium dodecylsulfate (SDS) and cationic cetyltrimethylammonium bromide (CTAB) surfactants were carried out. The formation of these one dimensional micelles were practically detected by following the sharp change of physical properties through the presence of a remarkable high viscosity peak. The results indicate there is no linear or systematic relationship between the effects of the presence of these alcohols with their chain lengths. It has been found that the presence of aliphatic alcohols causes a positive effect on the formation of one dimensional micelles. The ability of the transition from spherical to wormlike micelles increases with increasing their concentrations of alcohols except for that of 1-octanol. The obtained achievements were explained in terms of CIF theory which then offered a reasonable support to this theory. It was concluded that alcohols particularly ethanol and 1-octanol can be employed as adjustment reagents for controlling required viscosity of threadlike micelles.

Tracking Wormlike Micelle Formation in Solution: Unique Insight through Fluorescence Correlation Spectroscopic Study.

Although worm-like micelles were invented 35 years ago, its formation pathway remains unclear. Inspired by the fact that a single molecular level experiment could provide meaningful and additional information, especially in a heterogeneous subpopulation, herein, we present a single molecular level study on the formation of wormlike micelles by cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal) in water. Our results indicated a coexistence of normal spherical micelles along with a big wormlike micelle in its formation path. More interestingly, we have two unique insights into the formation mechanism, which are inaccessible in ensemble averaged experiments: (i) at extremely low concentrations of the surfactant, [CTAB]/[NaSal] ∼ 0.06, the wormlike micelle attains the highest size; and (ii) the relative concentration of wormlike micelles is highest when [CTAB]/[NaSal] ∼ 2.

Temperature-controlled dripping-onto-substrate (DoS) extensional rheometry of polymer micelle solutions.

Capillary-driven thinning of a liquid bridge is commonly used to measure the extensional rheology of macromolecular solutions for assessment of material sprayability, printability, and jettability. Methods like dripping-onto-substrate (DoS) rheometry are often preferred to methods like capillary breakup extensional rheometry (CaBER) due to low required sample volume and ability to measure low-viscosity fluids; however, DoS measurements to-date have been limited to ambient temperatures. Here, an environmental control chamber is developed to enable temperature-controlled DoS (TC-DoS) measurements, and the temperature-dependent extensional rheology of a model system of poloxamer 234 (P234) in NaF brine is examined. Spherical P234 micelles at ambient conditions exhibit inertiocapillary (IC) thinning; above the sphere-to-rod transition temperature, the liquid bridge evolves towards viscocapillary (VC) thinning as micelles lengthen and shear viscosity increases. Above 37 °C, wormlike micelle (WLM) formation results in pronounced elastocapillary (EC) thinning, and further WLM growth and entanglement results in three elasticity-dominated flow regimes: EC thinning, beads-on-a-string (BOAS) instability formation, and BOAS thinning. Despite having a substantially larger amphiphile molecular weight and micelle cross-sectional radius than surfactant WLMs, entangled P234 WLMs exhibit similar extensional behavior and achieve comparable maximum Trouton ratios. Comparing DoS measurements of P234 WLMs with prior studies on surfactant WLMs reveals that the maximum Trouton ratio depends on the ratio of shear and extensional relaxation times, a trend undetectable via CaBER due to pre-deformation during the initial step stretch. These findings reveal rich temperature-dependent flow behaviors in polymer micelles and highlight the importance of using a minimally-disruptive method such as TC-DoS when measuring the extensional rheology of microstructured and thermosensitive fluids.

Modification of turbulence caused by cationic surfactant wormlike micellar structures in two-dimensional turbulent flow

An experimental study is performed to investigate the effects of the extensional rheological properties of drag-reducing wormlike micellar solutions on the vortex deformation and turbulence statistics in two-dimensional (2-D) turbulent flow. A self-standing 2-D turbulent flow was used as the experimental set-up, and the flow was observed through interference pattern monitoring and particle image velocimetry. Vortex shedding and turbulence statistics in the flow were affected by the formation of wormlike micelles and were enhanced by increasing the molar ratio of the counter-ion supplier to the surfactant, ξ, or by applying extensional stresses to the solution. In the 2-D turbulent flow, extensional and shear rates were applied to the fluids around a comb of equally spaced cylinders. This induced the formation of a structure made of wormlike micelles just behind the cylinder. The flow-induced structure influenced the velocity fields around the comb and the turbulence statistics. A characteristic increase in turbulent energy was observed, which decreased slowly downstream. The results implied that the characteristic modification of the 2-D turbulent flow of the drag-reducing surfactant solution was affected by the formation and slow relaxation of the flow-induced structure. The relaxation process of the flow-induced structure made of wormlike micelles was very different from that of the polymers.