Wormlike Micellar Research Articles

Extensive tip-splitting of injected organic liquid into an aqueous viscoelastic fluid

The injection of a fluid into another fluid causes a spatiotemporal pattern along the injection front. Viscous fingering is a well-known example when the replaced material is a viscous fluid. Notably, most fluids are, in reality, viscoelastic, i.e., they behave as an elastic solid over short timescales. For this reason, it is important to study the situation when the replaced fluid is viscoelastic. In this study, we observed a dynamics of fluids when an incompressible organic liquid was injected into an oleophilic Hele–Shaw cell filled with an aqueous viscoelastic fluid made of a wormlike micellar solution. We found extensive tip splitting of the injection front, which led to thin fingers with a characteristic size comparable to four times the cell thickness. We examined the material properties and suggest that the thin fingering pattern observed in our system is due to the delamination of viscoelastic fluid from the bottom substrate surface. Our result shows that the effect of interfacial energy in the existing solid layer should be considered in the injection process.

Flow of wormlike micellar solutions over concavities.

We present a comprehensive investigation combining numerical simulations with experimental validation, focusing on the creeping flow behavior of a shear-banding, viscoelastic wormlike micellar (WLM) solution over concavities with various depths (D) and lengths (L). The fluid is modeled using the diffusive Giesekus model, with model parameters set to quantitatively describe the shear rheology of a 100 : 60 mM cetylpyridinium chloride:sodium salicylate aqueous WLM solution used for the experimental validation. We observe a transition from "cavity flow" to "expansion-contraction flow" as the length L exceeds the sum of depth D and channel width W. This transition is manifested by a change of vortical structures within the concavity. For L ≤ D + W, "cavity flow" is characterized by large scale recirculations spanning the concavity length. For L > D + W, the recirculations observed in "expansion-contraction flow" are confined to the salient corners downstream of the expansion plane and upstream of the contraction plane. Using the numerical dataset, we construct phase diagrams in L-D at various fixed Weissenberg numbers Wi, characterizing the transitions and describing the evolution of vortical structures influenced by viscoelastic effects.

CO2-induced viscoelastic fluids based on a C22-tailed tertiary amine under different pressure and temperature

CO2-switchable viscoelastic surfactant solutions have gained much attention over the last decade; however, large-scale application and the impact of temperature and pressure on such soft colloidal materials have not been explored yet. Here in this work, the impact of environmental parameters including shear rate, pressure, temperature, and salinity on CO2-thickening behavior of aqueous dispersion of N-erucamidopropyl-N,N-dimethylamine (UC22AMPM) was assessed rheologically. Preliminary immiscible water-alternating-CO2 (CO2-WAG) flooding tests with and without UC22AMPM were conducted to highlight the potential of CO2-switchable surfactants in the process of improved oil recovery. The results demonstrated that the dispersion can be viscosified by CO2 15 times higher at 60 °C and 5 MPa, and such thickening power can be switched “on” and “off” upon bubbling and removal of CO2. A universal double-plateau viscosity profile was found as a function of CO2 bubbling time irrespective environmental condition, but the higher the shear rate or the pressure, the shorter the time is needed to reach the plateau viscosity; the higher the temperature, the longer the time is needed to get to the second plateau viscosity. UC22AMPM is first protonated into a long-chain cationic surfactant to form wormlike micellar solution, then the first viscosity plateau; expansion of the viscoelastic solution with CO2 attributes to the second viscosity plateau. Under the simulated oil reservoir of Daqing oilfield, immiscible CO2-WAG flood alone produced an additional 10.5% oil over the initial waterflood recovery, while the addition of 2.1 wt% UC22AMPM in the water slug could recover incremental 5.1% oil relative to pure CO2-WAG. These unique features suggest that coupling injection of CO2 and UC22AMPM dispersion holds great promise for recovering oil during the immiscible CO2-WAG process.

Numerical Study on the Unstable Flow Dynamics of Wormlike Micellar Solutions past a Sphere in the Creeping Flow Regime.

The flow dynamics of wormlike micellar solutions around a sphere is a fundamental problem in particle-laden complex fluids but is still understood insufficiently. In this study, the flows of the wormlike micellar solution past a sphere in the creeping flow regime are investigated numerically with the two species, micelles scission/reforming, Vasquez-Cook-McKinley (VCM) and the single-species Giesekus constitutive equations. The two constitutive models both exhibit the shear thinning and the extension hardening rheological properties. There exists a region with a high velocity that exceeds the main stream velocity in the wake of the sphere, forming a stretched wake with a large velocity gradient, when the fluids flow past a sphere at very low Reynolds numbers. We found a quasi-periodic fluctuation of the velocity with the time in the wake of the sphere using the Giesekus model, which shows a qualitative similarity with the results found in present and previous numerical simulations with the VCM model. The results indicate that it is the elasticity of the fluid that causes the flow instability at low Reynolds numbers, and the increase in the elasticity enhances the chaos of the velocity fluctuation. This elastic-induced instability might be the reason for the oscillating falling behaviors of a sphere in wormlike micellar solutions in prior experiments.

On the startup behavior of wormlike micellar networks: The effect of different salts bound to the same surfactant molecule

We report on shear startup data for two wormlike micellar solutions, differing only in concentration and type of two binding aromatic sodium salts. The surfactant molecule is cetylpiridinium chloride at a fixed concentration (100 mM). Sodium salicylate (NaSal) and diclofenac sodium (Diclo) are used as binding salts at concentrations 68 mM NaSal and 52 mM Diclo such that both systems are fully entangled and their linear viscoelastic response is essentially identical. Both systems show the linear response typical of a wormlike micellar solution, with terminal behavior at low frequencies, a well-defined moduli crossover, and a plateau modulus. In the nonlinear regime, however, the behavior of the two systems is totally different, suggesting that the molecular structure difference of the salts and their binding activity to the surfactant molecule are both crucial to determine the fast flow behavior. The NaSal solution shows a very complex rheological response, with strain hardening and very sharp stress peaks, whereas the solution containing Diclo behaves much like ordinary linear polymers, exhibiting pronounced overshoots as well as moderate undershoots in the transient shear viscosity, before approaching the steady state. This polymerlike behavior has also been proved by successfully comparing data with predictions of a constitutive equation recently adopted for both entangled polymers and linear wormlike micelles. As far as NaSal is concerned, a phenomenological model based on rubber network theory is developed, which describes the flow singularities. A physical interpretation of the different behavior in the nonlinear regime is also suggested.

Prediction of the Viscoelastic Properties of a Cetyl Pyridinium Chloride/Sodium Salicylate Micellar Solution: (II) Prediction of the Step Rate Experiments.

The reliable viscoelastic characterization and prediction of micellar solution is still required in industrial applications of the solution, e.g., in surfactant flooding and pharmaceuticals. Based on the recent theoretical characterization of the viscoelastic properties of a cetyl pyridinium chloride/sodium salicylate (CPyCl/NaSal) wormlike micellar solution with a structuralized constitutive model in the work published in 2022, the present work predicted five groups of transient shear viscoelasticities of the solution experimentally obtained in 2010, which include the first normal stress difference (N1) versus time curve in the start-up experiment, the shear stress (τ12) in the start-up experiment, τ12 in the long-term start-up experiment, the stress relaxation upon cessation of steady shear flow, and the transient N1/τ12 in the step strain experiment. The study findings clearly show an improvement in the predictions of the viscoelastic properties of the micellar solution compared with those predicted previously. For example, the experimental N1/τ12 is 9 at the strain of 9 in the step strain experiment, and the corresponding previous and present predictions are 2.47 and 8.45, respectively.

Photo-Controlled Dynamics and Transport in Entangled Wormlike Micellar Nanocomposites Studied by XPCS

Dynamics of nanoparticles (NPs) in microscopic networks, in particular, localization and transport, play a key role in designing new functional nanocomposites and drug delivery systems. To this aim, it is crucial to understand the interplay between the network structure and dynamics on the microscopic scale which determines NP diffusion. Here, we study the localization and transport of spherical NPs in photorheological wormlike micellar nanocomposites where the mobility of the NPs is controlled by the network mesh size and the micelle length, which can be tuned by UV-illumination. The macroscopic viscoelastic properties are measured by classical rheology, while X-ray photon correlation spectroscopy and nanorheology provide information on the microscopic NP dynamics on length scales on the order of the network mesh size. On long time scales, the data reveal that transport through the network is determined by the ratio between the NP size and the network mesh size, while upon UV illumination, the NP mobility is drastically enhanced. On shorter time scales, the influence of the dynamical and structural micelle properties on the NP dynamics under confinement is explored and indicates an anomalous speed-up of the dynamics, which is discussed in the context of changes in the local structure and non-linear phenomena such as strain stiffening and hopping motion.

Evaluation of constitutive models for shear-banding wormlike micellar solutions in simple and complex flows

Wormlike micellar solutions possess complex rheology: when exposed to a flow field, the wormlike micelles may orientate, stretch, and break into smaller micelles. Entangled wormlike micellar solutions exhibit shear banding characteristics: macroscopic bands with different local viscosities are organized and stacked along the velocity gradient direction, leading to a non-monotonic flow curve in simple shear. We present a systematic analysis of four commonly used constitutive models that can predict a non-monotonic flow curve and potentially describe the rheology of entangled wormlike micellar solutions with shear-banding characteristics: the Johnson–Segalman, the Giesekus, the thixotropic viscoelastic, and the Vasquez–Cook–McKinley (VCM) models. All four constitutive models contain a stress diffusion term, to account for a smooth transition between the shear bands and ensure a uniqueness of the numerical solution. Initially, the models are fitted to shear and extensional experimental data of a shear-banding wormlike micellar solution. Subsequently, they are employed to solve three non-homogeneous flows: the Poiseuille flow in a planar channel, the flow in a cross-slot geometry, and the flow past a cylinder in a straight channel. Each of these flows exposes the wormlike micellar solution to different flow kinematics (shear, extensional, and mixed), revealing different aspects of its rheological response. The predictive capability of each model is evaluated by directly comparing the numerical results to previously published experimental data obtained from microfluidic devices with corresponding flow configurations. While all the models can describe qualitatively the characteristic features observed experimentally in the benchmark flows, such as plug-like velocity profiles and elastic instabilities, none of them yields a quantitative agreement. Based on the overall performance of the models and also accounting for their differing numerical complexity, we conclude that the Giesekus model is at present the most suitable constitutive equation for simulating shear banding wormlike micellar solutions in flows that exhibit both shear and extensional deformations. However, the quantitative mismatch between model predictions and experiments with wormlike micellar solutions demand that improved constitutive models be developed in future works. • Comparison of popular constitutive models for wormlike micellar solutions. • Evaluation of their performance in rheometric and inhomogeneous shear flows. • Investigation of their predictions in extensional flows and elastic instabilities. • The models can only make qualitative predictions.

Prediction of the viscoelastic properties of a cetyl pyridinium chloride/sodium salicylate micellar solution: (I) characterization

The characterization capability of constitutive equation on the viscoelastic property of micellar solution was checked in this study. A modified viscoelastic model was employed here to express the viscoelastic properties of a cetyl pyridinium chloride/sodium salicylate (CPyCl/NaSal) wormlike micellar solution at 22 °C that was experimentally obtained in 2010. The relaxation spectrum of the CPyCl/NaSal solution was fitted using the dynamic frequency sweep, the parameter in the Papanastasiou-Scriven-Macosko damping function was obtained using stress relaxation in step strain, the structuralized parameter f in the model was obtained using the steady shear stress, and the parameter ζ was obtained using the stress growth in the shear startup experiment. Afterwards, the first normal stress difference of the solution in the steady shear experiment was predicted. The study clearly shows improvement in the calculated viscoelastic properties of the micellar solution compared with those obtained by the Vasquez-Cook-Mckinley model, such as steady shear stress.

Stokes layers in oscillatory flows of viscoelastic fluids

<h2>Abstract</h2> Oscillatory fluid flows play an important role in industrial problems such as pumping, drilling, and oil recovery, and in physiological processes such as mucous flow in respiration and blood pumping. A key concept in oscillatory flows is the Stokes layer, i.e. the region of fluid adjacent to a wall that is in motion as a result of the wall oscillatory motion along its own plane. In this talk we will discuss oscillatory flows of viscoelastic fluids from the perspective of Stokes viscoelastic layers. We will first revisit Stokes' second problem for a viscoelastic fluid modelled by a single-mode Maxwell constitutive equation, and the generation of viscoelastic shear waves. Next, we will turn the attention to wall-bounded zero-mean oscillatory flows, for which we will identify the governing dimensionless variables and discuss the presence of resonances in the one-dimensional base flow. Finally, we will present experimental evidence of the destabilization of the one-dimensional base flow for a wormlike micellar solution in a cylindrical pipe as the oscillatory forcing increases, and sketch a perturbative analysis based on the Giesekus constitutive equation that links the first instability of the base flow to the divergence of normal radial stresses. If there is enough time, we will also present recent experimental results of oscillatory pipe flow of shear-thinning non-elastic solutions.

Non-Newtonian flows and instabilities in 3D glass microfluidic devices

<h2>Abstract</h2> In this virtual seminar we will discuss the use of selective laser-induced etching (SLE) for the fabrication of glass microfluidic devices, in particular for the study of non-Newtonian fluid dynamics problems [1]. SLE enables the precise monolithic fabrication of three-dimensional (3D) channels and structures embedded in a rigid and transparent fused-silica substrate, and thus provides opportunities for a wealth of novel microscale flow experiments. We will highlight various recent and ongoing microfluidic projects from our laboratory that have been made possible by SLE fabrication. Our focus will be on flows of viscoelastic wormlike micellar and polymer solutions in microscale geometries featuring, for instance, slender rigid or cantilevered posts [2,3]. 3D intersections [4,5], and axisymmetric contraction/expansions [6]. We make use of state-of-the-art quantitative techniques such as high-speed flow-induced birefringence imaging and volumetric 3-component micro-particle image velocimetry to characterize and understand purely-elastic and inertioelastic flow instabilities arising in these complex systems.

Bifurcation in flows of wormlike micellar solutions past three vertically aligned microcylinders in a channel

This study presents a numerical investigation of path switching and selection phenomena in flows of wormlike micellar solutions (WLMs) past three vertically aligned microcylinders in a channel in the creeping flow regime. The flow characteristics of the wormlike micellar solution are examined with the help of a two-species Vasquez–Cook–McKinley constitutive model, which considers both the breakage and re-formation dynamics of wormlike micelles. At low Weissenberg numbers (ratio of the elastic to that of the viscous forces, Wi), the flow field in the present system is found to be steady and symmetric. Furthermore, the WLM solution passes through all the passages present between the microcylinders and channel walls. However, as the Weissenberg number reaches a critical value Wicri, a transition in the flow field from steady to unsteady occurs. Furthermore, the flow field is found to be bifurcated (a transition from symmetric to asymmetric flow field also occurs) as the Weissenberg number gradually increases. However, we observe that all these transitions are strongly dependent on the micelle breakage rate (i.e., how easy or hard to break a micelle) and the intercylinder gap. This study is an extension of our earlier studies on the flow of WLMs past a single and two vertically aligned microcylinders, which are often considered as model porous media for studying the flow dynamics of various complex fluids. The results presented in this work will be relevant for understanding the path switching phenomena of complex fluids during their flow through a porous media.

Modeling Techniques for the Rheology of Wormlike Micellar Solutions

Modeling Techniques for the Rheology of Wormlike Micellar Solutions

Hydrogel Formation by Glutamic-acid-based Organogelator Using Surfactant-mediated Gelation.

Hydrogels formed by low-molecular-weight gelators have reversible sol-gel transition and responsiveness to various stimuli, and are used in cosmetics and drug applications. It is challenging to obtain hydrogels using novel gelators because subtle differences in their molecular architecture affect gelation. Organogelators (which form organogels) are insoluble in water, and their use as hydrogelators has not previously been considered. However, a surfactant-mediated gelation method was reported in which organogelators were solubilized in water by surfactants to form hydrogels using 12-hydroxyoctadecanoic acid. To investigate whether this method can be applied with other organogelators, the formation of hydrogel using a glutamic-acid-based organogelator was studied here. Hydrogels were formed by solubilizing 1:1 mixtures of glutamate-based organogelators, N-lauroyl-L-glutamic acid dibuthylamide, and N-2-ethylhexanoyl-L-glutamic acid dibutylamide in aqueous micellar solutions of anionic surfactant (sodium lauroyl glutamate) and cationic surfactant (cetyltrimethylammonium chloride). The minimum gelation concentration of the hydrogel was ~0.2-0.6 wt%. By changing the molar fraction of cetyltrimethylammonium chloride in the mixed surfactant, either spherical or wormlike micelles were formed. The hydrogel with wormlike micelles had a higher sol-gel transition temperature than that with spherical micelles and formed fine self-assembled fibrillar networks. Additionally, the hydrogel with the spherical micelles was elastic, whereas that with wormlike micelles was viscoelastic, suggesting that networks of the organogelators and wormlike micelles coexisted in the hydrogel from the wormlike micellar solution. Moreover, the hydrogel suppressed the reduction in the storage modulus at higher temperatures compared with the micellar aqueous solution, indicating that the elastic properties of the organogelator networks were maintained at high temperatures. The gel fibers of the hydrogel partially formed a loosely aggregated structure as the temperature increased, the fibers bundled via hydrophobic interactions, and new cross-linking points formed spontaneously. This phenomenon corresponded with an inflection point in the temperature-dependent storage modulus of the hydrogel.

Effect of blockage ratio on flow of a viscoelastic wormlike micellar solution past a cylinder in a microchannel.

We present experiments on the flow of a viscoelastic wormlike micellar solution around cylinders (radius R) confined in straight microchannels (width W). Thirteen flow geometries are tested where the blockage ratio is varied over a wide range 0.055 ≤ BR = 2R/W ≤ 0.63. Experiments are performed at negligible Reynolds number, and for Weissenberg numbers Wi = λU/R up to 1000, where U is the average flow speed and λ is the relaxation time of the fluid. Micro-particle image velocimetry is used to characterise the flow state at each BR and Wi. In all of the geometries, a first critical Weissenberg number marks a transition from symmetric flow to an asymmetric but time-steady flow state, while a second higher critical Weissenberg number marks the onset of time-dependent flows. However, we report a clear shift in behaviour over a narrow intermediate range of 0.33 ≲ BR ≲ 0.41. Channels with BR ≤ 0.33 fall in a 'low' BR regime, with instabilities that originate from the downstream stagnation point, while those with BR ≥ 0.44 fall in a 'high' BR regime, with instabilities developing at the upstream stagnation point. Behaviour within the newly-identified intermediate BR regime is complex due to the competing influence of the two stagnation points. We summarise all our results in a flow state diagram covering Wi-BR parameter space, clearly defining the different regimes of blockage ratio for the first time. Our results contribute to the understanding of the complexities of viscoelastic flow in this benchmark geometry.

Kinetics of shear banding flow formation in linear and branched wormlike micelles.

We investigate the flow evolution of a linear and a branched wormlike micellar solution with matched rheology in a Taylor-Couette (TC) cell using a combination of particle-tracking velocimetry, birefringence, and turbidity measurements. Both solutions exhibit a stress plateau within a range of shear rates. Under startup of a steady shear rate flow within the stress plateau, both linear and branched samples exhibit strong transient shear thinning flow profiles. However, while the flow of the linear solution evolves to a banded structure at longer times, the flow of the branched solution transitions to a curved velocity profile with no evidence of shear banding. Flow-induced birefringence measurements indicate transient birefringence banding with strong micellar alignment in the high shear band for the linear solution. The transient flow-induced birefringence is stronger for the branched system at an otherwise identical Wi. At longer times, the birefringence bands are replaced by a chaotic flow reminiscent of elastic instabilities. Visualization of the flow-induced turbidity in the velocity gradient-vorticity plane reveals quasi-steady banding with a turbidity contrast between high and low shear bands in the linear solution. However, the turbidity evolves uniformly within the gap of the TC cell for the branched solution, corroborating the non-banded quasi-steady velocimetry results. Finally, we show that while elastic instabilities in the linear solution emerge in the high shear band, the flow of branched solution at high Wi becomes unstable due to end effects, with growing end regions that ultimately span the entire axial length of the TC cell.

Correction to: Viscoelastic characterization and prediction of a wormlike micellar solution

Correction to: Viscoelastic characterization and prediction of a wormlike micellar solution

Viscoelastic characterization and prediction of a wormlike micellar solution

Viscoelastic characterization and prediction of a wormlike micellar solution

Deducing the Relation between Viscosity and Oil-Induced Structural Changes of Viscoelastic Surfactants Using a Kinetic Approach.

The present study relates viscosity reduction with time of a wormlike micellar solution to the micellar transitions that occur with time in the presence of three n-alkanes, namely, n-decane, n-dodecane, and n-hexadecane. Steady-shear rheology and small-angle X-ray scattering were used to deduce the relationship. The effect of n-alkane concentration was tested only with n-decane. There were at most three stages of viscosity reduction, which appeared in the following order: (i) the rising viscosity stage, (ii) the fast viscosity reduction stage, and (iii) the low-viscosity stage. The stages and rates of viscosity transition depended on the type of micelles present and the degree of micelle entanglement. Moreover, the rate of transition increased when the n-alkane concentration was increased and when the n-alkane molecular mass was reduced. n-Hexadecane induced only the first two stages of transition at a slower rate compared to the other oils.

Stress-gradient-induced migration effects on the elastic instabilities of wormlike micellar solutions in a cross-slot flow

Using the two-species VCM model, we report the effects of stress-gradient-induced migration on the elastic instabilities of a wormlike micellar solution flowing through a cross-slot microchannel. The model was solved using a mixed finite element method in the open-source platform FEniCS. The stress-gradient-induced migration due to the non-Fickian (conformation) fluxes changes the onset of the instability and symmetry breaking and the size of secondary flow patterns upstream of the cross-channel corner. Varying the chain scission and micellar extensibility parameter, we observed that the suppression of asymmetric flow instability occurs at a different rate with the effects of migration. These results suggest that conformation force-driven migration has an influence on the onset of flow instability and should not be neglected a priori.