Microfluidic flows of wormlike micellar solutions

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.

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.

A microfluidic approach to studying the injection flow of concentrated albumin solutions

Abstract

Subcutaneous injection by means of prefilled syringes allows patients to self-administrate high-concentration (100 g/L or more) protein-based drugs. Although the shear flow of concentrated globulins or monoclonal antibodies has been intensively studied and related to the injection force proper of SC processes, very small attention has been paid to the extensional behavior of this category of complex fluids. This work focuses on the flow of concentrated bovine serum albumin (BSA) solutions through a microfluidic “syringe-on-chip” contraction device which shares some similarities with the geometry of syringes used in SC self-injection. By comparing the velocity and pressure measurements in complex flow with rheometric shear measurements obtained by means of the “Rheo-chip” device, it is shown that the extensional viscosity plays an important role in the injection process of protinaceous drugs.

Article Highlights

A microfluidic “syringe on chip” device mimicking the injection flow of protinaceous drugs has been developed.

The velocity field of concentrated BSA solutions through the “syringe on chip” is Newtonian-like.

The extensional viscosity of concentrated protein solutions should also be considered when computing injection forces through needles.

Bifurcations in flows of complex fluids around microfluidic cylinders

Flow around a cylinder is a classical problem in fluid dynamics and also one of the benchmarks for testing viscoelastic flows. The problem is of wide relevance to understanding many microscale industrial and biological processes and applications, such as porous media and mucociliary flows. In recent years, we have developed model microfluidic geometries consisting of very slender cylinders fabricated in glass by selective laser-induced etching. The cylinder radius is small compared with the channel width, which allows the effects of the stagnation points in the flow to dominate over the effects of squeezing between the cylinder and the channel walls. Furthermore, the cylinders are contained in high aspect ratio microchannels that render the flow field approximately two-dimensional (2D) and therefore conveniently permit comparison between experiments and 2D numerical simulations. A number of different viscoelastic fluids including wormlike micellar and various polymer solutions have been tested in our devices. Of particular interest to us has been the occurrence of a striking, steady-in-time, flow asymmetry that occurs for certain non-Newtonian fluids when the dimensionless Weissenberg number (quantifying the importance of elastic over viscous forces in the flow) increases above a critical value. In this perspective review, we present a summary of our key findings related to this novel flow instability and present our current understanding of the mechanism for its onset and growth. We believe that the same fundamental mechanism may also underlie some important non-Newtonian phenomena observed in viscoelastic flows around particles, drops, and bubbles, or through geometries composed of multiple bifurcation points such as cylinder arrays and other porous media. Knowledge of the instability we discuss will be important to consider in the design of optimally functional lab-on-a-chip devices in which viscoelastic fluids are to be used.

pH-Responsive Rheological Properties and Microstructure Transition in Mixture of Anionic Gemini/Cationic Monomeric Surfactants

Surfactant aggregates have long been considered as a tool to improve drug delivery and have been widely used in medical products. The pH-responsive aggregation behavior in anionic gemini surfactant 1,3-bis(N-dodecyl-N-propanesulfonate sodium)-propane (C12C3C12(SO3)2) and its mixture with a cationic monomeric surfactant cetyltrimethylammonium bromide (CTAB) have been investigated. The spherical-to-wormlike micelle transition was successfully realized in C12C3C12(SO3)2 through decreasing the pH, while the rheological properties were perfectly enhanced for the formation of wormlike micelles. Especially at 140 mM and pH 6.7, the mixture showed high viscoelasticity, and the maximum of the zero-shear viscosity reached 1530 Pa·s. Acting as a sulfobetaine zwitterionic gemini surfactant, the electrostatic attraction, the hydrogen bond and the short spacer of C12C3C12(SO3)2 molecules were all responsible for the significant micellar growth. Upon adding CTAB, the similar transition could also be realized at a low pH, and the further transformation to branched micelles occurred by adjusting the total concentration. Although the mixtures did not approach the viscosity maximum appearing in the C12C3C12(SO3)2 solution, CTAB addition is more favorable for viscosity enhancement in the wormlike-micelle region. The weakened charges of the headgroups in a catanionic mixed system minimizes the micellar spontaneous curvature and enhances the intermolecular hydrogen-bonding interaction between C12C3C12(SO3)2, facilitating the formation of a viscous solution, which would greatly induce entanglement and even the fusion of wormlike micelles, thus resulting in branched microstructures and a decline of viscosity.

Viscoelastic Fluid Formed by Ultralong-Chain Erucic Acid-Base Ionic Liquid Surfactant Responds to Acid/Alkaline, CO2, and Light

As a leftover of grease processing, the efficient utilization of erucic acid is still a challenge. An alternative strategy is to develop erucic acid-derived surfactants. However, erucic acid-based ionic liquid surfactants were barely involved. Here, a novel ionic liquid surfactant, benzyltrimethylammonium erucate (ErBTA), was developed by a simple neutralization reaction, and its aggregations in the diluted and concentrated solution were systematically studied by surface tension, conductivity, rheology, and cryo-TEM techniques. The results showed that ErBTA has a very low metaling point (-7.03 °C) and possesses excellent water solubility (Krafft temperature <4 °C). ErBTA alone starts to form micelles at a very low concentration (0.028 mmol/L) and then to form worm-based viscoelastic fluid at 4.07 mmol/L without any additives, exhibiting excellent self-assembly ability and thickening ability. This viscoelastic fluid formed by ErBTA can simultaneously respond to three stimuli: common acid/alkaline, CO₂ gas, and light, accompanied by an interesting gel-sol conversion, reflecting microstructure transition from wormlike micelles to spherical micelles. Although in essence CO₂ and light also act as pH regulators in the current system, they provide more sophisticated approaches to tune pH. Such a viscoelastic fluid with the characteristics of easy availability, renewability of raw materials, the simplicity of fabrication, good water-solubility, and excellent thickening ability may be an attractive candidate for clean fracturing in oil/gas recovery and fluid drag reduction.

A CO2-responsive smart fluid based on supramolecular assembly structures varying reversibly from vesicles to wormlike micelles

CO2-responsive smart fluids have been widely investigated in the past decade. In this article, we reported a CO2-responsive smart fluid based on supramolecular assembly structures varying from vesicles to wormlike micelles. Firstly, oleic acid and 3-dimethylaminopropylamine reacted to form a single-chain weak cationic surfactant with a tertiary amine head group, N-[3-(dimethylamino)propyl]oleamide (NDPO). Then, 1,3-dibromopropane was used as the spacer to react with NDPO to form a gemini cationic surfactant, trimethylene α,ω-bis(oleate amide propyl dimethyl ammonium bromide) (GCS). By controlling the feed ratio of 1,3-dibromopropane and NDPO, we found that the mixtures of GCS and NDPO with the molar ratio of 7 : 3 approximately could form vesicles in aqueous solution by supramolecular self-assembly. After bubbling CO2, the tertiary amine of NDPO was protonated. The packing parameter of the mixed surfactants reduced accordingly, accompanied by the transition of aggregates from vesicles to wormlike micelles. As a result, the zero-shear viscosity of the solution increased by more than four orders in magnitude. When the solid content of GCS/NPDO mixtures was higher than 5 wt% in solution, the sample treated by CO2 behaved as a gel over a wide frequency range with shear-thinning and self-healing properties. In addition, the sol-gel transition could be repeatedly and reversibly switched by cyclically bubbling CO2 and N2. Our effort may provide a new strategy for the design of CO2-responsive smart fluids, fostering their use in a range of applications such as in enhanced oil recovery.

Viscoelastic micellar solution formed by a Se-based ionic liquid surfactant and its response to redox changes

The incorporation of a Se atom endows the surfactants with redox-sensitivity, and that its site plays a crucial role both in single surfactant solution and viscoelastic micellar solution formed by surfactant and NaSal.

Reversibly Switching Wormlike Micelles Formed by a Selenium-Containing Surfactant and Benzyl Tertiary Amine Using CO2/N2 and Redox Reaction

Multiresponsive wormlike micelles (WLMs) remain a significant challenge in the construction of smart soft materials based on surfactants. Herein, we report the preparation of a viscoelastic wormlike micellar solution based on a new redox-responsive surfactant, sodium dodecylselanylpropyl sulfate (SDSePS), and commercially available benzyl tertiary amine (BTA) in the presence of CO₂. In this system, SDSePS can be reversibly switched on (selenide) and off (selenoxide) by a redox reaction, akin to that previously reported for benzylselanyl or phenylselanyl surfactants. By alternately adding H₂O₂ and N₂H₄·H₂O, WLMs can be reversibly broken and formed because of the transformation of the hydrophilic headgroup of SDSePS, originating from the reversible formation of selenoxide. Moreover, WLMs can also be switched on and off by cyclically bubbling CO₂ and N₂ because of the variation of the binding interaction between SDSePS and BTA, resulting from the reversible protonation of BTA. This interesting and unique multiresponsive behavior makes the current WLMs a potential candidate for smart control of the "sol-gel" transition or substantial thickening of solutions.

Jetting of a shear banding fluid in rectangular ducts

Non-Newtonian fluids are susceptible to flow instabilities such as shear banding, in which the fluid may exhibit a markedly discontinuous viscosity at a critical stress. Here we report the characteristics and causes of a jetting flow instability of shear banding wormlike micelle solutions in microfluidic channels with rectangular cross sections over an intermediate volumetric flow regime. Particle-tracking methods are used to measure the three-dimensional flow field in channels of differing aspect ratios, sizes, and wall materials. When jetting occurs, it is self-contained within a portion of the channel where the flow velocity is greater than the surroundings. We observe that the instability forms in channels with aspect ratio greater than 5, and that the location of the high-velocity jet appears to be sensitive to stress localizations. Jetting is not observed in a lower concentration solution without shear banding. Simulations using the Johnson-Segalman viscoelastic model show a qualitatively similar behavior to the experimental observations and indicate that compressive normal stresses in the cross-stream directions support the development of the jetting flow. Our results show that nonuniform flow of shear thinning fluids can develop across the wide dimension in rectangular microfluidic channels, with implications for microfluidic rheometry.

Flow of wormlike micellar solutions around confined microfluidic cylinders

Wormlike micellar (WLM) solutions are frequently used in enhanced oil and gas recovery applications in porous rock beds where complex microscopic geometries result in mixed flow kinematics with strong shear and extensional components. Experiments with WLM solutions through model microfluidic porous media have revealed a variety of complex flow phenomena, including the formation of stable gel-like structures known as a Flow-Induced Structured Phase (FISP), which undoubtedly play an important role in applications of WLM fluids, but are still poorly understood. A first step in understanding flows of WLM fluids through porous media can be made by examining the flow around a single micro-scale cylinder aligned on the flow axis. Here we study flow behavior of an aqueous WLM solution consisting of cationic surfactant cetyltrimethylammonium bromide (CTAB) and a stable hydrotropic salt 3-hydroxy naphthalene-2-carboxylate (SHNC) in microfluidic devices with three different cylinder blockage ratios, β. We observe a rich sequence of flow instabilities depending on β as the Weissenberg number (Wi) is increased to large values while the Reynolds number (Re) remains low. Instabilities upstream of the cylinder are associated with high stresses in fluid that accelerates into the narrow gap between the cylinder and the channel wall; vortex growth upstream is reminiscent of that seen in microfluidic contraction geometries. Instability downstream of the cylinder is associated with stresses generated at the trailing stagnation point and the resulting flow modification in the wake, coupled with the onset of time-dependent flow upstream and the asymmetric division of flow around the cylinder.

Scanning-SAXS of microfluidic flows: nanostructural mapping of soft matter

The determination of in situ structural information of soft matter under flow is challenging, as it depends on many factors, such as temperature, concentration, confinement, channel geometry, and type of imposed flow. Here, we combine microfluidics and scanning small-angle X-ray scattering (scanning-SAXS) to create a two-dimensional spatially resolved map, which represents quantitatively the variation of molecular properties under flow. As application examples, mappings of confined amyloid fibrils and wormlike micelles under flow into various channel geometries are compared. A simple process to fabricate X-rays resistant chips, based on polyimide and UV-curing resin, is discussed. During experiments, these chips remained in high-energy synchrotron radiation for more than 24 hours, causing constant low background scattering. Thus, sufficient statistics were obtained from sample scattering at exposure times as low as 0.1 s, even with the small scattering volumes in microfluidic channels. Scanning-SAXS of microfluidic flows has many potential applications from biology to fundamental soft matter physics. In general, any fluid which has enough contrast for X-ray scattering can be measured to obtain the dependence of molecular shape, conformation, alignment and size on the flow field. Besides, dynamic processes of soft matter caused by flow, temperature, concentration gradient, and confinement, for example self-assembling, aggregation, mixing, diffusion, and disintegration of macromolecules, can be quantified and visualized on a single image by this mapping technique.

Formation and flow behavior of micellar membranes in a T-shaped microchannel

Understanding the formation and instability behavior of membranes is of fundamental interest and practical relevance to various biotechnological applications and self-assembly systems. Surfactant micellar membranes serve as a simple model system when surfactant molecules self-assemble into micellar structures under flow, but observing such process in real time is a major challenge due to limitations in spatiotemporal resolutions. We use a simple T-shaped microchannel to capture the formation and flow behavior of an ionic surfactant micro-micellar-membrane (μMM) when an aqueous stream of organic salt sodium salicylate (NaSal) meets a stream of cationic surfactant cetyltrimethylammonium bromide (CTAB). The μMM is shown to grow and become unstable depending on the flow rate, as characterized using micro-particle image velocimetry, fluorescence microscopy, flow birefringence, and bulk rheometry. We propose a simple model that accounts for the flow, elasticity and inertia of the μMM to analyze its flow behavior. Our experimental protocol can be easily replicated in conventional laboratories without the need of utilizing sophisticated equipment such as synchrotron small angle X-ray scattering and micro-electronics circuits. Our combined experimental and modeling results can be extrapolated to provide new insights to study the flow behavior and thermodynamic phases of lipid membranes, membrane proteins, and biological membranes.

Microfluidics: an enabling screening technology for enhanced oil recovery (EOR)

Oil production is a critical industrial process that affects the entire world population and any improvements in its efficiency while reducing its environmental impact are of utmost societal importance. The paper reviews recent applications of microfluidics and microtechnology to study processes of oil extraction and recovery. It shows that microfluidic devices can be useful tools in investigation and visualization of such processes used in the oil & gas industry as fluid propagation, flooding, fracturing, emulsification and many others. Critical macro-scale processes that define oil extraction and recovery are controlled by the micro-scale processes based on wetting, adhesion, surface tension, colloids and other concepts of microfluidics. A growing number of research efforts demonstrates that microfluidics is becoming, albeit slowly, an accepted methodology in this area. We propose several areas of development where implementation of microfluidics may bring about deeper understanding and hence better control over the processes of oil recovery based on fluid propagation, droplet generation, wettability control. Studies of processes such as hydraulic fracturing, sand particle propagation in porous networks, high throughput screening of chemicals (for example, emulsifiers and surfactants) in microfluidic devices that simulate oil reservoirs are proposed to improve our understanding of these complicated physico-chemical systems. We also discuss why methods of additive manufacturing (3D printing) should be evaluated for quick prototyping and modification of the three-dimensional structures replicating natural oil-bearing rock formations for studies accessible to a wider audience of researchers.

CO2/pH-Controllable Viscoelastic Nanostructured Fluid Based on Stearic Acid Soap and Bola-Type Quaternary Ammonium Salt

Fatty acid soaps such as sodium stearate (NaOSA) represent a class of cheap, environmentally friendly surfactants; however, their poor solubility seriously challenges their application in various fields. Herein, we describe a CO2/pH-controllable viscoelastic nanostructured fluid, which was developed by simple mixing of the commodity soap NaOSA with a bola-type quaternary ammonium salt (Bola2be) in a 2:1 molar ratio without the need for complex organic synthesis. The introduction of Bola2be increased NaOSA solubility and promoted micelle growth by forming a noncovalent pseudo-Gemini structure, 2NaOSA-Bola2be. Long aggregates are formed with increases in concentration, and these become entangled into a three-dimensional network at 10 times that of the critical micelle concentration (0.057 mM), showing strong thickening ability. Micellar branching occurs above 22.38 mM, as deduced by rheology and verified by cryo-transmission electron microscopy. The worm-based fluid formed from the noncovalent pseudo-Gemini surfactant is highly thermosensitive, and features a higher flow activation energy of 399.76 kJ·mol(-1) compared with common worm systems. Because of the pH-sensitivity of NaOSA, the viscoelastic fluid can respond to common pH stimuli or green CO2 gas, and shows a transition between a gel-like wormlike micellar network and a water-like dispersion with precipitate. However, the CO2-responsive behavior is irreversible.

Microfluidic-SANS: flow processing of complex fluids

Understanding and engineering the flow-response of complex and non-Newtonian fluids at a molecular level is a key challenge for their practical utilisation. Here we demonstrate the coupling of microfluidics with small angle neutron scattering (SANS). Microdevices with high neutron transmission (up to 98%), low scattering background (≲10⁻² cm⁻¹), broad solvent compatibility and high pressure tolerance (≈3-15 bar) are rapidly prototyped via frontal photo polymerisation. Scattering from single microchannels of widths down to 60 μm, with beam footprint of 500 μm diameter, was successfully obtained in the scattering vector range 0.01-0.3 Å(-1), corresponding to real space dimensions of ≃10-600 Å. We demonstrate our approach by investigating the molecular re-orientation and alignment underpinning the flow response of two model complex fluids, namely cetyl trimethylammonium chloride/pentanol/D₂O and sodium lauryl sulfate/octanol/brine lamellar systems. Finally, we assess the applicability and outlook of microfluidic-SANS for high-throughput and flow processing studies, with emphasis of soft matter.

Amphiphilic short peptide modulated wormlike micelle formation with pH and metal ion dual-responsive properties

Amphiphilic short peptides (ASPs) and surfactants (C₁₄DMAO) were employed to prepare dual-responsive wormlike micelles.

Influence of micelle properties on micellar-enhanced ultrafiltration for chromium recovery

An investigation of micelle properties on the recovery of chromium for micellar enhanced ultrafiltration (MEUF) process was conducted using cationic surfactant of cetyltrimethylammonium bromide (CTAB). The relationship between degree of ionization, micellar sizes and chromium removal were determined in this study. The results showed that the complete ionization for CTA+ and Br- was observed for CTAB lower than 0.72 mM and aggregation initiated at concentration of CTAB higher than 0.72 mM to yield attraction of counterion. The micellar sizes increased with increase in concentration of CTAB (higher than 4.02 mM) to generate micron-sized micelles. The distribution of micellar sizes was used to estimate the molecular weight cutoff of membrane used in the MEUF process. As chromium was added into aqueous CTAB solution, the chromate was dominant and bound on the micellar surface instead of Br-. Moreover, the presence of micelle formed a gel-layer to slightly shrink the membrane pore, therefore, UF membrane of 30k Da molecular weight cutoff (pore size≈7.9 nm) was selected in the MEUF process to achieve the removal efficiency of Cr(VI) higher than 95%.