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It is known that mixtures of CTAB and NaSal form viscoelastic solutions. For a 100 mM CTAB solution and increasing NaSal concentration, the viscosity increases strongly at a concentration of 10 mM NaSal, reaches a maximum at 60 mM NaSal, decreases then to a minimum at 100 mM, and, on further, increases passes over a second maximum at a NaSal concentration of 220 mM. The changes of the viscosity at the first maximum, at the minimum, and at the second maximum are due to changes of the mechanism that effect the structural relaxation times while the shear modulus remains constant. On addition of 1-propanol to the solutions at the maximum and at the minimum, the viscosities decrease strongly to the viscosity slightly above water while the shear modulus in the viscous solution remains constant. The data can be explained by assuming that part of the 1-propanol adsorbs at the surface of the three-dimensional network that is formed from wormlike micelles and makes the hydrophobic surface hydrophilic. On addition of 2-propanol to the solutions at the first maximum, the viscosity decreases also at first up to a concentration of 4% 2-propanol. On further increase of 2-propanol, a second relaxation effect with a different relaxation time and a different shear modulus appear. The sum of the two moduli remains about the same as the shear modulus of the solution without isopropanol. The second relaxation effect can be explained by assuming that only one methyl group of 2-propanol binds to the surface of the network while the other one sticks into water. The surface remains therefore hydrophobic and parts of the network can bind to each other what gives rise to the second relaxation effect.

The rheological properties and structure of aqueous solutions of wormlike micelles of a zwitterionic surfactant in the presence of an ultra-small amount of a cosurfactant with a similar structure, the charge of which depends on the pH, are studied. It is shown that the transition of the cosurfactant from the cationic form to the uncharged form leads to an increase in the viscosity, relaxation time, and elastic response of solutions by an order of magnitude. The formation of wormlike micelles in the case of charged and uncharged forms of cosurfactants is confirmed by small-angle neutron scattering. It is shown that even a small amount of charged cosurfactant leads to a decrease in the average contour length of wormlike micelles in comparison with a neutral cosurfactant. Thus, pH-sensitive viscoelastic solutions based on a zwitterionic surfactant in the presence of an ultra-small amount of a cosurfactant, the charge of which depends on the pH, are investigated.

Cyanobacteria have a wide range of impact on natural ecosystems, and have been recognized as potentially rich sources of pharmacological and structurally interesting secondary metabolites. To better understand the basic molecular processes and mechanisms that influence and regulate the growth (like length) of cyanobacteria, or connections between environment, genotype, and phenotype, it would be essential to separate shape-synchronized cyanobacterial cell populations with relatively uniform length and size. This work proposes a novel and efficient method to separate cyanobacterial Anabaena by shape (rod aspect ratio) using viscoelastic microfluidics in a straight channel with expansion-contraction cavity arrays (ECCA channel). The biocompatible viscoelastic solutions with dissolved polymer would induce a combined effect of inertial lift force, elastic force, and secondary drag force for Anabaena flowing in it. Therefore, Anabaena with different lengths reach different lateral equilibrium positions and flow out from different outlets. Factors including flow rate, fluid viscoelasticity, channel structure, and length on the shape-based cell separation were studied systematically. This work, for the first time, demonstrates continuous and sheathless shape-based separation of cyanobacteria using viscoelastic microfluidics. Moreover, its ability to manipulate objects with different morphologies and with a size of >100 μm will extend the capability of microfluidics to a completely new field that has never been reached and would be attractive across a range of new applications.

After water flooding, there is much residual oil left in the pores. This cannot be easily displaced, but polymer flooding is able to achieve this. The use of viscoelastic solutions such as polymer can enhance the oil recovery, because it can increase the sweep volume by improving the mobility ratio of polymer solution and oil. The use of viscoelastic solutions can also increase the displacement efficiency of residual oil in the swept area. This article, based on the momentum equation and Oldroyd-B constitutive model, investigates the influences of polymer solution properties on the stress distribution and deformation of residual oil adhering to the walls of micro-channels. The results show that the elasticity and viscosity of the polymer displacing fluid are the factors with the greatest impact on the stress distribution and deformation of residual oil. The stress computation shows that along the direction of flow, the oil film is compressed at both ends, and pulled in the center. This stress distribution helps the oil film to bulge from its center, leading to its deformation and break-up. Furthermore, by increasing the viscosity or elasticity the residual oil is exposed to greater stress and deformation during the displacement process, which is characterized by a large advancing angle, an increasing deformation index and a small receding angle. Comparison of the simulation and experimental results shows a good conformance. Therefore, to displace the oil more efficiently, the polymer solutions need to be more viscous and elastic, which can be achieved by changing their chemical properties. The results clarify the micro-mechanism of polymers displacing residual oil and quantify the influences of the chemical properties of the solution on the stress, deformation and activation of residual oil.

Viscoelastic liquid transfer from one surface to another is a process that finds applications in many technologies, primarily in printing. Here, cylindrical shaped capillary bridges pinned between two parallel disks are considered. Specifically, the effects of polymer mass fraction, solution viscosity, disk diameter, initial aspect ratio, final aspect ratio, stretching velocity and filling fraction (alike contact angle) are experimentally investigated in uniaxial extensional flow. Both Newtonian and viscoelastic polymer solutions are prepared using polyethylene glycol (PEG) and polyethylene oxide (PEO), with a wide variety of mass fractions. The results show that the increase in polymer mass fraction and solvent viscosity reduces the liquid transfer to the top surface. Moreover, the increase in the initial and final stretching height of the capillary bridge also decreases the liquid transfer, for both Newtonian and viscoelastic solutions. Finally, the shape of the capillary bridge is varied by changing the liquid volume. Now, Newtonian and viscoelastic solutions exhibit opposite behaviors for the liquid transfer. These findings are discussed in terms of interfacial shape instability and gravitational drainage., 11 pages, 5 figures

The phase behavior and rheological properties of the mixtures of wormlike micelles of the cationic surfactant erucyl-bis(hydroxyethyl) methyl ammonium chloride and oppositely charged stiff-chain polyelectrolyte xanthan are studied. It is shown that, in the presence of an excess of a low molecular weight salt, which suppresses complex formation between the surfactant and the polymer, mixing allows a synergistic increase in the elastic modulus of the system to be achieved, which may be due to the formation of entanglements between polymer chains and wormlike micelles.

It is of great significance to explore novel applications of renewable resources. In this study, a rosin-based anionic surfactant (abbreviated R-11-2-Na), which contains a large hydrophobic group of 30 carbon atoms, was synthesized. R-11-2-Na forms wormlike micelles in the presence of the equimolar organic salt choline chloride, endowing solutions with strong viscoelasticity. The wormlike micellar solutions were investigated using rheology, small-angle X-ray scattering, and freeze-fracture transmission electron microscopy (FF-TEM) methods at 25 °C. Due to the strong van der Waals interactions caused by the large hydrophobic group contained in R-11-2-Na, the zero-shear viscosity (η0) of solutions showed extremely strong dependence on the concentration with an exponent of 23.4. The cross-sectional diameter of the wormlike micelles in the present system was significantly larger than that of the wormlike micelles formed by surfactants containing conventional alkyl tails. This finding may be attributed to the steric hindrance brought by the bulky and rigid dehydroabietic acid unit in the hydrophobic part. The wormlike micelles also showed high tolerance to the organic salt concentration. The present study reveals the notable qualities of rosin-based derivatives in forming complex fluids and facilitates new utilizations of forest resources.

The phase behavior and aggregates induced by changes in pH in a sodium N-tetradecyl-maleimidepimaric carboxylate (C14-MPA-Na)/cetyltrimethylammonium bromide (CTAB) system have been investigated using rheology and cryogenic transmission electron microscopy (cryo-TEM). Viscoelastic solutions at a pH between 6.40 and 1.41, opalescent blue solutions at a pH between 1.41 and 1.32 and aqueous surfactant two-phase systems (ASTPs) at a pH between 1.32 and 0.56 are formed sequentially. The microstructures in the viscoelastic solutions are wormlike micelles that partially transform to vesicles in the opalescent blue solutions. The aggregates in the upper phase of the ASTPs are vesicles and lamellar micelles, and in the lower phase are spherical micelles. Further, Single-walled carbon nanotubes (SWNTs) were successfully hybridized into the wormlike micellar viscoelastic solutions. cryo-TEM, UV–Vis-IR, near-infrared photoluminescence (NIR-PL) and UV–Vis spectroscopy were used to study the dispersion of SWNTs in the viscoelastic solutions, and the results showed SWNTs can remain dispersed for more than 6 months at pH 6.40. The results obtained from rheology showed that the hybridization of SWNTs will not affect the aggregates and viscoelasticity compared that to the native viscoelastic solution. Above more, the SWNTs could be recycled by the pH-switched phase behaviors between viscoelastic solutions and ASTPs.

Tissue biopsies and implant analysis during animal testing or clinical studies are a requirement for development of new surgical materials and procedures. In this paper we report the use of vibrational OCT (VOCT) to evaluate the viscoelastic behavior of tissues, polymeric materials, biofilms, and viscoelastic solutions of macromolecules. Our results suggest that VOCT is a useful technique to characterize the behavior of cellular tissues and biofilms, polymeric implant materials and viscoelastic solutions used in medicine. It is demonstrated that the modulus and resonant frequency squared per unit thickness is a feature that can be used to characterize a variety of tissues. Further work is needed to understand the generalized behavior of synthetic polymers and viscoelastic solutions.

Viscoelastic polymer solutions have been widely employed as suspending liquids for a myriad of microfluidic applications including particle and cell focusing, sorting, and encapsulation. It has been recently shown that viscoelastic solutions can drive the formation of equally spaced particles called "particle trains" as a result of the viscoelasticity-mediated hydrodynamic interactions between adjacent particles. Despite their potential impact on applications such as droplet encapsulation and flow cytometry, only limited experimental studies on viscoelastic ordering are currently available. In this work, we demonstrate that a viscoelastic shear-thinning aqueous xanthan gum solution drives the self-assembly of particle trains on the centerline of a serpentine microfluidic device with a nearly circular cross section. After focusing, the flowing particles change their mutual distance and organize in aligned structures characterized by a preferential spacing, quantified in terms of distributions of the interparticle distance. We observe the occurrence of multi-particle strings, mainly doublets and triplets, that interrupt the continuity of the particle train. To account for the fluctuations in the number of flowing particles in the experimental window, we introduce the concept of local particle concentration, observing that an increase of the local particle concentration leads to an increase of doublets and triplets. We also demonstrate that using only a single tube to connect the sample to the microfluidic device results in a drastic reduction of doublets/triplets, thus leading to a more uniform particle train. Our findings establish the foundation for optimized applications such as deterministic droplet encapsulation in viscoelastic liquids and optimized flow cytometry.

International audience; This paper presents an experimental work on the determination of viscoelastic solution properties from the analysis of their behavior when subject to the capillary instability. Known as the jet rheometer method, such an approach is reserved for low-viscous viscoelastic solutions for which commercial techniques are unsuitable. In the present work, the injection conditions are free and cover a wide range of operating points. The analysis is statistical in nature and uses a multi-scale description tool that allows access to all dynamics involved and, in particular, to those of small characteristic scales. The results are numerous and evidence the presence of polymer mechanical degradation due to the injection. The influence of the operating conditions on this mechanism is highlighted.

This thesis deals with uniaxial stretching of capillary bridges of polymer solutions. It consists of two experimental parts investigating drop dynamics and liquid transfer. The stretching of viscoelastic polymer solutions can create the drops-on-a-filament. Solutions are prepared with a wide range of concentrations. The stretching is quantified using diameter-space-time diagrams, which demonstrate hierarchy, asymmetry, migration, oscillation, and merging of drops. Also, position of minimum diameter on the filament is determined, along with number, positions and diameters of the drops. The maximum number of drops can be predicted using the characteristic relaxation time. The extracted minimum diameter is used to calculate extensional viscosity. Then, the viscoelastic liquid transfer for cylindrical-shaped capillary bridges, pinned on circular parallel plates is studied. Specifically, the effects of polymer concentration, solvent viscosity, plate diameter, initial and final stretching height, stretching speed, and contact angle are investigated. With the increase in polymer concentration and solvent viscosity, the liquid transfer to the top plate significantly reduces. The increase in the initial and final stretching height of the capillary bridge decreases the final liquid transfer to the top plate. The shape of the initial capillary bridge is studied by varying liquid volume introduced, and for the Newtonian and viscoelastic solutions, an opposite liquid transfer behaviour is noticed. Finally, numerical simulations for a liquid capillary bridge are reported and show formation of filament and a big central drop having size similar to the experimental value., Cette thèse traite de l'étirement uniaxial de ponts capillaires de solutions de polymères. Celle-ci se compose de deux parties expérimentales étudiant la dynamique des gouttes et le transfert du liquide. L'étirement des solutions de polymères viscoélastiques peut créer des gouttes-sur-le-filament. Les solutions sont préparées pour une large gamme de concentrations. L'étirement est quantifié à l'aide de diagrammes diamètre-espace-temps, qui démontrent la hiérarchie, l'asymétrie, la migration, l'oscillation et la fusion des gouttes. La position du diamètre minimum sur le filament est déterminée, ainsi que le nombre, les positions et les diamètres des gouttes. Le nombre maximum de gouttes peut être prédit en utilisant le temps de relaxation. Le diamètre minimum est utilisé pour calculer la viscosité élongationnelle. Puis, le transfert de liquide viscoélastique pour des ponts capillaires de forme cylindrique, entre deux plaques circulaires parallèles est étudié. En particulier, les effets de la concentration, de la viscosité du solvant, du diamètre de la plaque, des hauteurs initiale et finale, de la vitesse et de l'angle de contact sont présentés. Avec l'augmentation de la concentration et de la viscosité du solvant, le transfert vers la plaque supérieure diminue. L'augmentation de la hauteur d'étirement initiale et finale du pont capillaire diminue le transfert de liquide. La forme du pont capillaire initial est variée, et on constate un comportement opposé du transfert pour les solutions newtonienne et viscoélastique. Enfin, des simulations numériques sont présentées et montrent la formation de filament et une grosse goutte centrale de taille similaire aux expériences.

When pushed out of a syringe, polymer solutions form droplets attached by long and slender cylindrical filaments whose diameter decreases exponentially with time before eventually breaking. In the last stages of this process, a striking feature is the self-similarity of the solution shape near the end of the filament. This means that shapes at different times, if properly rescaled, collapse onto one universal shape. A theoretical description inspired by this similarity observation and based on the Oldroyd-B model was recently shown to disagree with existing experimental results. By revisiting these measurements and analysing the interface profiles of very diluted polyethylene oxide solutions at high temporal and spatial resolution, we show that they are very well described by the model., Comment: 9 pages, 5 figures

The study of viscoelastic fluids as artificial tears dates back to the late 1970s. Healon, the first ophthalmic viscosurgical device, was approved in 1980, but studied extensively before then, exhibits very interesting shear-thinning properties that were found to be beneficial in both ophthalmic surgery and somewhat later as a tear replacement solution. Unlike the previous tear film replacements, which were mainly viscous in nature, viscoelastic solutions, particularly those based on hyaluronan, exhibited very interesting, potentially beneficial, rheological properties, especially when slightly altered to become elastoviscous. This review examines the rheological properties that are significant in artificial tear solutions. We define herein the necessary parameters that need to be further studied to design and formulate rheologically better artificial tears, which should provide enhanced efficacy compared with their predecessors.

We propose and demonstrate an on-chip optofluidic device allowing active oscillatory microrheological measurements with sub-μL sample volume, low cost and high flexibility. Thanks to the use of this optofluidic microrheometer it is possible to measure the viscoelastic properties of complex fluids in the frequency range 0.01–10 Hz at different temperatures. The system is based on the optical forces exerted on a microbead by two counterpropagating infrared laser beams. The core elements of the optical part, integrated waveguides and an optical modulator, are fabricated by fs-laser writing on a glass substrate. The system performance is validated by measuring viscoelastic solutions of aqueous worm-like micelles composed by Cetylpyridinium Chloride (CPyCl) and Sodium Salicylate (NaSal).

Microfluidic rheometry is considered to be a potential alternative to conventional rheometry for the rheological characterization of viscoelastic solutions having relatively low viscoelastic properties. None of the microfluidic platforms introduced so far, however, can be used for the measurements of multiple rheological properties in the same device. In this work, I present the first microfluidic platform, named the “μ-rheometer,” which allows for the simultaneous measurement of zero-shear viscosity η0 and longest shear relaxation time λ. This is achieved by transforming the original “flow rate controlled” platform presented by Del Giudice et al., “Rheometry-on-a-chip: Measuring the relaxation time of a viscoelastic liquid through particle migration in microchannel flows,” Lab Chip 15, 783–792 (2015) into a “pressure drop controlled” microfluidic device, by replacing a syringe pump with a pressure pump. The novel device has been tested by measuring both η0 and λ for a number of polyethylene oxide solutions in glycerol–water 25 wt. % and pure water, respectively. Its effectiveness has been corroborated by means of a direct comparison with a conventional rotational rheometer.

For the first time, the responsiveness of mixed micelles of cationic and anionic surfactants to hydrocarbons was investigated. The micelles were prepared at different molar ratios of potassium oleate and n-octyltrimethylammonium bromide in salt-free water. It was shown that the viscoelastic solutions of the mixed micelles demonstrate a tremendous drop of viscosity by 4–6 orders of magnitude upon addition of a tiny amount of n-decane, which is due to the transformation of wormlike micelles into microemulsion droplets as was evidenced by SANS. Comparison of linear and branched micelles shows that unexpectedly, much larger amount of hydrocarbon was needed to induce the disruption of the network of branched micelles suggesting preferential solubilization of hydrocarbon inside the branching points. According to SANS data, at the excess of hydrocarbon, in both systems, the ellipsoid emulsion droplets are formed, which can be due to irregular distribution of the two surfactants on their surface.

Aqueous solutions (∼0.01–0.2%) of long chain polymers are used during gravity thickening to aggregate particle suspensions. These polymers are often dosed into turbulent flow through submerged pipes (sparges) to maximise their distribution. At high concentrations, these viscoelastic solutions mix poorly with surrounding flows unless highly diluted. If varying the bluff body sparge shape can enhance mixing, such dilution may be avoided. Turbulent mixing of concentrated (0.2%) polymer solutions sparged into turbulent flows within a transparent channel was captured using high-speed imaging to understand the complex mixing phenomena. Square and round sparges were investigated at 0 or 180° to the channel flow. At 0°, vortex shedding from square sparge edges enhanced mixing, but the 180° orientation produced better overall mixing. Image analysis gave the first quantification of flow past bluff bodies issuing viscoelastic fluid jets at industrial flow regimes, from which more appropriate turbulence model closures for fluid dynamics modelling of the process may be derived.

The phase behavior and aggregates of a catanionic system have been investigated. The anionic surfactants in the mixed systems were sodium N-alkylmaleimidepimaric carboxylate (Cn -MPA-Na, n=12, 14, 16), which were prepared from rosin and contain a large rigid skeleton and a flexible alkane chain, and the cationic surfactant was cetyltrimethylammonium bromide (CTAB). The phase behavior of the C14 -MPA-Na/CTAB system transformed sequentially from a viscoelastic solution to an aqueous surfactant two-phase system (ASTP), an aqueous surfactant three-phase system (AS3P), and an anisotropic homogeneous phase as the concentration of C14 -MPA-Na was continuously increased from 10 to 35 mm. The C16 -MPA-Na/CTAB system showed similar phase behavior, whereas the C12 -MPA-Na/CTAB system did not form the AS3P system. The corresponding microstructures in the different phases were investigated by using rheology and cryogenic transmission electron microscopy (Cryo-TEM). The aggregates in the viscoelastic solutions are thread-like, annular, and worm-like micelles. The microstructures in the upper phase of the ASTP are worm-like micelles, and in the lower phase are spherical and rod-like micelles. The aggregates in the upper and lower phases of the AS3P are worm-like micelles and spherical and rod-like micelles, respectively. The aggregates in the middle phase of the AS3P and the anisotropic homogeneous phase are sponge-like micelles. The clear Cryo-TEM images of the sponge-like micelles are presented.

This research study presents analytical solutions for the stresses and displacements around deeply buried non-circular tunnels, taking into account the viscoelasticity of the ground, and the sequential excavation of the tunnels’ cross-sections. General initial far-field stress states are assumed, and the time-dependent pressures exerted at the internal tunnel boundaries are found to account for the support effects or water pressures of the hydraulic tunnels. Then, solutions are derived for tunnels with a time-varying sizes and/or shape, by assuming the time-dependent functions specified by the designers. The analytical solutions for the stresses and displacements around elliptical and square tunnels are specifically presented for linearly viscoelastic models using a Muskhelishvili complex variable method and Laplace transform techniques. For validation purposes, numerical analyses are performed for the excavations of elliptical and square tunnels in rock which are simulated by Poynting–Thomson or generalized Kelvin viscoelastic models. Good agreements are observed between the analytical and numerical results of this study. Then, parametric analyses are carried out in order to investigate the effects of the far-field shear stress, along with the distribution forms of the internal pressures, on the ground displacements and stresses. The proposed analytical solutions can be employed to accurately predict the stress concentrations, as well as the time-dependent displacements around deeply buried elliptical or square-shaped tunnels. Furthermore, it is confirmed that this study’s described methodology may be potentially applied to obtain analytical solutions for other arbitrary shaped tunnels sequentially excavated in viscoelastic rock.

Viscoelastic solutions are notoriously sensitive to temperature and ionic strength. In order to be applicable for use in oil reservoirs, they need to be resilient to higher temperatures as well as to saline content. We define the essential characteristics required. Refractory properties obtained under Couette testing do not necessarily provide the same performance under pressure-driven flow. Nonetheless, it is possible to formulate solutions which clearly indicate that subsurface application is practicable. We show examples where salinity enables significantly enhanced viscoelasticity above ambient temperatures.

Polymer solutions are considered dilute when polymer chains in a solution do not interact with each other. One important step in the characterization of these systems is the measurement of their longest relaxation times λ. For dilute polymer solutions in low-viscous solvents, this measurement can be very challenging through conventional techniques. Recently, several microfluidic platforms have been successfully employed to measure the rheological properties of weakly viscoelastic solutions. Nevertheless, a comparison between data generated from different microfluidic platforms has not yet been presented. In this work, we measure λ of dilute polymer solutions for concentrations down to a few parts per million, by using two distinct microfluidic platforms with shear and extensional flow configurations. We consider three representative polymer classes: Neutral polymers in near-theta and good solvents, and a biological polyelectrolyte in a good solvent in the presence of salt. Relaxation times in shear flow λshear are measured through the μ-rheometer based on the viscoelastic alignment of particles in a straight microchannel. Relaxation times in extensional flow λext are measured in a microfluidic optimized cross-slot configuration based on the onset of the flow-induced birefringence. A good agreement between experimental measurements from the two platforms is found. Experimental measures are also compared with available theories.

Viscoelastic microfluidics becomes an efficient and label-free hydrodynamic technology to enrich and separate micrometer-scale particles, including blood cells, circulating tumor cells, and bacteria. However, the manipulation of nanoscale particles by viscoelastic microfluidics remains a major challenge, because the viscoelastic force acting on the smaller particle decreases dramatically. In contrast to the commonly used polymer solutions of high molecular weight, herein we utilize the aqueous solutions of poly(ethylene oxide) (PEO) of low molecular weight with minimized shear thinning but sufficient elastic force for high-quality focusing and separation of various nanoparticles. The focusing efficiencies of 100 nm polystyrene (PS) nanoparticles and λ-DNA molecules are 84% and 85%, respectively, in a double spiral microchannel, without the aid of sheath flows. Furthermore, we demonstrate the size-based viscoelastic separation of two sets of binary mixtures-100/2000 nm PS particles and λ-DNA molecules/blood platelets-all achieving separation efficiencies of >95% in the same device. Our proposal technique would be a promising approach for enrichment/separation of the nanoparticles encountered in applications of analytical chemistry and nanotechnology.

In this study, the fluid hammer of viscoelastic flow in pipes is studied numerically. Here, the Oldroyd-B model is used as the constitutive equation. This model is suitable for dilute viscoelastic solutions and Boger liquids. The numerical solution is obtained using a two-step variant of the Lax–Friedrichs (LxF) method. The derived equations are non-dimensionalized, and the effect of Deborah and Reynolds numbers and the viscosity ratio on the behavior of viscoelastic flow during fluid hammer caused by the sudden closure of a downstream valve in a reservoir-pipe-valve system is investigated in detail. Present results show that the attenuation of the laminar fluid transient is affected by viscoelastic properties of the non-Newtonian fluid. Moreover, the results indicate that the shear stress in viscoelastic fluid hammer phenomena is significantly lower than those in Newtonian fluid with similar viscosity.

Label-free separation of particles by an intrinsic property can be implemented in microfluidic devices through either an externally imposed field or an inherent flow-induced force. Among the latter type of passive techniques, elastic or elasto-inertial lift-based particle separation in non-Newtonian fluids has received a rapidly growing interest in the past decade. However, current demonstrations of particle separation in non-Newtonian fluids have all taken place in viscoelastic polymer or biological solutions. We demonstrate for the first time a continuous sheath-free separation of polystyrene particles in the flow of weakly elastic xanthan gum (XG) solution through a simple straight rectangular microchannel. This separation is fundamentally different from that in the flow of viscoelastic solutions. We explain the observed particle migrations in XG solutions using the competition of a strong wall-directed (because of the strong shear thinning effect) and a small center-directed (because of the weak elasticity effect) lateral force induced by normal stresses in a Poiseuille flow.

Many cationic surfactants form highly viscoelastic solutions at concentrations of only a few percent. These solutions contain wormlike micelles (WLM) which can be several micrometers long. The structural relaxation time, τs in these solutions can be as long as many seconds, and the zero-shear viscosity can be in the range of 106 Pas. Electric birefringence measurements on such solutions showed four different relaxation times with increasing concentration. The two shortest ones, τ1 and τ2 in the μs region were due to the alignment of small rodlike or ringlike micelles. The third one, τ3, was observed in the viscoelastic region in the ms region and finally, a fourth one, τ4, which was the same as the structural relaxation time, τs. In this article, it is shown that the τ3 process is due to the formation and opening of contacts between the WLM. The reason for the contacts is the attraction between the WLM which is due to the hydrophobic surfaces of the micelles. The contacts crosslink the WLM and thus form a three-dimensional network. This network is the reason for the high viscosity and viscoelastic properties of the samples. If the attraction between the WLM is reduced by adding glycerol to the solution, the viscosity of the solution breaks down. The same happens if the surface of the WLM is made hydrophilic by the addition of small amphiphilic molecules. The WLM are not destroyed by these procedures.

2nd International Conference of Microfluidics, Nanofluidics and Lab-on-a-Chip -- JUN 08-10, 2018 -- Beijing, PEOPLES R CHINA Serhatlioglu, Murat/0000-0003-2083-6756; Elbuken, Caglar/0000-0001-8359-6871; GULER, MUSTAFA TAHSIN/0000-0002-0478-3183 WOS: 000461091000010 PubMed: 30632175 Elastic nature of the viscoelastic fluids induces lateral migration of particles into a single streamline and can be used by microfluidic based flow cytometry devices. In this study, we investigated focusing efficiency of polyethylene oxide based viscoelastic solutions at varying ionic concentration to demonstrate their use in impedimetric particle characterization systems. Rheological properties of the viscoelastic fluid and particle focusing performance are not affected by ionic concentration. We investigated the viscoelastic focusing dynamics using polystyrene (PS) beads and human red blood cells (RBCs) suspended in the viscoelastic fluid. Elasto-inertial focusing of PS beads was achieved with the combination of inertial and viscoelastic effects. RBCs were aligned along the channel centerline in parachute shape which yielded consistent impedimetric signals. We compared our impedance-based microfluidic flow cytometry results for RBCs and PS beads by analyzing particle transit time and peak amplitude at varying viscoelastic focusing conditions obtained at different flow rates. We showed that single orientation, single train focusing of nonspherical RBCs can be achieved with polyethylene oxide based viscoelastic solution that has been shown to be a good candidate as a carrier fluid for impedance cytometry. Scientific and Technological Research Council of Turkey (TUBITAK)Turkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) [215E086] The authors thank to Zeynep Erdogan, Ziya Isiksacan for contributions during rheometer measurements and preparation of the manuscript. The authors acknowledge support from Kutay Icoz, Urartu S. Seker, Recep Ahan and Elif D. Ergul for biological experiments. The detection system was acquired by financial support from the Scientific and Technological Research Council of Turkey (TUBITAK, Project No. 215E086).

Surfactants are used as organogelators, which form surfactant-based gels in nonpolar solvents. One of the well-known surfactants that form organogels is lecithin, which self-assembles into reverse spherical micelles in nonpolar solvents. When inorganic salts are added to lecithin solutions, the reverse spherical micelles transform into long reverse cylindrical micelles, giving rise to organogels. Although previous studies have shown that the addition of inorganic salts to lecithin solutions can induce the gel formation, the solvent effects on the gel formation have not been mapped out in detail. In this study, we systematically investigated the effects of hydrocarbon chain length of alkanes on the self-assembly of lecithin and monovalent ion mixtures. Our findings showed that LiCl, LiBr, LiI, NaBr, NaI, and KI salts formed organogels in the presence of lecithin, whereas NaCl and KBr formed viscoelastic solutions with lecithin. More importantly, with an increase in the hydrocarbon chain length of alkanes, the lecithin/salt mixtures formed gels more efficiently. The gel formation is closely related to the length of the reverse cylindrical micelles, as confirmed by small-angle X-ray scattering analysis. In addition, the length of the cylinders is affected by the interactions between the lecithin head groups and ions, as evidenced by Fourier transform infrared spectroscopy.

Viscoelastic solutions formed in the mixed aqueous solutions of 3-tetradecyloxy-2-hydroxypropyltrimethylammonium bromide (R14HTAB) and aromatic salts such as sodium salicylate (NaSal), sodium 1-hydroxynaphthalene-2-carboxylate (1SHNC), and sodium 2-hydroxynaphthalene-3-carboxylate (2SHNC) were systematically studied by steady and dynamic shear rheology in terms of concentration and temperature. In the absence of a salt, R14HTAB only produced spherical or short cylindrical micelles within a range of concentrations of 100–400 mmol kg–1. The addition of aromatic salts induced one-dimensional growth of micelles generating wormlike micelles. Zero-shear viscosity of the solutions shows viscosity maxima behavior in the examined range of salt concentration, where the strongest and the most stable network structures were formed. The changes in the viscoelastic behavior are a result of variation of the structural relaxation time, indicating that the flow behavior is primarily controlled by micellar kinetics. The mi...

The formation of unilamellar vesicles was successfully established by the addition of hydrotropic salt p-toluidine hydrochloride (PTHC) to solutions of the anionic surfactant sodium dodecyl sulfate (SDS) at high salt concentrations ([PTHC]/[SDS], xPTHC > 0.6). Further studies on the existing vesicles were conducted in terms of changes of concentrations or temperatures of the aqueous solutions. Upon dilution or heating these vesicles can transform into long, flexible wormlike micelles (WLMs). In this process, the solutions switch from an aqueous solution of bluish and nearly Newtonian liquids with low viscosity to clear and viscoelastic solutions with an ability to trap bubbles, which was called “dilution-thickening” or “thermo-thickening”. It should be noted that a microscopic phase separation always emerges at a narrow range of concentrations or temperatures during the transition. Rheological techniques, laser light scattering (LLS), transmission electron microscopy (TEM), micro-differential scanning calorimetry (micro-DSC), and transmittance measurements were used to confirm the formation of vesicles and their reversible transformation with WLMs. Finally, a tentative mechanism for the reversible vesicle-to-WLM transition was proposed to explain the results.

Viscoelasticity in fluids is present in many common substances, from paints and glues to ketchup and mayonnaise. The rheological behavior of fluids can be accurately studied with laboratory equipment and encompasses an active area of research. However, instead of precisely analyzing the viscoelastic parameters, some applications simply require the detection of presence of viscoelastic behavior in fluid. This is desirable in online biofilm monitoring, for instance, where it is important to discern inorganic accumulation (shown as purely Newtonian behavior) from biofilms (viscoelastic behavior). Therefore, this study presents a simple method for potentially detecting the presence of viscoelastic behavior in a fluidic sample. Two sample solutions were compared: glycerol as the Newtonian fluid and a solution of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal) as the viscoelastic fluid. They were compared by observing the rotational response of a magnetic particle in a rotating magnetic field, where the presence of underdamped oscillations was attributed to the viscoelasticity of the fluid. According to these preliminary results, the setup may be used to detect the presence of viscoelastic behavior in the fluid.

Many practically relevant materials combine properties of viscous fluids and elastic solids to viscoelastic behavior. Our focus is on the induced dynamic behavior of damped finite-sized particulate inclusions in such substances. We explicitly describe history-dependent interactions that emerge between the embedded particles. These interactions are mediated by the viscoelastic surroundings. They result from the flows and distortions of the viscoelastic medium when induced by the rigid inclusions. Both, viscoelastic environments of terminal fluid-like flow or of completely reversible damped elastic behavior, are covered. For illustration and to highlight the role of the formalism in potential applications, we briefly address the relevant examples of dragging a rigid sphere through a viscoelastic environment together with subsequent relaxation dynamics, the switching dynamics of magnetic fillers in elastic gel matrices, and the swimming behavior of active microswimmers in viscoelastic solutions. The approach provides a basis for more quantitative and extended investigations of these and related systems in the future., 12 pages, 7 figures

Characterization of viscoelastic materials from a mechanical point of view is often performed via dynamic mechanical analysis (DMA), consisting in the arousal of a steady-state undulated response in a uniaxial bar specimen, allowing for the experimental measurement of the so-called complex modulus, assessing both the elastic energy storage and the internal energy dissipation in the viscoelastic material. The existing theoretical investigations of the complex modulus’ influence on the contact behavior feature severe limitations due to the employed contact solution inferring a nondecreasing contact radius during the loading program. In case of a harmonic cyclic load, this assumption is verified only if the oscillation indentation depth is negligible compared to that due to the step load. This limitation is released in the present numerical model, which is capable of contact simulation under arbitrary loading profiles, irregular contact geometry, and complicated rheological models of linear viscoelastic materials, featuring more than one relaxation time. The classical method of deriving viscoelastic solutions for the problems of stress analysis, based on the elastic-viscoelastic correspondence principle, is applied here to derive the displacement response of the viscoelastic material under an arbitrary distribution of surface tractions. The latter solution is further used to construct a sequence of contact problems with boundary conditions that match the ones of the original viscoelastic contact problem at specific time intervals, assuring accurate reproduction of the contact process history. The developed computer code is validated against classical contact solutions for universal rheological models and then employed in the simulation of a harmonic cyclic indentation of a polymethyl methacrylate half-space by a rigid sphere. The contact process stabilization after the first cycles is demonstrated and the energy loss per cycle is calculated under an extended spectrum of harmonic load frequencies, highlighting the frequency for which the internal energy dissipation reaches its maximum.

The structure of micellar solutions has been a topic of intense research. Of particular interest is the relation between rheological properties and the nanostructure of the corresponding micelles. At compositions surrounding the 2nd CMC, the onset of micellar growth, spherical micelles typically transition into short rodlike micelles, and these assemblies continue to grow into long, entangled threadlike micelles, avoiding micellar ends. Another possibility to avoid unfavorable micellar ends is to form small closed structures—rings and loops. Here, we disclose with cryo-TEM the formation of these assemblies near the 2nd CMC, for a perfluorosurfactant C 8 F 17 SO 3 N(C 2 H 5 ) 4 , and we correlate the observations with electric birefringence (EB) data that show 4 separate processes near that critical composition.

Microflow cytometry is a powerful technique for characterization of particles suspended in a solution. In this work, we present a microflow cytometer based on viscoelastic focusing. 3D single-line focusing of microparticles was achieved in a straight capillary using viscoelastic focusing which alleviated the need for sheath flow or any other actuation mechanism. Optical detection was performed by fiber coupled light source and photodetectors. Using this system, we present the detection of microparticles suspended in three different viscoelastic solutions. The rheological properties of the solutions were measured and used to assess the focusing performance both analytically and numerically. The results were verified experimentally, and it has been shown that polyethlyene oxide (PEO) and hyaluronic acid (HA) based sheathless microflow cytometer demonstrates similar performance to state-of-the art flow cytometers. The sheathless microflow cytometer was shown to present 780 particles/s throughput and 5.8% CV for the forward scatter signal for HA-based focusing. The presented system is composed of a single capillary to accommodate the fluid and optical fibers to couple the light to the fluid of interest. Thanks to its simplicity, the system has the potential to widen the applicability of microflow cytometers.

WOS:000403628200001; International audience; A multi-scale analyzing tool is now available to investigate the temporal evolution of two phase flows such as liquid systems experiencing an atomization process. Thanks to its multi-scale and global nature, it allows identifying all dynamics simultaneously involved in the process with no restriction of the liquid system shape. In the present work this multi-scale tool is applied on 2D visualizations of free falling jets of a low-viscosity viscoelastic solution. The jets are produced from a cylindrical discharge orifice and the liquid is a very dilute polymer solution containing 5 ppm of Poly(ethylene oxide). High spatial resolution images of the free falling jets are performed as a function of the velocity and at several distances from the discharge orifice. For every operating condition, the liquid jet remains cylindrical first, then shows the development of a sinusoidal perturbation and finally adopts a beads-on-a-string pattern before breakup occurs. The multi-scale analysis is performed on a high number of images and at several spatial positions in order to return statistical and temporal information, respectively. The results of this analysis show that during the sinusoidal perturbation stage, the large-scale region follows an exponential increase as predicted by the linear stability theory and during the beads-on-a-string stage, the small-scale region follows an exponential decrease similar to an elasto-capillary regime from which the relaxation time of the polymer solution can be extracted. This work positions the multi-scale approach as a promising and complementary tool to the currently used techniques in order to probe complex liquid rheology, especially in the case of mobile viscoelastic solutions. (C) 2017 Elsevier B.V. All rights reserved.