Capillary Breakup Extensional Rheometer Research Articles

Understanding the Dynamics of Cellulose Dissolved in an Ionic Liquid Solvent Under Shear and Extensional Flows.

Ionic liquids (ILs) hold great potential as solvents to dissolve, recycle, and regenerate cellulosic fabrics, but the dissolved cellulose material system requires greater study in conditions relevant to fiber spinning processes, especially characterization of nonlinear shear and extensional flows. To address this gap, we aimed to disentangle the effects of the temperature, cellulose concentration, and degree of polymerization (DOP) on the shear and extensional flows of cellulose dissolved in an IL. We have studied the behavior of cellulose from two sources, fabric and filter paper, dissolved in 1-ethyl-3-methylimidazolium acetate ([C2C1Im][OAc]) over a range of temperatures (25 to 80 °C) and concentrations (up to 4%) that cover both semidilute and entangled regimes. The linear viscoelastic (LVE) response was measured using small-amplitude oscillatory shear techniques, and the results were unified by reducing the temperature, concentration, and DOP onto a single master curve using time superposition techniques. The shear rheological data were further fitted to a fractional Maxwell liquid (FML) model and were found to satisfy the Cox-Merz rule within the measurement range. Meanwhile, the material response in the non-LVE (NLVE) regime at large strains and strain rates has special relevance for spinning processes. We quantified the NLVE behavior using steady shear flow tests alongside uniaxial extension using a customized capillary breakup extensional rheometer. The results for both shear and extensional NLVE responses were described by the Rolie-Poly model to account for flow-dependent relaxation times and nonmonotonic viscosity evolution with strain rates in an extensional flow, which primarily arise from complex polymer interactions at high concentrations. The physically interpretable model fitting parameters were further compared to describe differences in material response to different flow types at varying temperatures, concentrations, and DOP. Finally, the fitting parameters from the FML and Rolie-Poly models were connected under the same superposition framework to provide a comprehensive description within the wide measured parameter window for the flow and handling of cellulose in [C2C1Im][OAc] in both linear and nonlinear regimes.

Synergistic Behavior of Anionic Surfactants and Hydrolyzed Polyacrylamide under an Extensional Field: Effect of Hydrophobicity.

Surfactant-polymer interaction has been studied by many academic and industrial researchers. Associative polymers have attracted attention, especially in enhanced oil recovery due to their ability to generate higher resistance than parental polyacrylamide (HPAM) at a lower concentration. The effect of hydrophobicity on the associative polymer-surfactant interaction has been studied through many means including rheology. Previous rheological studies were restricted to shear-based behavior, and no efforts were undertaken to study the effect of hydrophobicity on the extensional rheological behavior of the surfactant-HPAM system. In this work, the extensional behavior of anionic surfactant-polyacrylamide systems was studied for varying levels of hydrophobicity. The concentration of the surfactant used in the surfactant-polymer formulation ranged from 0 to 0.3%, and the polymer concentration was fixed at 1000 ppm. Extensional rheology was performed using a capillary breakup extensional rheometer. Surface tension studies were also conducted. The results revealed that the parental HPAM-surfactant system shows the maximum extensional viscosity for the concentration range studied here. This is contrary to shear behavior reported in the literature, and it appears that electrostatic repulsive interaction associated with HPAM-surfactant systems becomes dominant in the extensional field. Associative polymer-surfactant systems characterized by higher hydrophobicity showed the least maximum extensional viscosity, as opposed to the literature-reported behavior in the shear field. Hydrophobic interaction associated with associative polymer-surfactant systems appears to become weaker in the extensional field.

Instrumental characteristics from extensional rheology and tribology of polysaccharide solutions.

Instrumental characteristics from extensional rheology and tribology for aqueous xanthan gum (XG) and locust bean gum (LBG) solutions were studied in the presence or absence of simulated saliva. Extensional viscosity was calculated from the filament shrinkage behavior using a capillary breakup extensional rheometer, whereas the friction coefficient was measured using a set-up consisting of polydimethylsiloxane substrate and a glass ball bearing on a rotational rheometer. Increase in extensional viscosity was detected immediately after initiation of extensional flow, particularly XG, and also immediately before the filament rupture, particularly LBG. Extensional viscosity tended to decrease with increased addition of simulated saliva for XG, while to increase for LBG. In both cases, effect of cations in the saliva was greater than that of mucin. From the shape of the Stribeck curve (i.e., dependence of the friction coefficient on the entrainment speed) and comparison of the friction coefficient itself, lubricity of XG was greater than that of LBG. Simulated saliva added decreased the friction coefficient for each polysaccharide through functions of cations rather than mucin. Extensional viscosity and tribological measurements revealed mechanical properties of polysaccharide solutions which cannot be determined or quantified by shear viscosity alone.

Flow of hydrophobically associating polymers through unconsolidated sand pack: Role of extensional rheology and degree of association

Theoretically, both the polymer’s resistance to flow and retention are the reasons for the additional pressure drop and resistance factor (RF) generated during polymer flow in porous media. In the case of hydrolyzed polyacrylamide (HPAM), the increase in RF at high flux rates, a phenomenon called shear thickening were successfully correlated in our previous works using the combination of shear and extensional rheology alone. Associative polymers that have gained increased attention for enhanced oil recovery (EOR) applications have relatively a higher retention value in porous media than HPAM. No attempts were made in the literature to investigate whether the associative polymer’s flow behavior at various hydrophobicity in porous media could be correlated using the bulk rheology alone. Further, the role of hydrophobicity and concentration on the associative polymer’s shear thickening and extensional behavior were not studied.In this paper, high flux single phase porous media experiments were performed using three different associative polymers of varying hydrophobicity at 1000 ppm and 2000 ppm concentration in the unconsolidated sand pack. Extensional rheology was performed on these polymers using capillary break-up extensional rheometer. Extensional rheological parameters become lower with increasing hydrophobicity at both 1000 and 2000 ppm. Porous media result indicates that extensional rheology is directly correlated with the RF in associative polymer at 2000 ppm but not at 1000 ppm. At 1000 ppm, the highest RF is exhibited by higher hydrophobic polymer that possess least extensional resistance. Unusually higher retention value observed at 1000 ppm explain the increased RF associated higher hydrophobic polymer or else it could be due to the clustered hydrophobic species formation that appears to be active only if the concentration is not high enough to breakage. It is also our observation that higher retention cannot be predicted through bulk rheological measurements and therefore, this work signifies that a combination of retention and extensional rheological parameters are needed to model the flow behavior of associative polymers in porous media at higher fluxes. Moreover, the relative influence of retention and extensional rheology on the flow behavior of associative polymer is strictly a concentration and hydrophobicity dependent.

Highlighting the need for high-speed imaging in capillary breakup extensional rheometry

The capillary breakup extensional rheometer is commonly used to determine material properties of complex fluids. This is achieved by tracking the diameter evolution of a liquid bridge undergoing capillary thinning and breakup in a uniaxial extensional flow. Typically, the filament diameter evolution is tracked at the mid-plane between the two end-plates using a laser micrometer. We show using high-speed imaging that while this arrangement is satisfactory in flows where the filament is long (relative to its initial diameter), slender and approximately cylindrical, errors can be significant when the filaments are short (and with a non-negligible curvature) such as encountered when using the so-called slow-retraction-method and ‘Dripping-onto-Substrate’ rheometry. We will further highlight the need for high-speed imaging in CaBER experiments by considering errors induced when the laser micrometer is misaligned with the location of filament breakup. This latter source of error will be particularly relevant for capillary breakup experiments where the location of filament breakup is not typically known a priori, such as the case for many so-called ‘yield-stress’ fluids.

An improved Capillary Breakup Extensional Rheometer to characterize weakly rate-thickening fluids: Applications in synthetic automotive oils

We describe a customized Capillary Breakup Extensional Rheometer (CaBER) with improved dynamic performance and added features for temperature control over the range from room temperature up to 250 °C. The system is aimed at characterizing the extensional rheological behavior of weakly rate-thickening fluids that are widely utilized in the automotive industry. We examine the shear rheology and filament-thinning dynamics of two commercially available automotive fluids with the same viscosity index. Comparisons of the rheological properties of the two samples reveal that although they have identical shear viscosities, they exhibit significant and distinct rate-thickening behavior in the strong extensional flow that is generated close to filament breakup. For the more elastic sample, the exponential filament-thinning dynamics are well-described by the Oldroyd-B model; however, this viscoelastic model poorly describes the response of the more weakly rate-thickening fluid. To address this limitation, we propose a simple Inelastic Rate-Thickening (IRT) model that more robustly describes the measured material response. The two constitutive parameters of the model represent the zero-shear-rate viscosity of the fluid and the rate of extensional thickening in the fluid. Numerical calculations with the IRT model show that the radii of thinning fluid filaments deviate from a linear decay in time and approach a quadratic dependence very close to breakup. By carefully fitting the measured temporal evolution of the mid-plane radius we can therefore systematically differentiate the extensional rheological response of the two oils. More generally, we show that the weakly rate-thickening regime can be distinguished from the well-known elasto-capillary response predicted by the Oldroyd-B model, via a constraint on the relaxation time (or more specifically the elasto-capillary number) of the fluid. The weakly rate-thickening behavior documented in these oils is representative of the relatively unstudied extensional rheology of many industrial fluids at large extensional strain rates (100–1000 s-1) and influences many complex industrial processes such as jetting, coating and stamping.

Extensional Rheological Measurements of Surfactant-Polymer Mixtures.

Polymer solutions flowing in the porous media during enhanced oil recovery (EOR) processes are subjected to both shear and extensional rheological deformation. However, the previous rheological studies conducted on a surfactant–polymer (SP) system or polymer systems were only shear-based. In this paper, the extensional rheological performance of hydrolyzed polyacrylamide (HPAM) in the presence of an anionic surfactant at various concentrations (0, 0.01, 0.05, 0.1, 0.2, and 0.3%) is studied with deionized water and 1% NaCl. Further, the extensional rheological behavior of HPAM in the presence of NaCl and CaCl2 is studied at varying ionic strengths (1–10%). A capillary break-up extensional rheometer is used for performing extensional rheological characterization. Results revealed that the extensional resistance of HPAM is enhanced in the presence of a surfactant. Particularly, around the critical micelle concentration value of the surfactant (0.1%), HPAM showed higher extensional resistance. Higher extensional resistance for the SP system is observed with deionized water when compared to 1% NaCl. HPAM showed improved performance at 1% NaCl salinity when compared to the higher concentration of NaCl salinity. However, the presence of even 1% of calcium ions is detrimental to the extensional properties of HPAM.

The effect of end-plate wetting and unpinned contact lines on the filament thinning of strain hardening fluids

The Filament Extension Atomizer™ (FEA) is a unique technology designed for highly viscous or strain-hardening fluids that are otherwise difficult to atomize. The fluid is processed as a thin film between the contact points of two counter-rotating rollers of different materials. As the film is processed beyond the contact point, it is subject to an extensional flow that creates numerous thin filaments. As the filaments are stretched, they thin, and eventually, surface tension causes them to break into tightly dispersed droplets. Certain fluids, particularly those of low to moderate viscosity and high surface tension, can present challenges to atomize in FEA. Due to the tendency of these fluids to coalesce, their wetting on the rollers has been critical in optimizing film formation, though the impact of surface wetting on filament formation and breakup is not well understood. Accordingly, we studied the role of end-plate wetting for a high surface tension, aqueous, strain-hardening polymer solution on filament formation, thinning, and breakup, and fluid transfer to the end-plates, using a modified Capillary Breakup Extensional Rheometer (CABER). We found that filament formation and evolution were dramatically affected by both the wetting and wetting imbalances between the two end-plates, leading to different behavior across different end-plate combinations. The highly imbalanced wetting scenarios (i.e., combining a highly wetting and a non-wetting end-plate) gave rise to the most extreme deviations from classic behavior in conventional CABER experiments, such as long persisting filaments.

Quantifying separation energy with a modified Capillary Break-up Extensional Rheometer (CaBER) to study polymer solutions

ABSTRACT A Capillary Break-up Extensional Rheometer (CaBER) was modified to enable measurements of the axial force in combination with the related displacement. This makes it possible to determine the separation energy , the tackiness, of a polymer solution between two parallel plates with the CaBER. Separation energies of solutions of linear polystyrene, comb polystyrene and linear poly(methyl methacrylate) in dimethylformamide were measured as model systems. For linear polymers at a critical polymer concentration, there was a drastic change in slope of the separation energy . This critical concentration was shown to be the minimum concentration at which continuous bead-free fibers could be electrospun.

Viscoelastic properties of flexible and rigid polymers for turbulent drag reduction

The relation between the drag reduction (DR) performance of several water-soluble polymers and their viscoelastic properties was investigated. Polymers with a flexible molecular structure including three grades of polyacrylamides (PAM), and a polyethylene oxide (PEO) were investigated. Xanthan gum (XG) and carboxymethyl cellulose (CMC), each with a rigid molecular structure, were also considered. The rheology was characterized using steady shear-viscosity measurement, capillary break-up extensional rheometer (CaBER), and small-amplitude oscillatory shear measurement at the concentration of the drag-reduced solution. To isolate the effect of shear viscosity, the concentration of the polymers was adjusted to produce solutions with a similar shear viscosity at high shear rates. Using pressure drop measurements in a turbulent pipe flow, the DR of each polymer solution was determined. With identical high-shear-rate viscosities, the flexible PAM solutions resulted in an initial DR of 50–58%, while the initial DR of PEO was 44%, and the rigid polymers provided the least DR of 12%. The rigid polymers demonstrated negligible degradation of DR over a period of 2 h. Of the flexible polymers, PAM showed moderate degradation, while the DR of PEO quickly diminished after 20 min. Drag reduction correlated with extensional viscosity and Weissenberg number obtained from CaBER. A strong correlation was not observed between DR and the viscoelastic moduli obtained from small-amplitude oscillatory shear. The large mechanical degradation of PEO was associated with a continuous extensional thickening, in which extensional viscosity increased with decreasing strain rate until the filament broke up.

Extensional Magnetorheology as a Tool for Optimizing the Formulation of Ferrofluids in Oil-Spill Clean-Up Processes

In this study, we propose a new way of optimising the formulation of ferrofluids for oil-spill clean-up processes, based on the rheological behaviour under extensional flow and magnetic fields. Different commercial ferrofluids (FFs), consisting of a set of six ferrofluids with different magnetic saturation and particle concentration, were characterised in a Capillary Break-Up Extensional Rheometer (CaBER) equipped with two magnetorheological cells that allow imposing a homogeneous and tunable magnetic field either parallel or perpendicular to the flow direction. The filament thinning process with different intensities and orientation of the magnetic field with respect to the flow direction was analysed, and the results showed that the perpendicular configuration did not have a significant effect on the behaviour of the ferrofluids, as in shear magnetorheometry. However, the parallel configuration allowed to determine that the formulation of ferrofluids for oil-spill cleaning processes should consist of a 4% vol concentration of magnetic nanoparticles with a magnetic saturation of M s > 20 mT.

Extensional Effects during Viscoelastic Polymer Flooding: Understanding Unresolved Challenges

Summary Several studies have tried to relate polymers’ enhanced oil recovery (EOR) potential to their viscoelastic characteristics such as onset, rheo thickening, extensional viscosity, and Deborah number (De). Contradictions prevail when it comes to reduction in residual oil saturation (Sor) during polymer flooding and the role of extensional properties. De calculated using the oscillatory relaxation time fails to explain the different pressure profiles exhibited by the viscous and viscoelastic polymers. Extensional viscosity has been ignored in many studies as the reason for additional Sor reduction based on the core-scale apparent viscosity and core-scale capillary number (Nc). In recent studies, a significant oil mobilization was shown by the viscoelastic polymers even before the critical Nc, which indicates that the capillary theory breaks out under specific conditions during polymer flooding. Moreover, the additional residual oil recovery caused by the high-salinity polymer solutions cannot be explained by the oscillatory De. In this paper, we compile and examine many such unresolved challenges from various literature with rheological and petrophysical insights. The uniaxial bulk extensional rheology is performed on the relevant polymers using a capillary breakup extensional rheometer to measure the extensional relaxation time, maximum extensional viscosity at the critical De, and strain hardening index. A detailed analysis signifies the role of extensional rheology on the viscoelastic onset, rheo thickening, and Sor reduction even under varying salinity conditions. The results also highlight the advantages of extensional rheology over oscillatory rheology and validate the capillary theory using modified capillary number.

High temperature extensional rheology of commercially available polycarbonate mixed with flame retardant salts

In this paper, we present a study of the dripping properties of polycarbonate (PC) modified with combinations of earth metal salts of inorganic sulfur, potassium perfluorobutane sulfonate (Rimar); non-halogenated flame retardant additives, potassium diphenyl sulfone-3-Sulfonate (KSS); and block co-polymers-polytetrafluoroethylene encapsulated with styrene acrylonitrile resin (T-SAN). Measurements of the extensional rheology of polycarbonate with different concentration of each flame retardant additive were performed using a custom-built high temperature Capillary Breakup Extensional Rheometer (CaBER) at temperatures up to T=400°C. From these measurements, the evolution of the apparent transient extensional viscosity was monitored as a function of time and strain both in air and in an inert nitrogen environment. The evolution of extensional viscosity has been shown to be an excellent tool for predicting the dripping behavior of polymers exposed to heat and a valuable tool for understanding the mechanism of polymer degradation which is typically dominated by either crosslinking or charring. We show that extensional rheology measurements are significantly more sensitive to temperature-induced changes to the polymer microstructure than shear rheology measurements. We have also performed systematic concentration of specific flame retardant salts and through variation in extensional rheology and investigated the optimum concentration required to achieve a V0 rating. Finally, we will show that extensional rheology is a powerful method for predicting the effect of flame retardant modifiers and optimizing their use in new flame resistant materials.

Endogenous salivary citrate is associated with enhanced rheological properties following oral capsaicin stimulation.

New Findings What is the central question of this study? What are the relationships between physical properties of saliva, protein composition and metabolite composition? What is the main finding and its importance? Salivary citrate, one of the major endogenous metabolites in saliva, increased upon capsaicin stimulation and was associated with improved physical properties measured by extensional rheology. This suggests salivary gland citrate transporters might be a valuable area of future study. Saliva displays viscoelastic properties which enable coating, lubrication and protection of the oral mucosa and hard tissues. Individuals lacking saliva or perceiving oral dryness can manage their symptoms using artificial saliva preparations, but these often fail to mimic the sensation and functionality of natural saliva. It is widely acknowledged that mucins (MUC7 and MUC5B) confer saliva's rheological properties, but artificial saliva containing purified mucins is still often an inadequate substitute. This work aimed to explore salivary components that influence salivary extensional rheology to better understand how natural saliva could be replicated. Saliva was stimulated via control and capsaicin solutions in healthy volunteers. Extensional rheology was analysed using a CaBER‐1 (capillary breakup) extensional rheometer. Protein composition, including mucins, was measured by gel‐electrophoresis band densitometry and metabolites were measured by 1H nuclear magnetic resonance spectroscopy. Capsaicin stimulation significantly increased capillary breakup time, extensional viscosity and the abundance of most major salivary proteins. Stimulation also increased salivary citrate and choline concentrations. Significant correlations were found between capillary breakup time and amylase (r = 0.67, P < 0.05), statherin (ρ = 0.66, P < 0.05) and citrate (ρ = 0.81, P < 0.01). The relationship between citrate and salivary rheology was subsequently investigated in vitro. These results suggest that citrate and non‐mucin proteins are stronger predictors of salivary rheology than the more often studied mucin glycoproteins. Potential mechanisms are discussed and future work in this area could help formulate more effective saliva substitutes, more closely resembling natural saliva.

Numerical study on micro-scale extensional viscoelastic flows

The capillary thinning dynamics can be considered one of the fingerprints of extensionally-dominated viscoelastic flows. Notably, the rheological behavior of complex fluids in extensional flow has been investigated in different rheometric devices, as for instance in the Capillary Breakup Extensional Rheometer (CaBER), the Dripping-onto-Substrate (DoS) rheometry, or the Rayleigh Ohnesorge Jetting Extensional Rheometer (ROJER). In recent years, the Computational Rheology community has made a considerable effort to better understand the interplay of viscoelasticity and capillarity effects in such transient rheometric experiments. In this work, we present a numerical study on the dynamics of extensional flows of dilute polymeric solutions at small scales. Our numerical investigation focus primarily on the potential of using small scale two-phase extensional viscoelastic flows as suitable platforms for performing rheometry of weakly viscoelastic polymer solutions. In particular, we have adopted the setup used in the experiments of Sousa et al. [Rheol. Acta 56 (2017) 11–20]. In such set up, the filament stretching is conducted using oil as an outer phase, avoiding sample evaporation, or allowing visualization of the filament interior. In order to handle with the moving interface problem, we have employed a two-phase viscoelastic fluid flow solver, based on a finite differences scheme. In this methodology, the interface between the fluids is approximated by the volume-of-fluid interface reconstruction algorithm, and a second-order operator-split method is used to solve the advection equation. We observed a negligible influence of the use of different types of low viscosity oils as outer fluid on the measurement of the sample relaxation time. We also found that the use of an external low viscosity immiscible oil did not prevent the formation of beads-on-a-string structures.

Does Polymer's Viscoelasticity Influence Heavy-Oil Sweep Efficiency and Injectivity at 1 ft/D?

Summary For heavy-oil-recovery applications, mobility control is more important than interfacial-tension reduction, and therefore importance should be given to the recovery of remaining mobile oil by enhanced sweep efficiency. Although the relative roles of polymer viscosity and elasticity in capillary-trapped residual light-oil recovery have been studied extensively, their roles in sweeping mobile viscous oil have not been explored. Injectivity is vital for heavy-oil-recovery applications, and polymer selection is performed solely using criteria that is based on shear rheology. In this paper, the influence of viscous (shear) resistance and elastic (extensional) resistance of viscoelastic polymer on mobile-heavy-oil recovery and injectivity is investigated through the combination of bulk shear/extensional rheology and single-phase and multiphase coreflood experiments at a typical reservoir-flooding rate of 1 ft/D. Two polymer solutions with different concentrations and salinities are selected such that a polymer with low molecular weight (MW) [hydrolyzed polyacrylamide (HPAM) 3130] provides higher shear resistance than a high-MW polymer (HPAM 3630). Extensional characterization of these two polymer solutions performed using a capillary breakup extensional rheometer revealed that HPAM 3630 provided higher extensional viscosity than HPAM 3130. The results show that the behaviors of polymers in extension and shear are completely different. Two multiphase and two single-phase experiments are conducted at low flux rate to investigate the roles of extensional viscosity on mobile-heavy-oil recovery and high flux rates on injectivity. After 1 pore volume (PV) of polymer injections, higher-concentration and lower-MW HPAM 3130 contributes to approximately 17% higher incremental recovery factor vs. lower-concentration and higher-MW HPAM 3630. The core-scale pressure drop generated by HPAM 3130 is more than twice the pressure drop generated by HPAM 3630. Under low-flux-rate conditions at the core scale, shear forces dominate, and displacing fluid with higher shear viscosity contributes to better sweep. HPAM 3630 exhibits a shear-thickening phenomenon and possesses the apparent viscosity of approximately 90 cp at the flux rate of approximately 90 ft/D. In contrast, HPAM 3130 continued showing shear thinning and has the apparent viscosity of approximately 70 cp at approximately 90 ft/D. This signifies the role of extension rheology on the injectivity at higher flux rates. Results revealed that while the extensional rheological role toward sweeping the mobile heavy-oil recovery at low flux is lesser compared with the shear role, its negative role on the polymer injectivity is very significant. Polymer-selection criteria for heavy-oil-recovery applications should incorporate extensional rheological parameters.

Capillary breakup extensional electrorheometry (CaBEER)

A new electrorheological cell for the capillary breakup extensional rheometer (HaakeTM CaBER 1TM, Thermo Scientific) allowing the analysis of the rheological properties of complex fluids under the simultaneous application of an extensional flow and an electric field has been developed and tested. In this study, the add-on is described in full detail and different cornstarch suspensions in olive oil are characterized under different stretching conditions with and without an electric field. The results show that, under extensional flow and no electric field, the sample behaves as a Newtonian-like fluid, showing a linear relationship between time and filament minimum diameter. However, when the external electric field is imposed, the filament thinning process delays increasing the breakup time. If the external electric field is further increased, then the filament may even not break, depending on the imposed voltage, the Hencky strain, and the concentration of particles in the fluid sample.

Influence of the Polarity of the Electric Field on Electrorheometry

Uniaxial extensional flow is a canonical flow typically used in rheological characterization to provide complementary information to that obtained by imposing simple shear flow. In spite of the importance of having a full rheological characterization of complex fluids, publications on the rheological characterization of mobile liquids under extensional flow have increased significantly only in the last 20 years. In the case of the rheological characterization of electrorheological fluids, the situation is even more dramatic, as the ERFs have been exclusively determined under simple shear flow, where an electrorheological cell is attached to the rotational rheometer generating an electric field perpendicular to the flow direction and that does not allow for inverting the polarity. The very recent work published by Sadek et al., who developed a new electrorheological cell to be used with the commercial Capillary Breakup Extensional Rheometer (CaBER), allows for the very first time performing electrorheometry under extensional flow. By means of the same experimental setup, this study investigates the influence of the polarity of the imposed electric field on the filament thinning process of a Newtonian and an electrorheological fluid. Results show that a polarity against the gravity results in filament thinning processes that live longer or reach a stable configuration at lower intensities of the applied electric field.

Capillary breakup extensional magnetorheometry

This work introduces a novel design of a fixture to be adapted to the commercial version of the capillary breakup extensional rheometer (CaBER), here termed as an extensional magneto-rheological fixture (ExMRFx), that allows the characterization of magnetic fluids under uniaxial extensional flow. The fixture consists of four solenoids that generate a homogeneous magnetic field with a tunable intensity on the fluid sample, while characterized in the CaBER under uniaxial extensional flow. The ExMRFx is designed in two configurations, one to impose a magnetic field parallel, while the other to impose a magnetic field perpendicular to the deformation axis. As an example of possible application, the ExMRFx has been tested with a magnetorheological fluid (MRF) consisting of a suspension of carbonyl iron particles dispersed in glycerine. The experimental results show that the filament thinning process of the MRF sample is influenced by the concentration of carbonyl iron particles used, the orientation of the magnetic field (parallel or perpendicular), the intensity of the magnetic field applied, and finally the Hencky strain imposed. Depending on the combination of these latter parameters, the MRF sample may result in a very stable filament bridging the two measuring plates, which will remain stable, as long as none of the experimental parameters change.This work introduces a novel design of a fixture to be adapted to the commercial version of the capillary breakup extensional rheometer (CaBER), here termed as an extensional magneto-rheological fixture (ExMRFx), that allows the characterization of magnetic fluids under uniaxial extensional flow. The fixture consists of four solenoids that generate a homogeneous magnetic field with a tunable intensity on the fluid sample, while characterized in the CaBER under uniaxial extensional flow. The ExMRFx is designed in two configurations, one to impose a magnetic field parallel, while the other to impose a magnetic field perpendicular to the deformation axis. As an example of possible application, the ExMRFx has been tested with a magnetorheological fluid (MRF) consisting of a suspension of carbonyl iron particles dispersed in glycerine. The experimental results show that the filament thinning process of the MRF sample is influenced by the concentration of carbonyl iron particles used, the orientation of the mag...

Shear and extensional rheology of acid milk gel suspensions with varying ropiness.

Exopolysaccharides (EPS) synthesized by lactic acid bacteria during fermentation significantly affect the rheology of set-style acid milk gels and that of acid gel suspensions, produced from the gels by stirring. In this study, shear and uniaxial extensional flow of acid gel suspensions made with seven single strains of Streptococcus thermophilus or Lactococcus lactis was investigated. Six strains produced ropy EPS, and maximum filament length determined by using a continuous stretching method was up to four times higher than filament length of an EPS-negative control strain. The systems containing ropy EPS showed a different response to shear and extensional deformation. In shear rheology, higher apparent viscosities and an enhanced structural breakdown was observed for acid gel suspensions with more pronounced ropiness. Breakup time and extensional viscosity, determined by using a Capillary Breakup Extensional Rheometer (CaBER™), significantly increased with increasing ropiness. The increase of extensional viscosity with increasing ropiness was, however, much higher than the effects of ropiness on shear viscosity. As relaxation times also depended on ropiness, it is concluded that ropiness is caused by EPS-EPS interactions that can be better discriminated in extensional rheology. PRACTICAL APPLICATIONS: To improve the texture of fermented milk, lactic acid bacteria that are able to produce ropy exopolysaccharides (EPS) are increasingly used in the dairy industry. The EPS exhibit a significant influence on processing properties and sensory characteristics of the resulting products, which can be estimated by means of shear and extensional rheology. The current work provides information on these respective properties of acid gel suspensions, which facilitate product design by supporting the selection of appropriate starter cultures.