Capillary Breakup Extensional Rheometry Research Articles

Fiber spinning from polymer solutions

The thinning of a cylinder of a polymer solution in a volatile solvent is argued to be controlled by solvent diffusion through a dense polymer layer at the cylinder surface. This naturally leads to the exponential time dependence of cylinder radius that is observed in experiments using a fast camera, such as capillary breakup extensional rheometry (CaBER). The relaxation time is controlled by the thickness of the dense (and often glassy) polymer layer and the diffusion coefficient of solvent through that layer. If correct, this means that while CaBER is very useful for understanding fiber spinning, the relaxation time does not yield a measure of the extensional viscosity of polymer solutions in volatile solvents.

Extensional rheology of hydrophobically associating polyacrylamide solution used in chemical flooding: Effects of temperature, NaCl and surfactant

The study provides an insight into the oil washing mechanism of hydrophobically associating polyacrylamide (HAHPAM) in chemical flooding from the perspective of extensional rheology. The extensional rheology of HAHPAM solution was systematically investigated under different influencing factors (concentration, temperature, NaCl and surfactant) using capillary breakup extensional rheometry (CaBER). Compared with partially hydrolyzed polyacrylamide (HPAM), the HAHPAM solution shows better extensibility due to the additional hydrophobic interaction between molecular chains, particularly at high temperature and high salinity. By increasing the concentration and decreasing the temperature, the filament breakup time and extensional viscosity of the HAHPAM solution increase. NaCl delays the attenuation of the HAHPAM filament by increasing the solution extensional viscosity and surface tension. In the presence of sodium dodecyl sulfate (SDS), the reduced surface tension dominates the stability of the filament and weakens the extensibility of the HAHPAM/SDS composite solution.

A technical note on large normal-stress differences observed in a novel self-assembling functionalized dipeptide surfactant solution

A number of functionalised dipeptides self-assemble in water under specific conditions to give micellar aggregates. The micellar aggregates formed depend on the exact molecular structure and are important to understand as they control the properties both of the micellar phase and also of the gel phase which can be formed from these precursor solutions. Here, we investigate the rheological properties of a functionalised dipeptide which behaves as a surfactant at high pH. This solution has been shown previously to exhibit very “stringy” behaviour, and this has previously been characterised using capillary breakup extensional rheometry (CaBER). In the current technical note, we extend the rheological characterisation of an exemplar precursor solution via small-amplitude oscillatory shear and steady shear. Using a cone-and-plate geometry and a dedicated protocol, we can measure the first normal-stress difference N1 and using a parallel-plate geometry to also measure (N1-N2), subsequently determining the second normal-stress difference N2. In so doing, we confirm that these systems are highly elastic, e.g. for shear rates greater than ~ 30 s−1, corresponding to a Weissenberg number based on the longest relaxation time ~ 330, N1 > 10τ where τ is the shear stress, and also, we find that N2 can be significant, is negative and approximately equal in magnitude to ~ 0.36 ± 0.05 N1. Significant uncertainties associated with the normal-stress difference data led to us using a range of different rheometers (and geometries) and highlight the issues with determining N2 using this two-measurement approach. Despite these uncertainties, the non-negligible value of the second-normal stress difference is demonstrated for these fluids.

Instrumental characterization of xanthan gum and scleroglucan solutions: Comparison of rotational rheometry, capillary breakup extensional rheometry and soft-contact tribology

Xanthan gum and scleroglucan, two rod-like polysaccharide hydrocolloids, are compared using a wide range of instrumental techniques and methods: steady shear flow, small amplitude oscillatory shear, first normal stress difference, capillary break-up and soft-contact tribology. The aqueous solutions of these two hydrocolloids with similar flow and viscoelastic profile show marked differences in capillary break-up time and apparent extensional viscosity. This result correlates with differences in first normal stress difference and, to a lesser extent, Stribeck curve behaviour. Formulating the hydrocolloids in concentrated sucrose solution (40 wt%) shifts relaxation profiles to longer times which greatly diminishes differences in rheological and lubrication behaviour. With exception of capillary break-up tests, no other methods showed statistically significant differences between the polysaccharides dissolved in the viscosified matrix. The toolbox of techniques is also applied to probe interactions of xanthan gum and scleroglucan with human whole saliva and bovine submaxillary mucin. We report no specific interactions between either hydrocolloid and salivary proteins and suggest that any cumulative effects must stem from specific sets of linear and non-linear rheological properties of saliva/hydrocolloid mixtures. • Rheology and tribology of xanthan gum and scleroglucan are compared. • Capillary break-up provides sensitive discrimination between hydrocolloids. • Xanthan gum and scleroglucan have similar hydrodynamic volume. • Relaxation spectrum of xanthan features faster modes compared to scleroglucan.

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.

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.

Flow behavior of concentrated tricalcium phosphate suspensions in oil through injection for softgel encapsulation

Softgel encapsulation is an important manufacturing process used to produce convenient oral dosage forms of pharmaceuticals and dietary supplements. An important step in this process involves the injection of a fill material into a gelatin lined mold, and poor flow of the fill material during the injection may lead to undesired defects. We investigated the effect of fill composition on the injection behavior of an oil-based tricalcium phosphate (TCP) suspension containing lecithin by rheological measurements, capillary breakup extensional rheometry (CaBER), and high-speed flow visualization experiments. Shear rheology for TCP suspensions in soybean oil at concentrations of 27.8 wt% and 55.6 wt% showed shear thinning behaviors both with and without the addition of lecithin. Adding up to 0.5 wt% lecithin led to decreases in shear viscosity, while additions up to 1 wt% caused a slight increase. Results from CaBER and breakoff studies indicated that the length at which flow breakoff occurs was closely related to the Ohnesorge number, a dimensionless parameter representing the relative importance of viscous to surface-tension forces during breakoff, with higher values correlated to longer breakoff lengths. An optimal lecithin concentration exists for minimized TCP suspension viscosity which shortens flow breakoff times and improves softgel capsule seaming quality.

GO CaBER: Capillary breakup and steady-shear experiments on aqueous graphene oxide (GO) suspensions

Aqueous solutions of graphene oxide (GO) with a very large average flake aspect ratio, AR = O ( 10 4 ), are probed using capillary breakup experiments and shear rheological measurements for concentrations up to ϕ = 0.4 vol . %. GO, a chemically oxidized sheet of graphene that we obtained from graphite exfoliation, is suspended in distilled water to produce solutions with volume concentrations spanning 0.02 ≤ ϕ ≤ 0.4 vol . %. For concentrations between 0.1 ≤ ϕ ≤ 0.4 vol . %, the apparent shear yield stress, τ y (determined from creep tests), scales with concentration following a power-law relationship with exponent ∼ 2.2. Using capillary breakup extensional rheometry (CaBER), we demonstrate that for concentrations ϕ ≳ 0.1 vol . %, aqueous GO solutions exhibit power-law fluid behavior in uniaxial elongation. The deformation-rate-dependent transient Trouton ratio over the range where the deformation rates match in shear and capillary breakup experiments is independent of the deformation rate, but inversely proportional to the volume concentration with all values exceeding 3. An attempt is made to estimate the apparent elongational yield stress (for ϕ ≥ 0.3 vol . %) by using the Laplace pressure within the fluid filament as its surrogate, and we find that in the strain rate regime that we can observe, the apparent elongational yield stress, τ y , e, is not 3 times the shear yield stress, i.e., for our aqueous GO solutions, we are not able to observe agreement with the von Mises plasticity criterion. GO samples at a given concentration probed in capillary breakup extensional rheology experiments under different protocols, i.e., slow-retraction opposed to step-stretch, exhibited different responses confirming that GO is highly sensitive to the deformation history; however, they still exhibit power-law fluid behavior even given the different initial conditions. Although slightly limited in our ability to quantify the extension-thinning yield-stress-like behavior of aqueous GO in uniaxial elongational flow by the fact that we cannot prescribe the strain rates in CaBER experiments, we demonstrated that even a very small amount of GO flakes can significantly alter the shear rheology and the pinch-off dynamics of a liquid bridge in comparison to Newtonian fluids of equivalent shear viscosity.Aqueous solutions of graphene oxide (GO) with a very large average flake aspect ratio, AR = O ( 10 4 ), are probed using capillary breakup experiments and shear rheological measurements for concentrations up to ϕ = 0.4 vol . %. GO, a chemically oxidized sheet of graphene that we obtained from graphite exfoliation, is suspended in distilled water to produce solutions with volume concentrations spanning 0.02 ≤ ϕ ≤ 0.4 vol . %. For concentrations between 0.1 ≤ ϕ ≤ 0.4 vol . %, the apparent shear yield stress, τ y (determined from creep tests), scales with concentration following a power-law relationship with exponent ∼ 2.2. Using capillary breakup extensional rheometry (CaBER), we demonstrate that for concentrations ϕ ≳ 0.1 vol . %, aqueous GO solutions exhibit power-law fluid behavior in uniaxial elongation. The deformation-rate-dependent transient Trouton ratio over the range where the deformation rates match in shear and capillary breakup experiments is independent of the d...

Capillary breakup extensional rheometry of sodium carboxymethylcellulose solutions in water and propylene glycol/water mixtures

ABSTRACTThis article presents the results of capillary break‐up extensional rheometer experiments conducted for semidilute solutions of carboxymethylcellulose sodium salt (Na‐CMC) with degrees of substitution (DS) ranging from 0.62 to 1.04 in distilled water and propylene glycol (PG)/water mixtures. The partial aggregation of Na‐CMC chains with DS < 1 observed in aqueous solutions triggers an increase in apparent extensional viscosity and extension of break‐up time. The rheological properties of Na‐CMC solutions in propylene glycol/water mixture are determined by the solubility of the polymer and the physical crosslinking of chains. The disappearance of the elasto‐capillary regime during the filament thinning of Na‐CMC solutions with DS < 1 in propylene glycol/water mixture was linked to the physical crosslinking of polymer chains. The shape of the extensional viscosity curve for Na‐CMC solutions with DS = 1.04 in PG/water mixture was characteristic for semidilute polymer solutions with a low number of entanglements. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 1537–1547

What controls filament thinning in uniaxial extension?

Breakup of fluid threads is omnipresent in nature and highly relevant for technical processes such as atomization, printing, coating, or spinning. We discuss how to control the filament lifetime of shear-thinning, viscoelastic fluids during uniaxial extension without affecting their shear viscosity. Two commercial acrylic thickeners differing with respect to the co-polymerized hydrophobic monomers, and hence different hydrophobic intra- and intermolecular association, are mixed to obtain aqueous solutions with similar shear viscosity. The elongational relaxation time as determined by capillary breakup extensional rheometry, however, varies by almost two orders of magnitude. Filament lifetime of these solutions can be varied upon adding non-Brownian, plate-shaped particles, again without affecting shear viscosity. A trace amount of particles increases elongational relaxation time by about a factor of four. Car bodies are usually coated using high-speed rotary bell atomizers guaranteeing high transfer efficiency and high-quality appearance. We use the solutions and suspensions described above to investigate the effect of extensional viscosity on ligament formation at the bell edge as a decisive intermediate step prior to droplet formation. High-speed imaging reveals a logarithmic scaling of ligament length with extensional relaxation time for pure thickener solutions. In contrast, ligament length monotonically decreases with increasing particle concentration, i.e. extensional viscosity. Plate-shaped particles obviously act as defects promoting ligament breakup. Extended filament lifetimes are commonly observed during atomization and spraying as a consequence of fluid viscoelasticity. On the other hand, low viscosity fluid threads rupture quickly. Here, we demonstrate that a unique banding instability during extension of a low viscosity surfactant solution with no measurable elasticity leads to extremely long filament lifetimes and to the formation of remarkably long threads. Combining filament stretching and particle image velocimetry we found an unexpected heterogeneous, banded flow in opposing directions. This phenomenon is not limited to surfactant solutions but can also occur in biopolymer solutions, thus broadening the view on instabilities in complex elongational flows.

Instabilities in micro-contraction flows of semi-dilute CTAB and CPyCl solutions: rheology and flow instabilities

The flows of two semi-dilute surfactant solutions (CTAB and CPyCl) through several micro contractions/expansions are experimentally investigated, following an extensive rheological characterization in both shear and extensional flows. The shear rheology of the solutions shows strong shear thinning and shear banding, whereas Small Amplitude Oscillatory Shear and Capillary Break-up Extensional Rheometry indicate that they have high shear and extensional elasticities. Flow visualizations and micro-particle image velocimetry measurements show that the surfactant solutions exhibit three established types of flow patterns in contraction flows: Newtonian-like, asymmetric and disordered. Newtonian-like flow occurs at low flow rates and is preceded by a long transient flow in experiments starting from rest, which seems to be related to shear banding and the alignment of wormlike micelles. The asymmetric flow regime occurs at moderate flow rates and is characterized by an asymmetric upstream central jet with two adjacent vortices, features that change non-periodically, but slowly, in time. This flow pattern seems to be related with the high elasticity of the semi-dilute solutions. The disordered flow pattern is similar to the asymmetric flow in terms of broad characteristics, but the flow asymmetry changes with time much faster than in the asymmetric flow regime, resembling a chaotic-like flow. The disordered flow seems to be related with the breakdown of micellar structures. We concluded also that the flows in both the asymmetric and disordered flow regimes are globally stable in terms of flow patterns, but locally unstable in terms of flow characteristics, with power spectra of the velocity fluctuations having slopes that differ from those typically encountered in elastic turbulence.

Dripping-onto-substrate capillary breakup extensional rheometry of low-viscosity printing inks

In this manuscript, the capillary thinning dynamics of a series of solutions containing polymers commonly used in the coating industry as low-viscosity printing inks were studied. Four different polymer binders were studied in methyl-ethyl ketone. These included one acrylic polymer, one cellulose polymer and two vinyl polymers of different molecular weights. The dripping-onto-substrate capillary breakup extensional rheometry (CaBER-DoS) method was used to characterize the extensional rheology for solutions with viscosities as low as 3.5mPa.s. This technique is based on the measurement of the decay of a fluid filament under the influence of surface tension and has been shown to be capable of measuring relaxation times as low as 20 µs for weakly elastic liquids. The influence of the polymer concentration on the dynamics of filament breakup was investigated for each polymer solution and the results were compared to those obtained with different polymer binders. With an increase in polymer concentration, a critical polymer concentration was identified for the transition between the inertio-capillary, the visco-capillary and the elasto-capillary breakup regimes. With the onset of elasto-capillary breakup at moderate to high polymer concentrations, a delay in the filament breakup was observed due to an increase of the viscous and elastic stresses. This viscoelastic breakup delay would be detrimental to most ink jet printing applications. Within the elasto-capillary breakup regime, the transient extensional viscosity resulted in Trouton ratios ranging from just above the Newtonian limit of Tr = 3 to values close to Tr = 100.

Effects of particle stiffness on the extensional rheology of model rod-like nanoparticle suspensions.

The linear and nonlinear rheological behavior of two rod-like particle suspensions as a function of concentration is studied using small amplitude oscillatory shear, steady shear and capillary breakup extensional rheometry. The rod-like suspensions are composed of fd virus and its mutant fdY21M, which are perfectly monodisperse, with a length on the order of 900 nm. The particles are semiflexible yet differ in their persistence length. The effect of stiffness on the rheological behavior in both, shear and extensional flow, is investigated experimentally. The linear viscoelastic shear data is compared in detail with theoretical predictions for worm-like chains. The extensional properties are compared to Batchelor's theory, generalized for the shear thinning nature of the suspensions. Theoretical predictions agree well with the measured complex moduli at low concentrations as well as the nonlinear shear and elongational viscosities at high flow rates. The results in this work provide guidelines for enhancing the elongational viscosity based on purely frictional effects in the absence of strong normal forces which are characteristic for high molecular weight polymers.

Drop breakup dynamics of dilute polymer solutions: Effect of molecular weight, concentration, and viscosity

The large extensional viscosity of dilute polymer solutions has been shown to dramatically delay the breakup of jets into drops. For low shear viscosity solutions, the jet breakup is initially governed by a balance of inertial and capillary stresses before transitioning to a balance of viscoelastic and capillary stresses at later times. This transition occurs at a critical time when the radius decay dynamics shift from a 2/3 power law to an exponential decay as the increasing deformation rate imposed on the fluid filament results in large molecular deformations and rapid crossover into the elasto-capillary regime. By experimental fits of the elasto-capillary thinning diameter data, a relaxation time of less than 100 μs has been successfully measured. In this paper, we show that, with a better understanding of the transition from the inertia-capillary to the elasto-capillary breakup regime, relaxation times close to 10 μs can be measured with the relaxation time resolution limited only by the frame rate and spatial resolution of the high speed camera. In this paper, the dynamics of drop formation and pinch-off are presented using dripping onto substrate capillary breakup extensional rheometry (CaBER-DoS) for a series of dilute solutions of polyethylene oxide (PEO) in water and in a viscosified water and glycerin mixture. Four different molecular weights between 100 k and 1 M g/mol were studied with varying solution viscosities between 1 and 22 mPa s and at concentrations between 0.004 and 0.5 times the overlap concentration, c*. The dependence of the relaxation time and extensional viscosity on these varying parameters was studied and compared to the predictions of dilute solution theory while simultaneously searching for the lower limit in solution elasticity that can be detected. For PEO in water, this limit was found to be a fluid with a relaxation time of roughly 20 μs. These results confirm that CaBER-DoS can be a powerful technique characterizing the rheology of a notoriously difficult material to quantify, namely, low viscosity inkjet printer inks.

CaBER vs ROJER—Different time scales for the thinning of a weakly elastic jet

In this paper, we demonstrate that the capillary thinning dynamics of a weakly viscoelastic jet follow a different timescale than a liquid bridge of the same fluid between two stationary surfaces for similar geometrical scales. The thinning in the latter case observed with capillary breakup extensional rheometry (or CaBER) follows a well established scaling of the radius with time for an elasto-capillary (EC) balance of R∼exp⁡(−t/3λ). However, for the thinning of the filaments between droplets in a jet, it was so far just assumed that the same scaling law holds. In this paper, we experimentally demonstrate that the jet thinning in a Rayleigh–Ohnesorge jetting extensional rheometer (or ROJER) follows a different scaling of R∼exp⁡(−t/2λ). This is demonstrated by a direct comparison of the thinning dynamics of weakly viscoelastic (Oh<0.01) aqueous solutions of polyethylene oxide in the two experimental setups, covering a wide range of jetting velocities or Weber numbers of 1–70. We demonstrate outgoing from a momentum balance that includes inertia and elasticity that this difference in scaling is due to a constant axial tension in the jet arising from the constant creation rate of new surface at the nozzle. Numerical simulations using the FENE-P model support this theory and demonstrate that in the exponential thinning regime of the jet the elastic stresses are indeed balanced by the axial tension (rather than by capillary pressure as in the EC balance regime of the CaBER experiment). It is readily shown from the reduced stress balance that this axial-elastic balance regime in the ROJER experiment leads to a faster exponential thinning, following the new scaling of R∼exp⁡(−t/2λ) that was experimentally observed. Furthermore, we observe both in experiment and simulation that a jet thinning does not exhibit a self-similar structure of the corner region where the thinning filament connects to the drop as it is generally observed for a filament with an axial tension decaying with the filament radius as in the CaBER. The resulting difference of 50% in extensional relaxation time λ extracted from ROJER experiments might require one to revisit previously reported ROJER experiments and is required for the correct evaluation of future jetting rheometry experiments.

Extensional Flow Behavior of Methylcellulose Solutions Containing Fibrils.

The extensional properties of semidilute aqueous methylcellulose (MC) solutions have been characterized. Pure aqueous MC solutions are shear-thinning liquids at room temperature. With the addition of 8 wt % NaCl, a fraction of MC self-assembles into long fibrils, which modify the rheological properties of the original MC solution. Capillary Breakup Extensional Rheometry (CaBER) was used to characterize salt-free and 8 wt % NaCl solutions of MC at room temperature. The salt-free solutions exhibit only power-law behavior whereas solutions with NaCl exhibit both power-law and elastic regimes. As MC concentration increases, the extensional relaxation time also increases strongly, from 0.04 s at 0.5 wt % to 4 s at 1 wt %. In addition, the apparent extensional viscosity rapidly increases as a function of increasing MC concentration, from 40 Pa·s at 0.5 wt % to 1300 Pa·s at 1 wt %. This behavior is attributed to the presence of fibrils in the MC solutions containing NaCl.

Capillary breakup extensional rheometry of associative and hydrolyzed polyacrylamide polymers for oil recovery applications

ABSTRACTHigh molecular weight polymers used for heavy oil recovery exhibit viscoelasticity that can influence the oil recovery during chemical enhanced oil recovery. Different polymers having similar molecular weight and shear rheology may have different elongation flow behavior depending on their extensional properties. Displacing slugs are more likely to stretch than shear in tortuous porous media. Therefore, it is critical to seek an analytical tool that can characterize extensional parameters to improve polymer selection criteria. This article focuses on the extensional characterization of two polymers (hydrolyzed polyacrylamide and associative polymer) having identical shear behavior using capillary breakup extensional rheometer to explain their different porous media behavior. Maximum extensional viscosity at the critical Deborah number and Deborah number in porous media classified the associative polymer as the one having high elastic‐limit. Extensional characterization results were complemented by significantly higher pressure drop, marginally increased oil recovery of associative polymer in porous media. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46253.

Exploring sensitivity of the extensional flow to wormlike micellar structure

The non-linear dynamics of a series of salt-free surfactant solutions is investigated via a combination of TEM imaging, shear rheology and capillary breakup extensional rheometry. Surfactant solutions contain Hexadecyltrimethylammonium p-toluenesulfonate (CTAT) in de-ionized water. At low surfactant concentrations, 0.7 wt% < c < 1.1 wt%, solutions exhibit shear thickening behavior and the transient extensional measurements lead to formation of long lasting elastic filaments with a fairly constant extensional relaxation time λE ≈ 0.03 s. At higher concentrations, 1.1 wt% ≤ c < 2 wt%, solutions exhibit strong viscoelastic behavior evidenced by dramatic increase in shear relaxation time. However, for concentrations above 2 wt%, the shear relaxation time decreases, while the zero shear viscosity reaches an asymptotic value. TEM images suggest that below the critical concentration, a polydisperse micellar structure with fairly long, short and spherical micelles co-exist. However, above the critical concentration, a dense micellar network forms with no indication of branched micelles. Using shear rheology data, we demonstrate that the micellar chain length decreases beyond the critical concentration, while the entanglement number remains fairly constant. On the other hand, the maximum Trouton ratio measured in transient extensional experiments decreases as surfactant concentration is increased to c ≈ 3 wt%, and remains fairly constant around Tr ≈ 3, for higher concentrations. This finding is in-line with the structural parameters inferred from the shear rheology and also the TEM images. Therefore, we confirm that the extensional rheology is sensitive to structural transitions near the critical concentration. Finally, we show that the ratio of the extensional relaxation time to the shear relaxation time, λE/λ, is always less than unity regardless of whether relaxation is dominated by reptation or breakage of the wormlike micelles.

Extensional rheology of cellulose/NaOH/urea/H2O solutions

The extensional flow behaviors of cellulose/NaOH/urea/H2O solution were investigated by using capillary breakup extensional rheometry (CaBER). The effects of temperature, storage time and cellulose concentrations on both the storage modulus G′ and the loss modulus G″ were also analyzed. For 2 wt% cellulose solution, the G′, G″ and filament lifetime remained unchanged after long storage time. While, for 4 wt% cellulose solution, physical gels could form at either higher temperature or for longer storage time, and the filament lifetime, the relaxation time (λe) and the initial extensional viscosity (ηe0) first increased and then decreased with increase of the storage time. The transition points of the filament lifetime shifted to lower storage time with the increase of the temperature. The ηe0 is proportional to λe. The results presented suggest that the extensional properties of the cellulose/NaOH/urea/H2O solution first increase and then decrease during the gelation process, and the spinning time, which decreases linearly with the increase in the storage temperature, must be controlled below the time that ηe0 starts to decrease.

Extensional rheometry of cellulose ether solutions: flow instability

Capillary breakup extensional rheometry of semi-dilute hydroxyethyl cellulose (HEC) solutions was performed under several step-stretch conditions. The resulting parameters, i.e. terminal steady state extensional viscosity (η E ) and the timescale for viscoelastic stress growth, commonly referred to as the extensional relaxation time (λ E ) were found to be sensitive to the step-stretch conditions. The λ E decreased with increasing step-strain as opposed to the η E . Prior to the filament break-up, a ‘bead-on-string’ instability was observed close to the mid-plane. It is believed that this instability originated from the accumulation of viscoelastic stresses near the filament neck leading to the ‘elastic recoil’ of the extended polymer chains. The reasons for this belief are discussed in detail with the perspective of the past literature. Such type of flow instability has been reported for the first time for a cellulosic system. Various dimensionless numbers were plotted for the HEC solutions and compared with those obtained from past studies for various biopolymers as well as synthetic polymers.