Elongational Flow Field Research Articles

Hydrodynamic properties of DNA and DNA–lipid complex in an elongational flow field

The aim of this study was to determine the difference between hydrodynamic properties of DNA–cetyltrimethylammonium (CTA) complex and those of DNA, which may be related to the difference in fibre-forming ability of DNA–CTA from that of DNA. Responses of DNA and DNA–CTA complex to an elongational flow field were investigated. In both solution systems, results suggesting a coil–stretch transition were obtained. From a critical strain rate value, the radius of gyration of DNA–CTA molecules in ethanol–glycerol solution was revealed to be 0.3–0.5 times of that of DNA in aqueous NaCl solution. Shear viscosity of DNA–CTA solution was much smaller than that of DNA solution, also suggesting a smaller size of DNA–CTA in ethanol–glycerol solution than that of DNA in aqueous NaCl solution. The plateau birefringence value of the DNA–CTA system, a parameter that indicates the local molecular conformation and the molecular arrangement, was only about 1/10 of that of the DNA system. There is an empirically determined molecular model of DNA–CTA complex in which a DNA molecule is sheathed by a cylindrical crust made of CTA chains. This structure reduces the DNA molecular density in a pure elongational flow field region but cannot explain the observed reduction of birefringence intensity. The small plateau birefringence value of DNA–CTA compared with that of DNA was attributed to the reduced molecular polarizability by the particular conformation of DNA molecules and CTA chains in the DNA–CTA system such as that expected by the conformational models.

Constitutive equations for dilute bubble suspensions and rheological behavior in simple shear and uniaxial elongational flow fields

A theoretical model is proposed in order to investigate rheological behavior of bubble suspension with large deformation. Theoretical constitutive equations for dilute bubble suspensions are derived by applying a deformation theory of ellipsoidal droplet [1] to a phenomenological suspension theory [2]. The rate of deformation tensor within the bubble and the time evolution of interface tensor are predicted by applying the proposed constitutive equations, which have two free fitting parameters. The transient and steady rheological properties of dilute bubble suspensions are studied for several capillary numbers (Ca) under simple shear flow and uniaxial elongational flow fields. The retraction force of the bubble caused by the interfacial tension increases as bubbles undergo deformation. The transient and steady relative viscosity decreases asCa increases. The normal stress difference (NSD) under the simple shear has the largest value whenCa is around 1 and the ratio of the first NSD to the second NSD has the value of 3/4 for largeCa but 2 for smallCa. In the uniaxial elongational flow, the elongational viscosity is three times as large as the shear viscosity like the Newtonian fluid.

Steady state solutions of the Smoluchowski equation for rigid nematic polymers under imposed fields

We solve the Smoluchowski equation for steady state solutions of rigid nematic polymers and suspensions under imposed elongational flow, magnetic or electric fields, respectively. Under the three imposed fields, we show that (1) the Smoluchowski equation can be cast into a generic form, (2) the external field must parallel to one of the eigenvectors of the second moment tensor in steady states, and (3) the steady state solution of the Smoluchowski equation (probability density function or simply pdf) is of the Boltzmann type parameterized by material parameters and two order parameters governed by two algebraic-integral equations. Then, we present a complete bifurcation diagram of the order parameters with respect to the material parameters by solving the algebraic-integral equations. The stability of the pdf solutions is inferred from the minimum of the free energy density. The solution method is extended to dilute solutions of dipolar, rigid nematic polymers under an imposed electric field. The first moment of the steady state pdf is shown to be parallel to the external field direction at sufficiently strong permanent dipole or relatively weak dipole-dipole interaction. In this case, the steady solution of the Smoluchowski equation is parameterized by one order parameter and material parameters in the Boltzmann form. Otherwise, the first moment is not necessarily parallel to the external field direction.

Effect of flow on solutions of rod-coil block co-polymers

An elongational flow field is imposed on a solution of block copolymers consisting of semirigid macromolecules with rigid, rodlike sequences of units in combination with random coil (flexible) units. The problem is formulated according to the lattice treatment of Matheson and Flory. In this formulation, the system consists of rigid blocks whose lengths and locations are fixed by the structure within each macromolecule. These blocks are separated by random coiled units. An excess free energy other than the equilibrium Gibbs free energy of the quiescent solution has to be considered due to the flow field that tends to align the rods. This excess free energy is calculated from the potential energy of rods when a steady-state, homogeneous and irrotational flow field is applied to the solution. The effects of composition, polymer–solvent interaction, size of the co-polymer and flow rates are investigated. Depending on the size and number of rods, some of the chains studied exhibit a biphasic region at equilibrium that shifts to lower concentrations with increasing flow. Longer chains with shorter rods that are isotropic at equilibrium, exhibit a biphasic region at finite values of flow. The degree of orientation increases sharply when the system is biphasic. For larger flows, the orientation function is very close to unity which is perfect orientation.

Effect of the Flow Field on the Rheological Behavior of Aqueous Cetyltrimethylammonium p-Toluenesulfonate Solutions

It is well-known that solutions of cetyltrimethylammonium p-toluenesulfonate in water exhibit a pronounced shear-thickening phenomenon in a specific concentration range (0.1-0.8%) when they are subjected to simple-shear flows, as a consequence of flow-induced self-assembly of wormlike micelles. This work shows that a strong elongational flow field (opposed-jets flow), applied to the same solutions, does not lead to extension thickening because the extensional flow prevents or destroys micellar association. In flow through a porous medium, a substantial increase in apparent viscosity is observed beyond a critical apparent shear rate, which surpasses increases observed in simple-shear flows. This is explained as the result of a synergistic effect of shear and relatively weak elongation on the solution microstructure.

Transcrystalline Morphology of an in situ Microfibrillar Poly(ethylene terephthalate)/Poly(propylene) Blend Fabricated through a Slit Extrusion Hot Stretching‐Quenching Process

AbstractSummary: Isotactic poly(propylene) (iPP) transcrystallites are obtained in in situ microfibrillar polyethylene terephthalate (PET)/iPP blends during a slit extrusion‐hot stretching‐quenching process. Based on morphological information from X‐ray scattering and microscopy, three nucleation origins are proposed in microfibrillar reinforced blends under an elongational flow field: (a) the classical row nuclei model; (b) fiber nuclei; (c) nuclei induced by fiber assistant alignment. The last model provides a natural explanation for the case that transcrystallites only occur in some microfiber reinforced blends under flow rather than without the external field.AFM image for the transcrystalline layer of the microfibrillar blend.imageAFM image for the transcrystalline layer of the microfibrillar blend.

Principles of a nanoscale DNA scanner

A nanofluidic device for the routine stretching of single DNA molecules was hypothesised and tested. The device consists of an entrance channel leading to a post-field preceding an elongational flow field. The device facilitates each molecule's coil-to-stretch transition, counteracts its entropic recoil, and presents a stream of moving stretched molecules for detailed single-molecule time-of-flight measurements. The physics of DNA stretching was explored in a device where there was a juxtaposition of hexagonal upright post arrays with lithographically defined elongational flow fields.

Flow-induced chain scission as a physical route to narrowly distributed, high molar mass polymers

We present data showing a substantial narrowing of the polydispersity index (PDI) of high polymers occurring as a consequence of random chain scission events in a transient elongational flow field. In our experiments, semi-dilute aqueous solutions of high-molar mass, polydisperse polymers (PDI>1.4) were injected under pressure through an elongational flow field at the entrance of a capillary tube (i.d. 250 μm). Chain scission events occurring during multiple passes through the capillary entrance cause a marked decrease in PDI, to values as low as 1.12, along with the expected decrease of the average molar mass. The phenomenon appears to be entirely physical and independent of the chemical nature of the polymer, since similar results are obtained with polyacrylamide, polydimethylacrylamide, and poly(ethylene oxide). Statistical modeling of the evolution of the polymer molar mass distribution shows the results to be consistent with the random scission, near the mid-point, of those polymer chains that exceed a certain flow field-dependent critical chain length.

Sequential, Orthogonal Fields: A Path to Long-Range, 3-D Order in Block Copolymer Thin Films

The use of two orthogonal, external fields is shown to control the orientation of lamellar microdomains in three dimensions in diblock copolymer thin films. An elongational flow field was applied to obtain an in-plane orientation of the microdomains of the copolymer melt, and an electric field, applied normal to the surface, was then used to further align the microdomains. Thin films of symmetric diblock copolymers of poly(styrene-b-methyl methacrylate) with long-range order and orientation of the lamellar microdomains were obtained as evidenced by small-angle X-ray scattering and transmission electron microscopy.

Morphology Development of Polymer Blends in Complex Flow Fields

Polymer blends are an economically interesting alternative when it comes to the development of materials with new or improved properties, with these properties strongly dependent on the underlying morphology. The influence of typically complex flow fields in processing equipment, on the morphology, is still poorly understood. A completely immiscible concentrated PS/PMMA model blend was subjected to different types of shear, elongation and combined flow fields and the morphology development in a capillary entrance flow field, is described.

Analysis of the tensile modulus of poly(p‐hydroxybenzoate)/poly(ethylene terephthalate)/poly(ethylene 2,6‐naphthalate) ternary polyester composite fibers

AbstractLiquid crystalline polymer reinforced plastics were prepared by compounding (PHB/PEN/PET) blends. A fibrillar PHB structure was formed in situ in the PEN/PET matrix under a high elongational flow field during melt‐spinning of the composite fibers. The formation of PHB microfibrils in the composite fiber with different PHB contents and winding speeds was observed. The PHB microfibril reinforced PEN/PET composite fibers exhibited an unexpectedly low tensile modulus. We have evaluated the tensile modulus of the fibers using the non‐modified22 and a modified23 Halpin–Tsai model. From the analysis of both models, large differences were found between the theoretical and experimental values of the tensile modulus, and the low value of the tensile modulus of the composite fiber could not adequately be explained by either model. Thus, we analyzed the observed modulus values using the Takayanagi model,24 which describes the concept of mechanical discontinuities in semi‐crystalline polymers. Using the Takayanagi model, the effective fraction of continuous or discontinuous microfibrils was evaluated. Consequently, we could successfully explain the very low modulus of the PHB/PEN/PET composite fiber, having a large number of PHB microfibrils, using the Takayanagi model. Copyright © 2003 Society of Chemical Industry

Mechanical load on a particle aggregate in mono-axial elongational flow

Subject of the present contribution is the numerical simulation of the effect of a elongational flow field on a suspended particle-aggregate. The particle-aggregate consists of seven rigid spherical particles and is suspended in the flow field at low Re-numbers (Re=0.01–0.1). The ratio of particle and fluid densities varies between 1 and 15. The velocity and pressure distribution is obtained from the numerical solution of the Navier–Stokes equation and the continuity equation based on finite volume methods. The particle motion is obtained from the Euler equation of motion for rigid bodies. A comparison of classical solutions with the result of the numerical simulation for a spherical particle shows a very good agreement. It can be shown, that the interaction of the aggregate with the fluid differs clearly from that of a spherical particle. Furthermore, it has been found that the magnitude of stresses on the aggregate surface is increasing with time monotonously. Shear stress is maximum on the outer parts of the aggregate. Normal stress takes on maximum values on the upstream and downstream oriented faces. The maximum pressure drop across the particle results in an extensional force which increases in time within the considered period.

Phenomenological analysis of elongational flow birefringence in semidilute solutions of hydroxypropylcellulose

The relaxation time of a polymer chain in an elongational flow field was investigated for hydroxypropylcellulose (HPC) semidilute solution systems by two methods: phenomenological analysis of elongational flow-induced birefringence, and dynamic light scattering (DLS) and rheological measurements. To understand the relaxation time of an entangled semiflexible polymer solution in an elongational flow field, scaling analysis of the elongational flow-induced birefringence curve was performed. The results of both temperature and concentration scaling analyses showed that birefringence curves at different temperatures and at several HPC concentrations were described well by a universal birefringence–strain rate curve. This scaling behavior was compared with the "fuzzy cylinder" model. The critical strain rate corresponded to the correlation time of the slow relaxation mode determined by DLS measurement and the relaxation spectrum obtained by dynamic viscoelasticity measurement. The elongational flow-induced birefringence observed in an HPC semidilute solution was concluded to be attributed to the orientation of the HPC segment in the entangled molecular system, because the dominant relaxation mode is found to be the concentration fluctuation of an entangled molecular cluster in a quiescent state.

Stretching and Aligning Biopolymer Chains by a Flow Field.

One of the most useful method of understanding physicochemical properties of biopolymers as a thread of life is to pull and stretch them out. In this article, the method of manipulation of biopolymers by an elongational flow field was presented. As an elongational flow field generator, an opposed jets and a four roller mill apparatuses were shown. An ability of the elongational flow field of stretching flexible polymers was demonstrated by showing the elongational behavior of DNA molecules interacting with DNA binding protein HU. Orientation of monodisperse collagen molecules was shown as a model of rigid rod-like polymer orientation in the elongational flow field.

Cell neighbor list method for planar elongational flow: rheology of a diatomic fluid

We present simple changes to the cell method for neighbor list construction that enable it to be used in molecular dynamics studies of systems subject to a planar elongational flow field. The modifications for planar elongational flow are similar to those required for planar shear flow and should be easy to incorporate into any cell neighbor list method that is used in simulations of homogeneous shear. The execution time of the code at equilibrium is shown to be proportional to the number of particles N. The introduction of the modifications allowing shear, and more importantly, elongational flow are shown to affect the performance of the code in both CPU time and memory usage. The modifications to enable the simulation of planar elongational flow using the cell method of neighbor list construction will not introduce any higher order dependency if applied to code that is N-dependent in planar shear flow. We use this code to study large systems of diatomic molecules at low strain-rates, and find that the linear regime in planar elongational flow can be determined by the ratio of the two planar elongational viscosity functions. The properties investigated in planar shear flow, such as angular velocity and alignment angle, were inconsistent with the shear viscosity results in their evaluation of where the linear regime ends. The high precision of the results allowed us to accurately determine the coefficients in the retarded-motion expansion.

Shaping of Gelling Biopolymer Drops in an Elongation Flow

Shaping, defined as deformation in combination with gel formation of gelatine and κ-carrageenan drops in an elongation flow, was studied. The focus was to investigate the possibility of shaping and fixating small drops in the diameter range 20 to 229 μm. In the shaping progress and the influence of experimental properties, the viscosity, temperature, and flow of the deforming fluid were examined on the final drop shape. In the experiments a hot emulsion of an aqueous biopolymer solution in silicone oil was injected into cold silicone oil where a deforming elongation flow field existed. After injection, a temperature decrease in the drops resulted in a gel formation of the biopolymer and a fixation of the deformed drop in the flow. The shape was measured and the effect on the drop aspect ratio was determined by image analysis. Over the total drop diameter range, κ-carrageenan was more ellipsoid-shaped than gelatine, with a maximum aspect ratio of 6 compared to 4 for gelatine. For small drops, around 22 μm, it is possible to shape κ-carrageenan, but for gelatine small drops tend to be unaffected. An increase in viscosity, temperature, and flow resulted in an increase in the final fixated shape of the drops. The differences in drop deformation between the biopolymers were explained by drop–viscosity/oil differences and differences in the kinetics of gel formation. The different gel formation kinetics resulted in a short, well-defined, shaping process for κ-carrageenan, while for gelatine the process was more complex, with both deformation and relaxation present at different stages.

Elongational Flow of Solutions of Poly(ethylene oxide) and Sulfonated Surfactants

In this work, the elongational flow behavior of aqueous solutions of poly(ethylene oxide) (PEO) was studied in the presence of sulfonated surfactants. The technique of opposed-jets flow was used to generate an elongational flow field in which pressure drops were measured as a function of strain rates. The surfactants used were sodium dodecyl benzene sulfonate (SDBS) and an α-olefin sulfonate (AOS). Solutions of PEO and other flexible polymers exhibit extension thickening in opposed-jets flow due to the formation of transient networks of entangled molecules. This effect is present at concentrations below the static coil overlap concentration, due to the changes in molecular conformation induced by the flow. When SDBS or AOS are added to PEO solutions at low concentrations, the extension thickening weakens due to an increase in PEO intramolecular interactions that lead to coil contraction. This occurs until the surfactant concentration is close to the critical aggregation concentration reported in the literature. Further addition of surfactant induces the formation of intermolecular interactions as the PEO molecules are expanded by the electrostatic repulsion between attached micellar aggregates, with an associated strengthening of extension thickening. Intramolecular effects were not seen beyond a specific PEO concentration.

Flow through a convergence. Part 2: Mixing of high viscosity ratio polymer blends

AbstractDispersive mixing of high viscosity ratio blends was studied in a converging flow using a capillary rheometer equipped with dies having different entry profiles. Three inhomogeneous bimodal polyolefin blends, with stress‐dependent viscosity ratios ranging from 8 to 450, were used in this work. Such magnitudes of viscosity ratio indicate that the dispersed droplets can be mixed only in an elongational flow field. The mixing efficiency was found to be dependent on both the profile of the convergence and flow rate. At the lowest flow rates, the dispersive mixing efficiency was very low, but it increased with an increasing flow rate until a profile‐dependent maximum. This maximum mixing efficiency was observed prior to fracture of the matrix material, after which the efficiency decreased. Stress and deformation fields within different profiles were estimated by numerical simulation using the K‐BKZ equation, and the results were used to interpret experimental results. The dispersive mixing efficiency was found to be proportional to the maximum elongational stress within the converging section, and to the length of the region where the critical conditions for elastic fracture of droplet material were met. It is shown that the dispersive mixing mechanism in high viscosity ratio blends is mainly dictated by elastic fracture of the droplet, and hence is applicable over a wide range of polymer blending processes.

Rheology–Structure Relationship of Polymer/Layered Silicate Hybrids

Linear and non-linear rheology of polymer/layered silicate hybrids(nanocomposites) of organically modified smectite clay in poly(methylmethacrylate) (PMMA) were studied in oscillatory shear and elongationflow for understanding the relation between rheology and structure inthe lights of time-dependent phenomena. Various rules for the linearviscoelastic response were proved by a comparison of the theoreticalpredictions with the experimental data at the low amplitude oscillatoryshear. Time-temperature superposition principal, as well as the relationbetween the relaxation time spectrum and the linear functions, asrelaxation modulus G(t) and elongation viscosity η E (t) were verified.Relaxation spectrum is shifted towards the longer relaxation time scalesfor 10 and 15 wt% SPN/PMMA hybrids, if compared with that of thematrix PMMA. Both the large-amplitude oscillatory shear and the fastelongation produced much stronger non-linear effects of hybrids than thatof pure PMMA. It is proposed that, the non-linear damping of hybridsunder step strain is a result of the strain-induced alignment of themultilayered clay domains in a shear flow field. Moreover, theretardation of the polymer relaxation of hybrids probably gives rise tothe strain-hardening properties under fast elongation. Highly orientedand elongated multilayered domain structure of hybrids in an elongationflow field has been proposed as a microstructural origin of both strainhardening and birefringence.

Elongation flow studies of DNA as a function of temperature

The response of T4-phage DNA molecules to an elongational flow field was monitored by flow-induced birefringence as a function of temperature. The flow-induced birefringence observed in this study was localized in the pure elongational flow area with a critical strain rate, indicating that the birefringence was attributed to a coil–stretch transition of DNA molecules. The slight decrease in the birefringence intensity with increases in temperature to 40°C was explained by a thermal-activation process. At temperatures above 50°C, flow-induced birefringence decreased remarkably, and no birefringence was observed at temperatures above 60°C. After the flow experiments, ambient temperature was reduced back to room temperature, and flow experiments at room temperature were performed again. Flow-induced birefringence was recovered almost completely in samples for which the first flow measurements were made at temperatures below 53°C. Irreversible changes were observed for samples for which the first flow experiments were performed at temperatures above 55°C. The temperature dependence of UV-absorption spectra revealed that the double-strand DNA helix began to partially untwine at a temperature over 50°C, and duplexes became almost completely untwined at a temperature over 55°C. A comparison of electrophoresis patterns for untwined molecules showed that flow-induced scission of DNA molecules occurred in a sample solution in flow experiments performed at 65°C, while no molecular weight reduction was observed in the sample solution at 55°C. In this article, this difference between the untwined DNA molecules is discussed on the basis of the thermally activated bond scission (TABS) model. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 1357–1365, 2002