Flow Birefringence Studies Research Articles

Birefringent properties of aqueous cellulose nanocrystal suspensions

Birefringence measurements of aqueous cellulose nanocrystal (CNC) suspensions are reported. Seven suspensions with concentrations between 0.7 and 1.3% per weight are sheared in a Taylor-Couette type setting and characterized using a birefringent measurement technique based on linear polarized light and acquisition with a polarization camera. Steady state measurements with shear rates up to 31 1/s show extinction angles of 23°–40° and birefringence in the order of 1e–5. The findings demonstrate the utility of CNC suspensions for flow birefringence studies.

Rheological and flow birefringence studies of rod-shaped pigment nanoparticle dispersions

We study rheological and rheo-optical properties of suspensions of anisometric pigment particles in a non-polar fluid. Different rheological regimes from the dilute regime to an orientationally arrested gel state were characterized and compared with existing theoretical models. We demonstrate the intricate flow behaviour in a wide range of volume fractions. A unique combination of the optical properties of the particles results in a giant rheo-optical effect: an unprecedentedly large shear stress-induced birefringence was found in the isotropic range, exhibiting a sharp pre-transitional behaviour.

Optical and hydrodynamic properties of solutions of copolymers of N,N-dimethylaminoethyl methacrylate and 2-deoxy-2-methacrylamido-D-glucose that contain silver particles

Ag0 nanoparticles are prepared via the reduction of silver ions by copolymers of N,N-dimethylaminoethyl methacrylate and 2-deoxy-2-methacrylamido-D-glucose of various compositions in water at room temperature. It is found that an insignificant part of the macromolecules is involved in the reduction of Ag+ to Ag0 and stabilization of the resulting nanoparticles. As shown by flow-birefringence studies, the optical properties of solutions of stabilized silver nanoparticles change abruptly relative to the optical properties of the solutions of the initial copolymers.

Flow-Induced Conformational Changes in Gelatin Structure and Colloidal Stabilization

Flow can change the rate at which solutes adsorb on surfaces by changing mass transfer to the surface, but moreover, flow can induce changes in the conformation of macromolecules in solution by providing sufficient stresses to perturb the segmental distribution function. However, there are few studies where the effect of flow on macromolecules has been shown to alter the structure of macromolecules adsorbed on surfaces. We have studied how the local energy dissipation alters the adsorption of gelatin onto polystyrene nanoparticles ( r = 85 nm). The change in the nature of the adsorbed layer is manifest in the change in the ability of the nanoparticles to resist aggregation. Circular dichroism spectroscopy was used to assess conformational changes in gelatin, and dynamic light scattering was used to assess the colloid stability. Experiments were conducted in a vortex jet mixer where energy density and mixing times have been quantified; mixing of the gelatin and unstable nanoparticles occurs on the order of milliseconds. The adsorption of the gelatin provides steric stabilization to the nanoparticles. We found that the stability of the gelatin-adsorbed nanoparticles increased with increasing mixing velocities: when the mixing velocities were changed from 0.9 to 550 m/s, the radius of the nanoclusters (aggregates) formed 12 h after the mixing decreased from 2620 to 600 nm. Increasing temperature also gave rise to similar trends in the stability behavior with increasing temperature, leading to increasing colloid stability. Linear flow birefringence studies also suggested that the velocity fields in the mixer are sufficiently strong to produce conformational changes in the gelatin. These results suggest that the energy dissipation produced by mixing can activate conformational changes in gelatin to alter its adsorption on the surfaces of nanoparticles. Understanding how such conformational changes in gelatin can be driven by local fluid mechanics and how these changes are related to the adsorption behavior of gelatin is very important both industrially and scientifically.

Cross-slot extensional flow birefringence observations of polymer melts using a multi-pass rheometer

This paper reports initial experimental flow birefringence studies of cross-slot flow for a range of molten polyethylenes using a multipass rheometer (MPR). An MPR was modified to accommodate a cross-slot center section insert thereby enabling small quantities of molten polymer to be evaluated in a near two-dimensional pure shear extensional flow. The results show that it was possible to obtain precise processing data for cross-slot flow using 10g quantities of polymer. Experimental observations on different polyethylenes showed that in all cases the flow commenced from a Newtonian situation where the optical birefringent fringes around the central stagnation point were essentially symmetric. As the flow developed, the fringe pattern became progressively more asymmetric about the exit symmetry plane and the extent of the viscoelasticity of the polymer could be quantified by the observed level of anisotropy between the birefringence fringes on the entry and exit symmetry planes of the flow. The progressive development of anisotropy and stress with increasing extensional flow rate and type of polymer is presented for a number of flow conditions and it was found that increasing viscoelasticity of the polymer resulted in a progressive increase in viscoelastic center-line extensional stresses. The data provide benchmark large strain extensional flow experiments for certain polymer melts that can in the future be used to compare with numerical simulation.

Flow birefringence study of the stick–slip instability during extrusion of high-density polyethylenes

The flow behavior of two molten linear high-density polyethylenes (HDPE) is carefully studied using a single screw extruder equipped with a transparent slit die. Flow curves are drawn for both resins and compared. Slit die experiments are performed using birefringence technique to obtain stress fields, and image analysis is developed to visualize the stress transients during the instability. At low flow rate, the extrusion is stable. At more important flow rate, the so-called spurt or stick–slip instability appears. During the instability, periodic birefringence fringes pulsations are observed, synchronized with pressure oscillations, suggesting a periodic stick–slip transition mechanism. Correlation with velocity oscillations is discussed.

The application of the multi-pass rheometer for precise rheo-optic characterisation of polyethylene melts

This paper presents an overview of the application of the Multi-Pass Rheometer (MPR) for the study of polymer processing with particular reference to flow birefringence measurements. The MPR is a two-piston capillary rheometer recently developed at the University of Cambridge. We review previous experiments that have been carried out on molten polymers using the MPR and we report on recent results of flow birefringence experiments for a polyethylene (PE) melt. Using a specially designed optic flow cell, molten PE was passed within a Double Cavity Die. A field-wise flow-induced birefringence technique was used to obtain the stress field in a complex flow of a flowing molten polymer. In general, for the PE tested, the entry stress field in the second cavity was found to be different from that in the first. The differences observed in the overall entry stress fields in the two slit configuration could be explained as resulting from the extensional and shear strain history experienced by the polymer. We have carried out matching K-B.K.Z.–Wagner numerical simulations and found that reasonable consistency can be achieved for certain polymers. As a consequence of these experiments, we have found that the MPR is a very good platform for experimental flow birefringence studies that can be matched with numerical simulation and in addition experiments can be carried out with small 20 g quantities of material.

Flow birefringence study of sharkskin and stress relaxation in polybutadiene melts

In this paper we report rheo-optical and rheological observations made through a transparent slit die attached to a capillary rheometer. We find that the flow birefringence signal oscillates periodically near the die exit when sharkskin-like extrudate distortion is present. In contrast, steady behavior is observed in the die inland region. Specifically, the flow birefringence varies at the die exit with a period identical to that measured directly from the sharkskin extrudate. We also show that the exit flow instability leading to sharkskin can be observed directly through cross-polarizers in terms of the temporal change of the retardation order. We demonstrate that the same kind of interfacial flow instability can occur at a boundary discontinuity within the die land where the upper portion of a clean die wall meets the lower portion of a polysiloxane-coated die wall. Finally, stress relaxation upon the cessation of the slit die flow of two polybutadiene melts is studied through time-dependent flow birefringence measurements. The stress relaxation is then correlated with sharkskin time scales to describe the role of relaxation in sharkskin ridge formation.

Flow birefringence and computational studies of a shear thinning polymer solution in axisymmetric stagnation flow

Flow of a shear thinning, moderately concentrated solution of monodisperse high molecular weight polystyrene has been studied in a nonhomogeneous axisymmetric stagnation flow. Shear and extensional rheometry have been used to characterize the rheology of the solution. Multimode Giesekus and Phan–Thien–Tanner (PTT) models, and a single mode PTT model with a White–Metzner rate dependent relaxation time (PTT–WM) were fit to the rheological data. Transient uniaxial elongational measurements show significant extension hardening, which could only be predicted by the PTT model. The Giesekus model provided the best representation of shear flow data, but substantially underpredicted the measured extensional viscosity. Flow birefringence was used to measure integrated axisymmetric shear and normal stress profiles as a function of position. These measurements were compared with the results of numerical simulations, performed using an adaptive finite element technique, with the three constitutive models. The computed results with the 3-mode Giesekus model accurately portrayed experimental shear and normal stresses in both forward and rear stagnation flow up to Weissenberg numbers ( We) of O(1). At higher We, the Giesekus model underpredicts normal stresses in the rear stagnation flow, where elongational gradients dominate. Both PTT and PTT–WM models gave rise to unrealistically large extensional stresses in the rear stagnation flow even at moderate We which led to poor agreement with experimental data. On the basis of these comparisons, it appears that the elongational properties of the solution are intermediate between the Giesekus and PTT, but much closer to the Giesekus, predictions. This is in conflict with the experimentally measured uniaxial viscosity, which suggests that the PTT model should be superior in flows with significant uniaxial extension.

Photoelastic tomography for three-dimensional flow birefringence studies

The possibility of applying photoelastic tomography (integrated photoelasticity) for investigating a three-dimensional birefringent flow is studied. Tomographic equations of the strain rates are reduced to a vector analogue of the Radon equation by dividing the flow-velocity field into an axial component, a transversal rotational, and a transversal potential component. It is shown how the axial and potential parts can be determined by tomographic photoelastic measurements.

Gaussian chains with excluded volume and hydrodynamic interaction: shear rate dependence of radius of gyration, intrinsic viscosity and flow birefringence

The effect of excluded volume (EV) and hydrodynamic interaction (HI) on a dilute solution of flexible polymer chains situated in a shear field has been studied by means of a Brownian dynamics simulation. The polymer was modelled as a Gaussian chain, accounting for the excluded volume effect by means of an exponentially decaying repulsive potential. The hydrodynamic interaction was included by means of the Rotne-Prager-Yamakawa tensor. The parameters studied were the mean square radius of gyration, the intrinsic viscosity, and the differential polarizability and extinction angle that are parameters relevant in flow birefringence studies. The EV-effect is seen to be most important at low shear rate. This is followed by an intermediate region where EV falls off and HI still has considerable effect, ending up with a chain that is so stretched out that neither EV nor HI has influence on conformational or hydrodynamical properties, giving parameter values in accordance with theory for the no-EV, no-HI case.

Flow birefringence in axisymmetric geometries

Flow birefringence has become increasingly popular as a tool in non‐Newtonian fluid mechanics, where it provides a means of measuring stress distributions in complex polymer flows. In order to access a wider range of kinematic complexity in the testing of constitutive models, we explore the application of flow birefringence in axisymmetric geometries. Unlike traditional applications of flow birefringence in two‐dimensional flows, there is no direct relationship between measured optical properties and stresses in the material. Rather, the optical properties reflect the cumulative effect of the axisymmetric refractive index (and hence stress) tensor on the polarization state of light. Since a typical goal of flow birefringence studies is comparison between experiment and computation, we propose that optical property distributions could serve as the basis for comparison rather than attempting to invert optical data to yield radial stress distributions. In the limit of small total retardance, there are particularly simple relationships between experimental observables and integrals of the stress distributions. Experiments are performed on an index‐matched Newtonian test fluid in a prototype axisymmetric geometry (tube flow past an obstruction with a hemispherical cap) in order to illustrate the principles of the technique.

Rheological and flow birefringence studies of a shear‐thickening complex fluid—A surfactant model system

A common surfactant solution system is investigated to demonstrate the characteristic and generic features observed in shear‐thickening self‐assembling fluids. Rheological and flow birefringence measurements on the aqueous CTAB/sodium‐salicylate solutions disclose a number of unusual flow behaviors under different flow conditions. All solutions with concentrations ranging from 0.5 to 5 mM exhibit shear thickening above certain critical shear rates, accompanied by an increase in flow birefringence over a range of temperature from T=14 to 36 °C. An extraordinarily long induction period, which is found to be inversely proportional to the applied shear rate, precedes the establishment of the shear‐thickening state. This dynamic state is highly flow birefringent, strongly nonlinear viscoelastic and possesses long relaxation times. We deduce that shear‐induced coagulation results in self‐assembling of surfactant molecules to form new micelles which are of exceedingly large sizes and which can survive against th...

Flow birefringence studies of a concentrated polystyrene solution in a two‐roll mill. I. Steady flow and start‐up of steady flow

AbstractWe have studied a 4.8 volume percent solution of a narrow distribution polystyrene with molecular weight 3.84 × 106 in flows generated by a co‐rotating two‐roll mill. These flows have a stagnation point at the midpoint between the roller axes. Further, they are linear, two‐dimensional, and the magnitudes of the strain‐rates are greater than the vorticity. The overall objective of our studies is to explore the dynamics of concentrated polymer solutions which are in the highly deformed state that is generated in the two‐roll mill. Birefringence data are presented for both steady flow and start‐up of steady flow in the two‐roll mill. The steady and transient data are used to analyze the linear and nonlinear viscoelastic regions of material behavior. In the nonlinear regime, the birefringence upon start‐up shows an initial overshoot followed by a strong undershoot that is enhanced as the ratio of elongation to rotation is increased (i.e., the flow becomes increasingly extensional in character.) We attribute this undershoot, which does not seem to appear in simple shear flows, or flows close to simple shear flow, to polymer segment stretching following an initial period of segment reorientation. Model studies are currently underway to test this notion. © 1992 John Wiley & Sons, Inc.

Viscoelastic and flow birefringence studies of compatible polymer blends (abstract)

Viscoelastic and flow birefringence studies of compatible polymer blends (abstract)

Flow birefringence studies in transient flows of a two roll mill for the test-fluid M1

We have studied the test-fluid M1 in flows generated by a two-roll mill. These flows are linear, and two-dimensional, and the magnitudes of the strain-rates are greater than the vorticity. The polymeric solution is shown to degrade significantly, even for small values of the velocity gradient, as measured by the changes in the macroscopic relaxation time-scales. Consequently, the determination of macroscopic properties such as elongational viscosity based upon the overall pressure difference, or the total force as measured in a spin-line rheometer, may not be representative of the properties of the fresh, undegraded fluid. In this work, the degradation of the test-fluid has been studied as a function of the strength of the velocity gradient for different values of the ratio of the strain-rate to vorticity of the flow. Furthermore, the observed relaxation time-scales have been evaluated based on data for steady state flows, and several transient flow histories such as start-up and cessation of flows, and double-step flows. These flow histories provide us with some insight into the relaxation dynamics of the polymeric structure for different initial conformation states.

Annealing of Gelsolin-severed Actin Fragments by Tropomyosin in the Presence of Ca2+: Potentiation of The Annealing Process by Caldesmon

Previous results from our laboratory have shown that 1) cultured rat cells contain two classes of tropomyosin (TM), one (high Mr TMs) with higher Mr values and greater affinity for actin than the other (low Mr TMs); 2) presaturation of F-actin with high Mr TMs, but not with low Mr TMs, inhibits both actin-severing and actin binding activities of gelsolin; and 3) nonmuscle caldesmon not only enhances the inhibitory effects of high Mr TMs but also makes low Mr TMs capable of inhibiting the severing activity of gelsolin (Ishikawa, R., Yamashiro, S., and Matsumura, F. (1989) J. Biol. Chem. 264, 7490-7497). These results suggest that gelsolin has much lower affinity for F-actin-TM-caldesmon complexes than for pure F-actin. We have therefore examined whether addition of TM and/or caldesmon to gelsolin-severed actin filaments can make gelsolin dissociate from barbed ends of actin filaments, resulting in annealing of short actin filaments into long ones. Flow birefringence and electron microscopic studies have suggested that high Mr TMs slowly and partially anneal gelsolin-severed actin fragments in 3 h, whereas low Mr TMs have no effects. Nonmuscle caldesmon greatly potentiates the effects of high Mr TMs and accelerates the process to 20 min, whereas nonmuscle caldesmon alone shows no effects. Furthermore, nonmuscle caldesmon makes low Mr TMs capable of reversing gelsolin-severing action. Actin binding assay has shown that gelsolin (or a gelsolin-actin complex) is dissociated from these annealed actin filaments. Smooth muscle TM and smooth muscle caldesmon also appear to anneal gelsolin-severed actin fragments as do high Mr TMs and nonmuscle caldesmon. Calmodulin decreases the potentiation effects of caldesmon as calmodulin inhibits actin binding of caldesmon. These results suggest that tropomyosin and caldesmon may regulate both capping and severing activities of gelsolin.

Flow birefringence study at the transition from laminar to Taylor vortex flow

We have studied the birefringence induced by the flow in a liquid containing very fine particles in the vicinity of the transition between the Couette flow and the Taylor vortex flow. We derive at first an analytical expression of the axisymmetrical hydrodynamical field and then the angular velocity of a particle in the flow. The resolution of the diffusion equation leads to the orientation distribution function F. On condition of some simplifying but realistic hypotheses, we show that the extinction angle χ and the birefringence Δn of the solution are the same as the properties of an equivalent stratified medium composed of very thin layers the orientation of which varies continuously. The results concerning the velocity profile and the critical Taylor number are in good agreement with other sources ; the theoretical computation of χ and Δn are then compared to experimental curves realized with solutions of bentonite and T.M.V. ; interesting conclusions can be drawn on the influence of the shape of the particles on the behavior of the angle of extinction and of the birefringence intensity in the region of the Taylor vortex flow.

A study of solutions of copolymers of higher alkyl methacrylates by flow birefringence and electron microscopy methods

Viscometry, flow birefringence, and electron microscope study of solutions of these copolymers with vinyl acetate show that solutions of these copolymers form solvates. The supermolecular formations thus arising have an ordered character in solutions of paraffin-naphthalene solvent only when the comonomers are copolymerized in this solvent.

Flow visualization and birefringence studies on planar entry flow behavior of polymer melts

Both flow visualization and flow birefringence results are presented for polystyrene and low density polyethylene (LDPE) in planar entry flow for contraction ratios of 4 : 1 and 8 : 1. It was found that vortex growth was a different function of Weissenberg number ( We) for each polymer and hence fluid elasticity could not be solely used to account for the onset and growth of vortices. Based on extensional stress measurements along the centerline,a normalized by means of the down-stream wall shear stress, it was proposed that vortex growth and intensity are a function of the extensional viscosity and to some degree the shear viscosity. In particular, LDPE, which exhibits a strain hardening extensional viscosity and an early onset of shear-thinning, exhibits large and intense vortices while polystyrene, which does not exhibit strain hardening, exhibits only small low intensity vortices. The purpose of the study was to emphasize the importance of a fluid's extensional properties, relative to its shear properties, in determining entry flow behavior.