Flow-induced Birefringence Research Articles

The effect of high pressure microfluidization on the structure and length distribution of whey protein fibrils

The effect of high pressure microfluidization on native β-lactoglobulin (β-lg) or whey protein isolate (WPI), both before and after heat-induced protein fibril formation at pH 2.0, was investigated using atomic force microscopy (AFM), shear birefringence, reversed phase-high pressure liquid chromatography, attenuated total reflectance–Fourier transform infrared spectroscopy and fluorescence spectroscopy. The morphology and length distribution of the fibrils were determined using AFM and flow-induced birefringence, respectively. High pressure (≥50 MPa) microfluidization treatment of β-lg induced ∼30% protein denaturation, accompanied by changes in secondary structure. Fibrils formed from high pressure treated β-lg or WPI were similar in length to fibrils formed from non-pressure treated proteins. High pressure (≥50 MPa) microfluidization of fibrils formed from β-lg or WPI resulted in their breakup into more uniformly sized and much shorter fibrils. Microfluidization pressures of up to 170 MPa resulted in slightly shorter fibrils but did not completely dissociate them.

Mechanical and thermal stabilities of peptide amphiphile fibres

In this study the thermal and mechanical stabilities of self-assembled fibres of palmitoyl functionalized GANPNAAG have been examined. The thermal stability was investigated with circular dichroism, dynamic light scattering and fluorescence measurements monitoring the disassembly upon heating and reassembly upon cooling. The determination of the temperature dependence of the critical aggregation concentration allowed us to estimate the thermodynamic parameters involved in the fibre formation process. The mechanical stability of the fibres was studied using flow-induced birefringence. The peptide based self-assembled structures were found to be thermally stable up to 70 °C, while being fractured by moderate shear flow.

Extensional rheology of human saliva

We have developed an oscillatory cross-slot extensional rheometer capable of performing measurements with unprecedentedly small volumes of test fluids (∼10–100 μL). This provides the possibility of studying exotic and precious or scarce bio-fluids, such as synovial fluid. To test our system, we have looked at a relatively abundant and accessible biological fluid, namely human saliva; a complex aqueous mixture of high molecular weight mucin molecules and other components. The results represent our first attempts to by this technique and as yet we have only sampled a small dataset. However, we believe we have produced the first successful quantitative measurements of extensional viscosity, Trouton ratio, and flow-induced birefringence made on saliva samples. The results significantly add to the scant literature on saliva rheology, especially in extension, and demonstrate the important role of saliva extensibility in relation to function.

Frozen-in birefringence and anisotropic shrinkage in optical moldings: II. Comparison of simulations with experiments on light-guide plates

The thermally-, flow-induced and total birefringence components and anisotropic shrinkages in LGP moldings were simulated by using a combination of a CV/FEM/FDM technique nonlinear viscoelastic and photoviscoelastic constitutive equations and orientation functions, as described in Part I of this study. The simulated results were compared with measurements on LGP moldings of a polystyrene (PS) and two optical grade polycarbonates (PCs) OQ1030 and OQ3820 having low and high molecular weights. The thermally-induced birefringence was simulated by a combination of constrained and free cooling during molding. In LGP moldings of PS, the simulated thermally-induced birefringence indicated a minor variation with location in the mold plane, a parabolic shape in the core region and an increase towards the wall. Compared to the flow-induced birefringence, the thermal birefringence provided a minor contribution to the total transverse birefringence Δn12. In LGP moldings of PCs, the simulated thermally-induced birefringence showed a significant variation with location in the mold plane, nearly constant value in the core region and high value in the wall region. In LGP moldings of both PCs, the contributions of the thermally- and flow-induced birefringence to the total transverse birefringence Δn12 were significant. The effect of processing conditions on the development of the normal birefringence in LGP moldings of PCs was ranked from most to least: the packing pressure, mold temperature, melt temperature, injection speed and packing time. However, in LGP moldings of PS the packing time effect was significant due to a longer gate freezing time. Simulated and measured normal birefringence along the flow direction was in fair agreement, but simulations were unable to describe the observed birefringence maximum arising near the gate. The averaged luminance of LGP moldings exhibited some correlation with the averaged normal birefringence. LGP moldings of PC OQ1030 indicated a pronounced maximum in the simulated transverse flow birefringence in the core but a low value near the wall. In contrast, the LGP molding of PC OQ3820 showed a high simulated birefringence near the wall and a low value of maximum in the core. The simulated and measured total transverse birefringence in LGP moldings was in fair agreement. LGP molding of both PCs showed similar tendency in shrinkage variation with processing conditions. However, the thickness shrinkage was higher in LGP moldings of PC OQ3820. The effect of processing conditions on the development of shrinkage in LGP moldings of both PCs was ranked from most to least: the packing pressure, melt temperature, mold temperature, injection speed and packing time. In LGP moldings of PS, the thickness shrinkage slightly increased with increasing melt temperature and significantly increased with reducing packing time. A good agreement between the simulated and measured anisotropic shrinkages in LGP moldings at various processing conditions was observed.

Flow of wormlike micelle solutions past a confined circular cylinder

In this paper, we present the results of an investigation into the flow of a series of viscoelastic wormlike micelle solutions past a confined circular cylinder. Although this benchmark flow has been studied in great detail for polymer solutions, this paper reports the first experiments to use a viscoelastic wormlike micelle solution as the test fluid. The flow kinematics, stability and pressure drop were examined for two different wormlike micelle solutions over a wide range of Deborah numbers and cylinder to channel aspect ratios. A combination of particle image velocimetry and pressure drop measurements were used to characterize the flow kinematics, while flow-induced birefringence measurements were used to measure the micelle deformation and alignment in the flow. The pressure drop was found to decrease initially due to the shear thinning of the test fluid before increasing at higher flow rates as elastic effects begin to dominate the flow. Above a critical Deborah number, an elastic instability was observed for just one of the test fluids studied, the other remained stable for all Deborah number tested. Flow-induced birefringence and velocimetry measurements showed that observed instability originates in the extensional flow in the wake of the cylinder and appears not as periodic counter-rotating vortices as has been observed in the flow of polymer solutions past circular cylinders, but as a chaotic rupture event in the wake of the cylinder that propagates axially along the cylinder. Reducing the cylinder to channel aspect ratio and the degree of shearing introduced by the channel walls had a weak impact on the stability of the flow. These measurements, when taken in conjunction with previous work on flow of wormlike micelle solutions through a periodic array of cylinders, definitively show that the instability can be attributed to a breakdown of the wormlike micelle solutions in the extensional flow in the wake of the cylinder.

Frozen-in birefringence and anisotropic shrinkage in optical moldings: I. Theory and simulation scheme

An unconditionally stable upwinding scheme was proposed to improve the efficiency of the viscoelastic simulation in molding of optical products using a CV/FEM/FDM technique. A significant computation time saving was achieved due to an elimination of subdivision of the time step as required in the conventional numerical scheme. The approach was applied to simulate the flow-induced birefringence and anisotropic shrinkage in disk moldings using a nonlinear viscoelastic constitutive equation, orientation functions and equation of state. The two-dimensional triangular finite element meshes were used in the disk cavity and the one-dimensional tubular elements were utilized in the delivery system. Good agreement was shown between the simulated pressure traces and flow birefringence in the molding using the unconditionally stable upwinding scheme of the present study and the conventional numerical scheme of the earlier study. In addition, an algorithm for simulation of the thermal stresses and birefringence in moldings using linear viscoelastic and photoviscoelastic constitutive equations was presented by combining constrained and free quenching approaches. The proposed numerical scheme for viscoelastic simulation of injection molding is more suitable for future commercial applications.

Velocity measurements for shear flows of CTAB/NaSal aqueous solutions between parallel plates

Shear flows of wormlike micellar solutions between parallel plates were experimentally investigated using aqueous solutions of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal) as test fluids. The solution having a molar concentration ratio of NaSal to CTAB of 2.0 was primarily considered. The velocity between the plates was measured by particle tracking velocimetry. In addition, flow-induced birefringence and shear stress were measured using a parallel-plate rheometer. The measurements were carried out for shear rates at which shear thinning or shear thickening was observed. Banded velocity profiles were found at every shear rate considered in the present experiments, and the profile was stable at relatively low shear rates but fluctuated with time at high shear rates. Furthermore, multi-band velocity profiles were observed in the shear-thinning region. The results of velocity measurements in the shear-thickening region suggest that the velocity profile largely varies with time, accompanying the movement of the position of a band interface. Moreover, both birefringence and shear stress fluctuated with time at high shear rates. These results suggest that the fluctuation of the velocity profile is related to the structural change in the micellar network that leads to the change in the thickness fraction of oriented and isotropic layers or to three-dimensional flows induced by material instabilities.

Sphere settling in an aging yield stress fluid: link between the induced flows and the rheological behavior

The flow pattern induced by the settling of a non-Brownian sphere in a fluid depends on the rheological properties of that fluid. For instance, at small Reynolds numbers, the pattern presents a fore–aft symmetry in a Newtonian fluid, whereas, in some viscoelastic polymer solutions, it can exhibit a negative wake, i.e., an upward flow in the sphere’s wake. This study is an experimental work on the settling of a sphere in a suspension of a synthetic colloidal clay, laponite. The fluid is a yield stress fluid that ages, i.e., whose rheological properties evolve over time. We show that the settling velocity of a given sphere, as well as the induced flow pattern, are strongly modified as the fluid ages. In particular, the flow pattern asymmetry increases with the age of the fluid, and a negative wake eventually forms. We relate those modifications to rheological measurements and suggest, in line with works dealing with polymer solutions, that it is the increase in the fluid viscoelasticity that is responsible for the formation of a negative wake. The flow field measurements are also compared with flow-induced birefringence measurements, and we show that very slow relaxation processes are involved in the sphere settling.

An experimental evaluation of the behaviour of mono and polydisperse polystyrenes in Cross-Slot flow

The behaviour of a number of mono and polydisperse polystyrenes are probed experimentally in complex extensional flow within a Cross-Slot geometry using flow-induced birefringence. Polystyrenes with similar molecular weight (M w) and increasing polydispersity (PD) illustrated the effect of PD on the principal stress difference (PSD) pattern in extensional flow. Monodisperse materials exhibited only slight asymmetry at moderate flowrates, although increased asymmetry and cusping was observed at high flowrates. The response of monodisperse materials of different M w at various flowrates is presented and characterised by Weissenberg numbers for both chain stretch and orientation using a theory for linear entangled polymers. The comparison of stress profiles against Weissenberg number for each process is used to determine whether the PSD pattern observed is independent of M w and elucidate which relaxation mechanism is dominant in the flow regimes probed. For monodisperse materials, at equivalent chain orientation Weissenberg number (We τd), different molecular weight materials were seen to exhibit similar steady state PSD patterns independent of We τR (chain stretch We). Whilst no obvious critical Weissenberg number (We) was found for the onset of increased asymmetry and cusping, it was found to occur in the “orientating flow without chain stretch” regime.

高分子水溶液の微小円柱まわりの流れ中における流動複屈折分布(<小特集>非ニュートン性,機能性を示す複雑流体の流動と応用)

Flow-induced birefringence distribution and velocity field were measured in a slit flow containing a tiny cylinder, which was 550μm in diameter and 200μm in height. Xanthan gum solution, which has a non-Newtonian property owing to network structure in the liquid, was used for a test fluid. The laser beam was collected, resulting beam diameter was 6μm, and the birefringence distribution was measured around the tiny cylinder in every 100μm step in space. Consequently, in the upstream region of the cylinder, birefringence became small as the fluid particle approached the cylinder. This is because the flow is decelerating upstream of the cylinder and the fluid particle is elongated in the direction perpendicular to the flow. Furthermore, just upstream of the cylinder the birefringence became large and the molecular orientation was aligned in the direction perpendicular to the flow. On the other hand, downstream of the cylinder the flow is accelerating, thus the fluid particle is elongated in the flow direction. As a result, the birefringence becomes large on the centerline behind the cylinder. This result shows that the strain of the fluid particle generated by flow behind the cylinder is not disappeared even at the fully downstream region.

Thioflavin T fluorescence assay for β-lactoglobulin fibrils hindered by DAPH

The molecule 4,5-dianilinophthalimide was recently found to be an efficient compound in disaggregating amyloid fibrils involved in the Alzheimer's disease. In this study we have investigated whether the compound 4,5-dianilinophthalimide was able to disaggregate fibrils derived from beta-lactoglobulin. In addition to a Thioflavin T fluorescence assay, flow-induced birefringence was used as an independent technique to measure the total length concentration of the fibrils. An additional advantage of the latter technique is that not only the total length concentration, but also the length distribution of the fibrils can be measured. The results from flow-induced birefringence showed that the total amount of fibrils and also the length distribution of the fibrils was not influenced by the addition of 4,5-dianilinophthalimide, even though this was suggested by the results of the Thioflavin T assay. The results of flow-induced birefringence were confirmed by rheological measurements and transmission electron microscopy. Our findings show that the use of a Thioflavin T assay in order to probe the possible disaggregating effect of certain compounds can give misleading results.

Effects of Flow on Hen Egg White Lysozyme (HEWL) Fibril Formation: Length Distribution, Flexibility, and Kinetics

The effect of steady shear and turbulent flow on the formation of amyloid fibrils from hen egg white lysozyme (HEWL) was studied. The conversion and size distribution of fibrils obtained by heating HEWL solutions at pH 2 were determined. The formation of fibrils was quantified using flow-induced birefringence. The size distribution was fitted using decay of birefringence measurements and transmission electron microscopy (TEM). The morphology of HEWL fibrils and the kinetics of their formation varied considerably depending on the flow applied. With increasing shear or stirring rate, more rod-like and shorter fibrils were obtained, and the conversion into fibrils was increased. The size distribution and final fibril concentration were significantly different from those obtained in the same heat treatment at rest. The width of the length distribution of fibrils was influenced by the homogeneity of the flow.

Flow of branched polymer melts in a lubricated cross-slot channel: a combined computational and experimental study

Numerical simulations have been performed to evaluate the accuracy of the multimode Giesekus model in predicting the flow behavior of a rheologically well characterized low-density polyethylene melt in a lubricated cross-slot channel. Specifically, the fidelity of the numerical results is established by detailed comparison with flow-induced birefringence measurements in a new optical rheometer with lubricated side walls that allows the creation of ideal two-dimensional flow kinematics that lead to the elimination of end effects commonly encountered in flow birefringence measurements. Based on these comparisons, the ability of the multimode Giesekus model to capture the flow characteristics with reasonable accuracy in the experimentally available Wi range of 21 to 29 has been established. However, it should be noted that the model predictions are, at best, qualitative in the vicinity of the stagnation point. The discrepancy between numerically predicted and experimentally observed stresses in this region is mainly attributed to the inaccuracy of the experimental data that stem from the occurrence of multiple orders of retardation within the measurement volume. Overall, these studies have paved the way for the development of a hi-fidelity lubricated cross-slot channel rheometer.

Properties of protein fibrils in whey protein isolate solutions: Microstructure, flow behaviour and gelation

We studied how protein fibrils (∼1 μm in length, prepared by heating at pH 2) can be used to modify structural properties of concentrated whey protein isolate (WPI) solutions. Using transmission electron microscopy, flow-induced birefringence and Thioflavin T fluorescence, we observed that the fibrils become shorter upon increasing the pH, and clusters were present at pH 5–7. For pH 3.5 and 7, the effect of adding fibrils to WPI solutions was studied. Different behaviour was observed, which makes this system rather complex. Rheological measurements showed that the presence of fibrils induced shear thickening (only at pH 3.5) and shear thinning behaviour, and resulted in an increase in viscosity and gel strength. Unstable flow regimes were observed for both pH values, which hint in the direction of phase separation. For pH 7, a phase separated dispersion was observed in the macrostructure after gelation using scanning electron microscopy.

Investigation of LDPE converging flows using fieldwise measurements techniques

This study is devoted to the analysis of the secondary flow of a low-density polyethylene melt occurring in the entrance region of extrusion dies. From a processing point of view, the appearance of secondary flows can lead to flow instabilities (as observed for helical defect in circular channels) and polymer degradation (due to long residence times). The flow is investigated in the entrance region of a flow channel by means of both Laser Doppler Velocimetry (LDV) and Flow Induced Birefringence (FIB) using a transparent slit die with a planar contraction of 14:1. We focus on the influence of temperature T and flow rate Q on the vortex size of the secondary flow, which strongly depends on balance between shear and extensional rheological properties of the material, as previously observed by means of LDV. The same dependence with T and Q is also observed on FIB patterns, in which one can identify a boundary line which exhibits a strong contrast in the birefringence pattern. This contrast could be related to the fact that molecular conformations (and consequently the birefringence pattern) change locally due to a different thermomechanical history, which indeed would occur close to the vortex boundary.

Lubricated cross-slot flow of a low density polyethylene melt

Flow-induced birefringence and particle tracking velocimetry are used to investigate the lubricated flow of a low density polyethylene melt in a cross-slot geometry. The numerical predictions of the extended Pom-Pom (XPP) model in its original and modified (mXPP) version as well as those of the Giesekus model are analyzed along selected streamlines in two-dimensional (2D) complex flows involving a mixture of shear and planar extensional deformations at two Weissenberg numbers of W i = 21 and 29. Oil film light reflections perturbing the particle tracking velocimetry data analysis together with multiple orders of retardation occurring within the laser beam close to the stagnation point prevent a conclusive discrimination between the mentioned models. Although the agreement with the experimental data is mostly qualitative, the Pom-Pom model does not overestimate the data along the cross-slot symmetry lines contrary to what was observed in other cross-slot experiments without lubrication. This is a clear indicator that end effects play a central role in unlubricated cross-slot geometries having a large aspect ratio.

Localised flow-induced crystallisation of a polyethylene melt

The sensitivity of flow-induced crystallisation (FIC) to the nature of flow type is demonstrated using a high-density polyethylene (HDPE) for two different flow geometries. A contraction-expansion slit geometry was used to create a mixed, but primarily simple shear flow, while a cross-slot geometry provided a region within the flow of high extension. Flow-induced birefringence was captured at a melt processing temperature of 155 °C to identify the principal stress difference within the two flows and determine regions of higher stress within the HDPE. The experiments were then repeated at 125 °C, and FIC was identified using bright-field observation. Crystallisation was observed within the regions that previously exhibited high stress levels. It was found that lower deformation rates in pure shear were required when compared with simple shear to create the crystal filaments.

Micrometer-Sized Fibrillar Protein Aggregates from Soy Glycinin and Soy Protein Isolate

Long, fibrillar semiflexible aggregates were formed from soy glycinin and soy protein isolate (SPI) when heated at 85 degrees C and pH 2. Transmission electron microscopy analysis showed that the contour length of the fibrils was approximately 1 microm, the persistence length 2.3 microm, and the thickness a few nanometers. Fibrils formed from SPI were more branched than the fibrils of soy glycinin. Binding of the fluorescent dye Thioflavin T to the fibrils showed that beta-sheets were present in the fibrils. The presence of the fibrils resulted in an increase in viscosity and shear thinning behavior. Flow-induced birefringence measurements showed that the behavior of the fibrils under flow can be described by scaling relations derived for rodlike macromolecules. The fibril formation could be influenced by the protein concentration and heating time. Most properties of soy glycinin fibrils are comparable to beta-lactoglobulin fibrils.

Lubricated optical rheometer for the study of two-dimensional complex flows of polymer melts

We describe a novel optical cross-slot channel rheometer generating two-dimensional and isothermal complex flows of polymer melts. This is made possible by lubricating the channel front and back viewing windows. Flow-induced birefringence and particle tracking velocimetry are reviewed and used to investigate the cross-slot flow of a low density polyethylene melt involving mixed shear and planar extensional deformations. This new device solves the issue of end effects in flow birefringence experiments where no variations of the optical properties along the light path are expected. It greatly facilitates the interpretation of stress field data by providing reliable measurements of the polymer melt extinction angle χ and retardation δ , with a spatial resolution of one tenth of a millimeter. At the same time, it offers an enhanced temperature control and an increased optical accuracy due to an improved laser beam shaping. The capabilities and performances of this unique type of lubricated rheometer are discussed in detail and compared with previous approaches.

Transient elongational viscosities of aqueous polyacrylamide solutions measured with an optical rheometer

An anionic polyacrylamide solution was characterized in elongational flow by combining laser-Doppler velocimetry to determine the strain rate in the flow direction and the two-color flow-induced birefringence method to measure the first normal stress difference along the axial centerline of a hyperbolic die. The elongational rate was constant along the axial centerline of the planar hyperbolic die as long as vortices at the die entrance did not occur. The transient elongational viscosity μ+ was determined as a function of the elongational rate. The parameters varied are the Hencky strain rate and the polymer concentration. μ+ showed a pronounced increase over the linear viscoelastic behavior \( \mu ^{ + }_{0} \) above critical Hencky strains of 1.2 to 1.5; that is, a significant strain hardening could be observed for polyacrylamide solutions. This strain hardening is stronger the higher the elongational rate. A slight enhancement of strain hardening was found by increasing the concentration from 0.5 to 1 g/l. The stress optical coefficient was determined as 1.8 × 10−7 Pa−1 (0.5 g/l) and 1.2 × 10−7 Pa−1 (1 g/l).