Showing papers in "Korea-australia Rheology Journal in 2017"

A sequence of physical processes quantified in LAOS by continuous local measures

Abstract: The response to large amplitude oscillatory shear of a soft colloidal glass formed by a suspension of multiarm star polymers is investigated by means of well-defined continuous local measures. The local measures provide information regarding the transient elastic and viscous response of the material, as well as elastic extension via a shifting equilibrium position. It is shown that even when the amplitude of the strain is very large, cages reform and break twice per period and exhibit maximum elasticity around the point of zero stress. It is also shown that around the point of zero stress, the cages are extended by a nearly constant amount of approximately 5% at 1 rad/s and 7% at 10 rad/s, even when the total strain is as large as 420%. The results of this study provide a blueprint for a generic approach to elucidating the complex dynamics exhibited by soft materials under flow.

A comparative study of the effects of cone-plate and parallel-plate geometries on rheological properties under oscillatory shear flow

Abstract: In this study, the effects of cone-plate (C/P) and parallel-plate (P/P) geometries were investigated on the rheological properties of various complex fluids, e.g. single-phase (polymer melts and solutions) and multiphase systems (polymer blend and nanocomposite, and suspension). Small amplitude oscillatory shear (SAOS) tests were carried out to compare linear rheological responses while nonlinear responses were compared using large amplitude oscillatory shear (LAOS) tests at different frequencies. Moreover, Fourier-transform (FT)-rheology method was used to analyze the nonlinear responses under LAOS flow. Experimental results were compared with predictions obtained by single-point correction and shear rate correction. For all systems, SAOS data measured by C/P and P/P coincide with each other, but results showed discordance between C/P and P/P measurements in the nonlinear regime. For all systems except xanthan gum solutions, first-harmonic moduli were corrected using a single horizontal shift factor, whereas FT rheology-based nonlinear parameters (I3/1, I5/1, Q3, and Q5) were corrected using vertical shift factors that are well predicted by single-point correction. Xanthan gum solutions exhibited anomalous corrections. Their first-harmonic Fourier moduli were superposed using a horizontal shift factor predicted by shear rate correction applicable to highly shear thinning fluids. The distinguished corrections were observed for FT rheology-based nonlinear parameters. I3/1 and I5/1 were superposed by horizontal shifts, while the other systems displayed vertical shifts of I3/1 and I5/1. Q3 and Q5 of xanthan gum solutions were corrected using both horizontal and vertical shift factors. In particular, the obtained vertical shift factors for Q3 and Q5 were twice as large as predictions made by single-point correction. Such larger values are rationalized by the definitions of Q3 and Q5. These results highlight the significance of horizontal shift corrections in nonlinear oscillatory shear data.

Improved diffusing wave spectroscopy based on the automatized determination of the optical transport and absorption mean free path

Abstract: Diffusing wave spectroscopy (DWS) can be employed as an optical rheology tool with numerous applications for studying the structure, dynamics and linear viscoelastic properties of complex fluids, foams, glasses and gels. To carry out DWS measurements, one first needs to quantify the static optical properties of the sample under investigation, i.e. the transport mean free path l* and the absorption length la. In the absence of absorption this can be done by comparing the diffuse optical transmission to a calibration sample whose l* is known. Performing this comparison however is cumbersome, time consuming, and prone to mistakes by the operator. Moreover, already weak absorption can lead to significant errors. In this paper, we demonstrate the implementation of an automatized approach, based on which the DWS measurement procedure can be simplified significantly. By comparison with a comprehensive set of calibration measurements we cover the entire parameter space relating measured count rates (CR t , CR b ) to (l*, la). Based on this approach we can determine l* and la of an unknown sample accurately thus making the additional measurement of a calibration sample obsolete. We illustrate the use of this approach by monitoring the coarsening of a commercially available shaving foam with DWS.

Viscoelastic properties of PLA/PCL blends compatibilized with different methods

Abstract: The aim of this study was to observe changes in the viscoelastic properties of PLA/PCL (80/20) blends produced using different compatibilization methods. Reactive extrusion and high-energy radiation methods were used for blend compatibilization. Storage and loss moduli, complex viscosity, transient stress relaxation modulus, and tan δ of blends were analyzed and blend morphologies were examined. All compatibilized PLA/PCL blends had smaller dispersed particle sizes than the non-compatibilized blend, and well compatibilized blends had finer morphologies than poorly compatibilized blends. Viscoelastic properties differentiated well compatibilized and poorly compatibilized blends. Well compatibilized blends had higher storage and loss moduli and complex viscosities than those calculated by the log-additive mixing rule due to strong interfacial adhesion, whereas poorly compatibilized blends showed negative deviations due to weak interfacial adhesion. Moreover, well compatibilized blends had much slower stress relaxation than poorly compatibilized blends and didn’t show tan δ plateau region caused by slippage at the interface between continuous and dispersed phases.

Prediction of rheology of shear thickening fluids using phenomenological and artificial neural network models

Abstract: Prediction models for the viscosity curve of a shear thickening fluid (STF) over a wide range of shear rate at different temperatures were developed using phenomenological and artificial neural network (ANN) models. STF containing 65% (w/w) silica nanoparticles was prepared using polyethylene glycol (PEG) as dispersion medium, and tested for rheological behavior at different temperatures. The experimental data set was divided into training data and testing data for the model development and validation, respectively. For both the models, the viscosity of STF was estimated for all the zones with good fit between experimental and predicted viscosity, for both training and testing data sets.

Effect of urea on heat-induced gelation of bovine serum albumin (BSA) studied by rheology and small angle neutron scattering (SANS)

Abstract: This paper reports the effects of urea on the heat-induced gelation of bovine serum albumin (BSA), which was studied by the tube inversion method, rheological measurements, and small-angle neutron scattering (SANS) An increase in the urea concentration accelerated the rate of gelation because the protein molecules have already been unfolded to some extent during sample preparation in the urea solution In addition, the BSA solution in the presence of urea underwent a sol-gel-sol transition during the time sweep test at a constant temperature of 80oC On the other hand, the BSA solution without urea turned into a hard and brittle gel that did not return to the solution state during isothermal heating at a constant temperature of 80oC Aggregation and re-bonding of the denatured and unfolded protein chains led to gel formation Urea added to the protein denatures its tertiary and secondary structures by simultaneously disrupting the hydrogen bonds, hydrophobic interactions, and altering the solvent properties Furthermore, urea induces thermoreversible chemical interactions in BSA solutions leading to the formation of a gel with dynamic properties under these experimental conditions

Natural convection in Bingham plastic fluids from an isothermal spheroid: Effects of fluid yield stress, viscous dissipation and temperature-dependent viscosity

Abstract: In this work, the buoyancy-induced convection from an isothermal spheroid is studied in a Bingham plastic fluid. Extensive results on the morphology of approximate yield surfaces, temperature profiles, and the local and average Nusselt numbers are reported to elucidate the effects of the pertinent dimensionless parameters: Rayleigh number, 102 ≤ Ra ≤ 106; Prandtl number, 20 ≤ Pr ≤ 100; Bingham number, 0 ≤ Bn ≤ 103, and aspect ratio, 0.2 ≤ e ≤ 5. Due to the fluid yield stress, fluid-like (yielded) and solid-like (unyielded) regions coexist in the flow domain depending upon the prevailing stress levels vis-a-vis the value of the fluid yield stress. The yielded parts progressively grow in size with the rising Rayleigh number while this tendency is countered by the increasing Bingham and Prandtl numbers. Due to these two competing effects, a limiting value of the Bingham number (Bn max) is observed beyond which heat transfer occurs solely by conduction due to the solid-like behaviour of the fluid everywhere in the domain. Such limiting values bear a positive dependence on the Rayleigh number (Ra) and aspect ratio (e). In addition to this, oblate shapes (e 1) impede it. Finally, simple predictive expressions for the maximum Bingham number and the average Nusselt number are developed which can be used to predict a priori the overall heat transfer coefficient in a new application. Also, a criterion is developed in terms of the composite parameter Bn∙Gr-1/2 which predicts the onset of convection in such fluids. Similarly, another criterion is developed which delineates the conditions for the onset of settling due to buoyancy effects. The paper is concluded by presenting limited results to delineate the effects of viscous dissipation and the temperature-dependent viscosity on the Nusselt number. Both these effects are seen to be rather small in Bingham plastic fluids.

Numerical modelling on pulsatile flow of Casson nanofluid through an inclined artery with stenosis and tapering under the influence of magnetic field and periodic body acceleration

Abstract: The present study investigates the pulsatile flow of Casson nanofluid through an inclined and stenosed artery with tapering in the presence of magnetic field and periodic body acceleration. The iron oxide nanoparticles are allowed to flow along with it. The governing equations for the flow of Casson fluid when the artery is tapered slightly having mild stenosis are highly non-linear and the momentum equations for temperature and concentration are coupled and are solved using finite difference numerical schemes in order to find the solutions for velocity, temperature, concentration, wall shear stress, and resistance to blood flow. The aim of the present study is to analyze the effects of flow parameters on the flow of nanofluid through an inclined arterial stenosis with tapering. These effects are represented graphically and concluded that the wall shear stress profiles enhance with increase in yield stress, magnetic field, thermophoresis parameter and decreases with Brownian motion parameter, local temperature Grashof number, local nanoparticle Grashof number. The significance of the model is the existence of yield stress and it is examined that when the rheology of blood changes from Newtonian to Casson fluid, the percentage of decrease in the flow resistance is higher with respect to the increase in the parameters local temperature Grashof number, local nanoparticle Grashof number, Brownian motion parameter, and Prandtl number. It is pertinent to observe that increase in the Brownian motion parameter leads to increment in concentration and temperature profiles. It is observed that the concentration of nanoparticles decreases with increase in the value of thermophoresis parameter.

A two-layered suspension (particle-fluid) model for non-Newtonian fluid flow in a catheterized arterial stenosis with slip condition at the wall of stenosed artery

Abstract: The primary concern of the present investigation is to study blood flow in a porous catheterized artery with an axially asymmetric and radially symmetric stenosis (constriction). In the present study, blood is characterized as a two-fluid system containing a cell-rich zone of suspension of blood cells described to be a particle-fluid suspension (Jeffrey fluid) and a cell-free plasma (Newtonian fluid) layer near the wall. The systematic expressions for flow characteristics such as fluid phase and particle phase velocities, flow rate, wall shear stress, resistive force, and frictional forces on walls of arterial stenosis and catheter are derived. It is recorded that the wall shear stress, flow resistance, and frictional forces are found to be increased with catheter size, red cell concentration, and slip parameter. When blood obeys the law of constitutive equation of a Jeffrey fluid, the flowing blood experiences lesser wall shear stress, flow resistance and frictional forces as compared to the case of blood being categorized as a Newtonian fluid. The increase in Darcy number, blood rheology as Jeffrey fluid, and the presence of peripheral plasma layer near the wall serves to reduce substantially the values of the flow characteristics (wall shear stress, flow resistance and frictional forces).

Determination of continuous relaxation spectrum based on the Fuoss-Kirkwood relation and logarithmic orthogonal power-series approximation

Abstract: In this study, we suggest a new algorithm for inferring continuous spectrum from dynamic moduli data The algorithm is based on the Fuoss-Kirkwood relation (Fuoss and Kirkwood, 1941) and logarithmic powerseries approximation The Fuoss-Kirkwood relation denotes the existence of the uniqueness of continuous spectrum If we know the exact equation of dynamic moduli, then continuous spectrum can be inferred uniquely We used the Chebyshev polynomials of the first kind to approximate dynamic moduli data in double-logarithmic scale After the approximation, a spectrum equation can be derived by use of the complex decomposition method and the Fuoss-Kirkwood relation We tested our algorithm to both simulated and experimental data of dynamic moduli and compared our result with those obtained from other methods such as the fixed-point iteration (Cho and Park, 2013) and cubic Hermite spline (Stadler and Bailly, 2009)

New multifunction materials with both electrorheological performance and luminescence property

Abstract: Novel multifunctional materials, the composites AlOOH-NaYFTb5 and AlOOH-NaYFTb10, containing AlO(OH) and β-NaYF4:5%Tb3+, have been synthesized via a facile hydrothermal route and a simple grinding method. The boehmite [AlO(OH)], yttrium nitrate [Y(NO3)3·6H2O], terbium nitrate, [Tb(NO3)3·6H2O], sodium citrate (Na3C6H5O7·2H2O) and sodium fluoride (NaF) were used as starting materials. The composition, electrorheological (ER) performance, and luminescence property of the functional materials were studied. Our results show that the composites not only have good electrorheological (ER) performance, but also have good optics property. The relative shear stress τ r (τ r = τ E/τ 0, τ E and τ 0 are the shear stresses at the electric field strength E = 4 and 0 kV/mm, respectively) values of the suspension (25 wt.%) of AlOOHNaYFTb5 material in silicone oil are all larger than 50 in a shear rate ranging from 0.06 to 26 s−1, the τr value reaches 1333 at a shear rate of 0.06 s−1. The material with such high ER activity and favorable luminescence performance is advantageous in its application as a multifunctional material.

Effects of molecular architecture on the rheological and physical properties of polycaprolactone

Abstract: The molecular modification of polycaprolactone (PCL) is of great importance for producing optimum physical properties for a given application. Linear polycaprolactone (L-PCL) and 4-arm star polycaprolactone (4-PCL) with similar molecular weights were prepared, and their rheological, thermal, and morphological properties were investigated in relation to their molecular architecture. In dilute solutions, L-PCL exhibited a higher intrinsic viscosity than 4-PCL. In the molten state, the former displayed a higher viscosity and greater temperature dependence of molecular relaxation time than the latter. DSC thermograms showed that molecular architecture had little effect on the melting/crystallization temperature and crystallinity. Thermogravimetric analysis indicated that the introduction of a branched structure deteriorated the thermal stability of PCL, which might be associated with the increased number of hydroxyl end groups in the polymer chains. In isothermal crystallization under shear at two different temperatures, 4-PCL exhibited longer crystallization times than L-PCL. A more notable difference in dynamic crystallization behavior caused by the chemical architecture was observed at 40°C than at 45°C. Examination with a wide angle X-ray diffractometer revealed that shear and temperature applied during isothermal crystallization, as well as chemical architecture, had little effect on the crystal structure.

Mathematical analysis on linear viscoelastic identification

Abstract: This paper is focused on mathematical analysis related with problems of linear viscoelastic identification such as intrinsic errors of conventional test methods, conversion between data of various viscoelastic functions, resolution and informativeness of viscoelastic functions and new test methods which are faster than dynamic test and more accurate than static tests. This paper provides various mathematical and numerical tools: Some of them are newly introduced here and the others are described for the reviewing previous works in this field.

Numerical simulation of FENE-P viscoelastic fluids flow and heat transfer in grooved channel with rectangular cavities

Abstract: The forced convection heat transfer for non-Newtonian viscoelastic fluids obeying the FENE-P model in a parallel-plate channel with transverse rectangular cavities is carried out numerically using ANSYS-POLYFLOW code. The flow investigated is assumed to be two-dimensional, incompressible, laminar and steady. The flow behavior and temperature distribution influenced by the re-circulation caused by the variation of cross-section area along the stream wise direction have been studied. The constant heat flux condition has been applied and the effects of the different parameters, such as the aspect ratio of channel cavities (AR = 0.25, 0.5), the Reynolds number (Re = 25, 250, and 500), the fluid elasticity defined by the Weissenberg number (We), and the extensibility parameter of the model (L 2), on heat transfer characteristics have been explored for channels of three successive cavities configuration. Different levels of heat transfer enhancement were obtained and discussed.

Numerical analysis of the heat transfer and fluid flow in the butt-fusion welding process

Abstract: Butt-fusion welding is an effective process for welding polymeric pipes. The process can be simplified into two stages. In heat soak stage, the pipe is heated using a hot plate contacted with one end of the pipe. In jointing stage, a pair of heated pipes is compressed against one another so that the melt regions become welded. In previous works, the jointing stage that is highly related to the welding quality was neglected. However, in this study, a finite element simulation is conducted including the jointing stage. The heat and momentum transfer are considered altogether. A new numerical scheme to describe the melt flow and pipe deformation for the butt-fusion welding process is introduced. High density polyethylene (HDPE) is used for the material. Flow via thermal expansion of the heat soak stage, and squeezing and fountain flow of the jointing stage are well reproduced. It is also observed that curling beads are formed and encounter the pipe body. The unique contribution of this study is its capability of directly observing the flow behaviors that occur during the jointing stage and relating them to welding quality.

Simulating deposition of high density tailings using smoothed particle hydrodynamics

Abstract: Tailings are a slurry of silt-sized residual material derived from the milling of rock. High density (HD) tailings are tailings that have been sufficiently dewatered to a point where they exhibit a yield stress upon deposition. They form gently sloped stacks on the surface when deposited; this eliminates or minimizes the need for dams or embankments for containment. Understanding the flow behaviour of high density tailings is essential for estimating the final stack geometry and overall slope angle. This paper focuses on modelling the flow behaviour of HD tailings using smoothed particle hydrodynamics (SPH) method incorporating a ‘bi-viscosity’ model to simulate the non-Newtonian behaviour. The model is validated by comparing the numerical results with bench scale experiments simulating single or multi-layer deposits in two-dimensions. The results indicate that the model agreed fairly well with the experimental work, excepting some repulsion of particles away from the bottom boundary closer to the toe of the deposits. Novel aspects of the work, compared to other simulation of Bingham fluids by SPH, are the simulation of multilayer deposits and the use of a stopping criteria to characterize the rest state.

A strategy to synthesize graphene-incorporated lignin polymer composite materials with uniform graphene dispersion and covalently bonded interface engineering

Abstract: Graphene-incorporated polymer composites have been demonstrated to have excellent mechanical and electrical properties. In the field of graphene-incorporated composite material synthesis, there are two main obstacles: Non-uniform dispersion of graphene filler in the matrix and weak interface bonding between the graphene filler and polymer matrix. To overcome these problems, we develop an in-situ polymerization strategy to synthesize uniformly dispersed and covalently bonded graphene/lignin composites. Graphene oxide (GO) was chemically modified by 4,4'-methylene diphenyl diisocyanate (MDI) to introduce isocyanate groups and form the urethane bonds with lignin macromonomers. Subsequential polycondensation reactions of lignin groups with caprolactone and sebacoyl chloride bring about a covalent network of modified GO and lignin-based polymers. The flexible and robust lignin polycaprolactone polycondensate/modified GO (Lig-GOm) composite membranes are achieved after vacuum filtration, which have tunable hydrophilicity and electrical resistance according to the contents of GOm. This research transforms lignin from an abundant biomass into film-state composite materials, paving a new way for the utilization of biomass wastes.

Process viscometry in flows of non-Newtonian fluids using an anchor agitator

Abstract: In this work, we present a viscosity measurement technique for inelastic non-Newtonian fluids directly in flows of anchor agitators that are commonly used in highly viscous fluid mixing particularly with yield stress. A two-blade anchor impeller is chosen as a model flow system and Carbopol 940 solutions and Xanthan gum solutions with various concentrations are investigated as test materials. Following the Metzner-Otto correlation, the effective shear rate constant and the energy dissipation rate constant have been estimated experimentally by establishing (i) the relationship between the power number and the Reynolds number using a reference Newtonian fluid and (ii) the proportionality between the effective shear rate and the impeller speed with a reference non-Newtonian fluid. The effective viscosity that reproduces the same amount of the energy dissipation rate, corresponding to that of Newtonian fluid, has been obtained by measuring torques for various impeller speeds and the accuracy in the viscosity prediction as a function of the shear rate has been compared with the rheological measurement. We report that the process viscometry with the anchor impeller yields viscosity estimation within the relative error of 20% with highly shear-thinning fluids.

Non-Newtonian behavior observed via dynamic rheology for various particle types in energetic materials and simulant composites

Abstract: The rheological properties of polymer composites highly filled with different filler materials were examined using a stress-controlled rheometer with a parallel-plate configuration, for particle characterization of the filler materials in plastic (polymer) bonded explosive (PBX). Ethylene vinyl acetate (EVA) with dioctyl adipate (DOA) was used as the matrix phase, which was shown to exhibit Newtonian-like behavior. The dispersed phase consisted of one of two energetic materials, i.e., explosive cyclotrimethylene trinitramine (RDX) or cyclotetramethylene tetranitramine (HMX), or a simulant (Dechlorane) in a bimodal size distribution. Before the test, preshearing was conducted to identify the initial condition of each sample. All examined filled polymer specimens exhibited yield stress and shear-thinning behavior over the investigated frequency range. The complex viscosity dependence on the dynamic oscillation frequency was also fitted using an appropriate rheological model, suggesting the model parameters. Furthermore, the temperature dependency of the different filler particle types was determined for different filler volume fractions. These comparative studies revealed the influence of the particle characteristics on the rheological properties of the filled polymer.

Modeling and numerical simulation of multiflux die in the multilayer co-extrusion process

Abstract: It is of great importance to understand the stretching and folding mechanism in the multiflux co-extrusion die to get uniform multilayer distribution at the end of die lip in the multilayer co-extrusion processes. In this work, to understand the mechanism of the layer distribution, modeling and numerical simulation were carried out for three-dimensional flow analysis in the multilayer co-extrusion die. The multilayer flow fields were numerically visualized and analyzed on the arbitrary cross-section of the multiflux die. In addition, numerical results for the multiflux die characteristics were obtained for non-Newtonian fluids in terms of power-law index for the cross model, which will be useful for the optimal design of screw and die, simultaneously, in the multilayer co-extrusion process.

Conformations of gelatin in trivalent chromium salt solutions: Viscosity and dynamic light scattering study

Abstract: An investigation of the influences of pH, salt type, and salt concentration on the conformations of gelatin molecules in trivalent chromium salt solutions was performed by viscosity and dynamic light scattering (DLS) techniques. It was found that the viscosity behaviors as polyelectrolytes or polyampholytes depended on the charge distribution on the gelatin chains, which can be tuned by the value of pH of the gelatin solution. The intrinsic viscosity of gelatin in basic chromium sulfate aqueous solution at pH = 2.0 first decreased and then increased with increasing Cr(OH)SO4 concentration, while a monotonic decrease of the intrinsic viscosity of gelatin was observed in CrCl3 solution. However, the intrinsic viscosity of gelatin at pH = 5.0 was found to be increased first and then decreased with an increase in salt concentration in Cr(OH)SO4 solution, as well as in CrCl3 solution. We suggested that the observed viscosity behavior of gelatin in trivalent chromium salt solutions was attributed to the comprehensive effects of shielding, overcharging, and crosslinking (complexation) caused by the introduction of the different counterions. In addition, the average hydrodynamic radius (R h ) of gelatin molecules in various salt solutions was determined by DLS. It was found that the change trend of R h with salt concentration was the same as the change of intrinsic viscosity. Based on the results of the viscosity and DLS, a possible mechanism for the conformational transition of gelatin chains with external conditions including pH, salt concentration, and salt type is proposed.

Numerical investigation of the dynamics of Janus magnetic particles in a rotating magnetic field

Abstract: We investigated the rotational dynamics of Janus magnetic particles suspended in a viscous liquid, in the presence of an externally applied rotating magnetic field. A previously developed two-dimensional direct simulation method, based on the finite element method and a fictitious domain method, is employed to solve the magnetic particulate flow. As for the magnetic problem, the two Maxwell equations are converted to a differential equation using the magnetic potential. The magnetic forces acting on the particles are treated by a Maxwell stress tensor formulation, enabling us to consider the magnetic interactions among the particles without any approximation. The dynamics of a single particle in the rotating field is studied to elucidate the effect of the Mason number and the magnetic susceptibility on the particle motions. Then, we extended our interest to a two-particle problem, focusing on the effect of the initial configuration of the particles on the particle motions. In three-particle interaction problems, the particle dynamics and the fluid flow induced by the particle motions are significantly affected by the particle configuration and the orientation of each particle.

Analysis of the static yield stress for giant electrorheological fluids

Abstract: Cheng et al. (2010)'s experimental results for the static yield stress of giant electrorheological (GER) fluids over the full range of electric field strengths were reanalyzed by applying Seo’s scaling function which could include both the polarization and the conductivity models. The Seo’s scaling function could correctly fit the yield stress behavior of GER suspensions behavior after if a proper normalization of the yield stress data was taken which collapse them onto a single curve. The model predictions were also contrasted with recently proposed Choi et al.’s scaling function to rouse the attention for a proper consideration of the GER fluid mechanisms.

Elastic effects of dilute polymer solution on bubble generation in a microfluidic flow-focusing channel

Abstract: Recently, two-phase flow in microfluidics has attracted much attention because of its importance in generating droplets or bubbles that can be used as building blocks for material synthesis and biological applications However, there are many unresolved issues in understanding droplet and bubble generation processes, especially when complex fluids are involved In this study, we investigated elastic effects on bubble generation processes in a flow-focusing geometry and the shapes of the produced bubbles flowing through a microchannel We used dilute polymer solutions with nearly constant shear viscosities so that the shear-thinning effects on bubble generation could be precluded We observed that a very small amount of polymer (poly(ethylene oxide) at ~O(10) ppm) significantly affects bubble generation When the polymer was added to a Newtonian fluid, the fluctuation in bubble size increased notably, which was attributed to the chaotic flow dynamics in the flow-focusing region In addition, it was demonstrated that the bubbles were thinner along the minor axis in the viscoelastic fluid than they were in the Newtonian fluid We expect that the current results will contribute to understanding the dynamics of two-phase flow in microchannels and the design and operation of the microfluidic devices to generate microbubbles

Rheology and gel point of the enzymatic hydrolysis of urea in the presence of urease

Abstract: This study reports on the rheology of the gelation kinetics of raw chitosan (CTS) solutions (2% w/v) produced by enzymatic hydrolysis of urea at different urea concentrations (40, 50, 60, 80, and 100 mM) in the presence of urease at 1 U/mL. Viscoelastic parameters and pH values were evaluated during gelation process and the rheological properties of CTS hydrogels produced were monitored after 24 h at 37°C to simulate human body temperatures. pH measurements suggest that above some critical urea concentration (50 mM) the time required (tgel) to reach the critical pH gelation shows no dependence on urea concentration (tgel was ca. 70 minutes). Above 50 mM of urea concentration, CTS hydrogels exhibit an elastic modulus G′ higher than the viscous modulus G″ with no frequency dependence characteristic of a gel behavior. Gelation kinetics analyzed by rheology suggest that the G′ (i.e., structure) development depends on urea concentration during solution neutralization.

A nonlinear rheological model for the ultrasonically induced squeeze film effect in variable friction haptic displays

Abstract: A squeeze film induced by ultrasonic vibration between two solid surfaces in contact can dramatically reduce the friction between them. This phenomenon, so-called the squeeze film effect, has been utilized in variable friction tactile displays for texture rendering purposes. Such tactile displays can provoke a haptic sensation to a finger pad in a controllable way. A real-time adjustment of the coefficient of lateral friction between the human finger pad and the tactile display can be accomplished by modulating the vibration amplitude of the tactile panel. Therefore, driving a reliable friction model is a key step towards designing and controlling tactile displays utilizing the squeeze film effect. This paper derives a modified Herschel- Bulkley rheological model to express the lateral friction exerted on a human fingertip via an air squeeze film as a function of the operating parameters such as the driving voltage amplitude, the finger sliding speed, and the contact pressure. In contrast to the conventional Coulomb friction model, such a rheology model can account for the sliding velocity dependence. This modeling work may contribute to the optimal control of the ultrasonic variable friction tactile displays.

Multi-step cure kinetic model of ultra-thin glass fiber epoxy prepreg exhibiting both autocatalytic and diffusion-controlled regimes under isothermal and dynamic-heating conditions

Abstract: As packaging technologies are demanded that reduce the assembly area of substrate, thin composite laminate substrates require the utmost high performance in such material properties as the coefficient of thermal expansion (CTE), and stiffness. Accordingly, thermosetting resin systems, which consist of multiple fillers, monomers and/or catalysts in thermoset-based glass fiber prepregs, are extremely complicated and closely associated with rheological properties, which depend on the temperature cycles for cure. For the process control of these complex systems, it is usually required to obtain a reliable kinetic model that could be used for the complex thermal cycles, which usually includes both the isothermal and dynamic-heating segments. In this study, an ultra-thin prepreg with highly loaded silica beads and glass fibers in the epoxy/amine resin system was investigated as a model system by isothermal/dynamic heating experiments. The maximum degree of cure was obtained as a function of temperature. The curing kinetics of the model prepreg system exhibited a multi-step reaction and a limited conversion as a function of isothermal curing temperatures, which are often observed in epoxy cure system because of the rate-determining diffusion of polymer chain growth. The modified kinetic equation accurately described the isothermal behavior and the beginning of the dynamic-heating behavior by integrating the obtained maximum degree of cure into the kinetic model development.

Time-dependent viscoelastic properties of Oldroyd-B fluid studied by advection-diffusion lattice Boltzmann method

Abstract: Time-dependent viscoelastic properties of Oldroyd-B fluid were investigated by lattice Boltzmann method (LBM) coupled with advection-diffusion model. To investigate the viscoelastic properties of Oldroyd-B fluid, realistic rheometries including step shear and oscillatory shear tests were implemented in wide ranges of Weissenberg number (Wi) and Deborah number (De). First, transient behavior of Oldroyd-B fluid was studied in both start up shear and cessation of shear. Stress relaxation was correctly captured, and calculated shear and normal stresses agreed well with analytical solutions. Second, the oscillatory shear test was implemented. Dynamic moduli were obtained for various De regime, and they showed a good agreement with analytical solutions. Complex viscosity derived from dynamic moduli showed two plateau regions at both low and high De limits, and it was confirmed that the polymer contribution becomes weakened as De increases. Finally, the viscoelastic properties related to the first normal stress difference were carefully investigated, and their validity was confirmed by comparison with the analytical solutions. From this study, we conclude that the LBM with advection-diffusion model can accurately predict time-dependent viscoelastic properties of Oldroyd-B fluid.

Which is more informative between creep and relaxation experiments

Abstract: We present mathematical analysis which compares linear viscoelastic measurements such as creep and relaxation The analysis is focused on which one is more informative Since the intervals of relaxation time (or retardation time) of most polymeric materials are much wider than the interval of observation time of conventional rheological measurement, it is not possible to extract perfectly the whole information of material from any rheological measurement The mathematical analysis is to manifest which experimental method can obtain more information within the same interval of observation Although the analysis of Jackle and Richert (2008) means that the width of retardation is wider than that of relaxation, the results of the analysis hold for only viscoelastic solid because their analysis is based on dielectric relaxation Our analysis shows that creep experiment is more informative than relaxation experiment as for viscoelastic fluid

Sensitivity analysis of two-dimensional viscoelastic film casting processes using transfer functions by transient frequency response method

Abstract: The sensitivity of two-dimensional (2-D) film casting processes for viscoelastic fluids is investigated using a transfer function approach based on the transient frequency response method. The transient responses of state variables to step-changed film tension have been conveniently changed into corresponding transfer functions via the Laplace transform in a tension-controlled system. The transfer function between take-up velocity and film tension plays a key role in elucidating both sensitivity and the stability of film drawing systems. Various amplitude ratios of state variables are effectively evaluated over a wide range of frequencies when an ongoing disturbance is imposed at take-up velocity. The aspect ratio (up to 1 in this study) tends to make the system less sensitive to disturbances. The effect of viscoelasticity on the sensitivity is related to stability patterns, which depend on the aspect ratio and Deborah number.