Showing papers in "Korea-australia Rheology Journal in 2020"

Controlling the emulsion stability of cosmetics through shear mixing process

Abstract: The manipulation of emulsion stability for kinetically sustainable cosmetic emulsions is an important technology in cosmetic industry, however the relationship between emulsifying process and long-term emulsion stability has not been elucidated. Herein, the effect of shear mixing process on the stability of oil-in-water cosmetic emulsions is investigated by varying the shear mixing rate, emulsification time, and water phase temperature. The analysis on droplet size distribution and shear viscosity revealed that the strong viscous forces at optimal shear mixing rate of 4000 rpm afforded the fine and uniform droplets for cosmetic emulsions, leading to the improvement of long-term emulsion stability. Moreover, since the prolonged shear mixing induced the destabilization of emulsion droplets through droplet coalescence, optimal shear mixing time of 3 min could improve the kinetic stability of cosmetic emulsions. The dependence of long-term emulsion stability on emulsification temperature was relatively low. The theoretical analysis using the Derjaguin-Landau-Verwey-Overbeek theory demonstrated that the shear mixing rate played a major role in sustaining fine and uniform cosmetic emulsions with long-term stability. The present study can greatly contribute to the fabrication of functional cosmetic emulsions with long-term stability by controlling the shear mixing parameters in simple emulsification process.

Mathematical model on magneto-hydrodynamic dispersion in a porous medium under the influence of bulk chemical reaction

Abstract: The mathematical model of hydrodynamic dispersion through a porous medium is developed in the presence of transversely applied magnetic fields and axial harmonic pressure gradient. The solute introduce into the flow is experienced a first-order chemical reaction with flowing liquid. The dispersion coefficient is numerically determined using Aris’s moment equation of solute concentration. The numerical technique employed here is a finite difference implicit scheme. Dispersion coefficient behavior with Darcy number, Hartmann number and bulk flow reaction parameter is investigated. This study highlighted that the dependency of Hartmann number and Darcy number on dispersion shows different natures in different ranges of these parameters.

Performance of OMMT/SBS on the rheological properties of asphalt binder

Abstract: Nanoclays have been successfully introduced into the asphalt binder, either separately or into the polymer modified asphalt binder resulting in the improved mechanical and rheological properties of the asphalt binder. The present research study was undertaken to evaluate the influence of organically modified montmorillonite clay (OMMT) in pure asphalt binder and also in styrene butadiene styrene (SBS) modified asphalt binder. The quantity of OMMT and SBS was varied from 2–6 percent by the weight of the base asphalt binder. The addition of OMMT in an appropriate amount can dramatically enhance the compatibility between SBS and base asphalt. It was observed from the results that modified binders exhibited higher Superpave rutting parameter (G*/sinδ) indicating an improvement in the stiffness of the binder. It was also observed that modified binders (OMMT/SBS) does not exhibit phase separation under high temperatures. Thus high temperature stoage stability could be improved by choosing a proper amount of OMMT and forming an exfoliated structure. Thus it can be concluded that OMMT added to the polymer modified binder (SBS) helps in obtaining better physical and rheological properties on a condition that clay scatters into the binder at the nanoscopic level.

Change of rheological/mechanical properties of poly(caprolactone)/CaCO 3 composite with particle surface modification

Abstract: In order to investigate how particle aggregation affects tensile mechanical performance of composite, a ductile biopolymer (poly(caprolactone)) was melt-compounded with CaCO3 particles over a wide concentration range from 10 to 60 wt.%. The aggregation of CaCO3 particles in poly(caprolactone) (PCL) is investigated depending on particle concentration and surface modification (with stearic acid (2.5 wt.%)) based on rheological assessment. If the composite is mixed with a high concentration of particles (> 30 wt.%), morphological observations and a thermal behavior analysis do not find a difference in the particle aggregation regardless of particle surface modification. However, the linear viscoelastic moduli of the composites distinguishes the difference in particle aggregation regarding to surface modification, indicating induced aggregation behavior with surface-modified CaCO3 (sCC). The composite with sCC starts to form network structure of particles at a lower concentration (30 wt.%) than that with unmodified particles (40 wt.%). When particles form the network structure above the particle percolation threshold, the yield strength of the composite begins to decrease even though Young’s modulus is still increasing. In contrast to the expectation of the better dispersion of particles by surface modification as well as improved tensile mechanical performance with better dispersion, sCC rather induced aggregation with a lower concentration of particle than unmodified particles which resulted in decrease in yielding performance. This study showed that rheological study, especially for the composite with high concentration of particles, is useful to figure out the particle dispersion against a limit at morphology observation.

Mucoadhesive and pH-responsive behavior of gelatin containing hydrogels for protein drug delivery applications

Abstract: Novel gelatin-containing polymer hydrogels that can be used as mucoadhesive delivery systems were developed. Poly(acrylic acid) hydrogels were modified by copolymerizing gelatin as adhesion promoter, to improve the adhesion to the mucus layer and the synthesized copolymer of acrylic acid (AA) and methacrylated gelatin (GelMA) were designated as P(AA-co-GelMA). The pH-sensitivity and the mucoadhesive property of the P(AA-co-GelMA) hydrogel were investigated as carries of an oral protein delivery system activated by pH changes of the human GI tract. There was a drastic change in the weight swelling ratio of P(AA-co-GelMA) hydrogels at a pH of around 5, that is, low swelling ratios at a pH below 5, while high swelling ratios at a pH greater than 5. In addition, the swelling ratio increased at a pH above 5, when the AA content in the hydrogel increased. In mucoadhesive experiments using the rheometer, when the GelMA concentration in the P(AA-co-GelMA) hydrogel increased, the maximum force of detachment increased, indicating that the mucoadhesion of the hydrogel was improved. The P(AA-co-GelMA) hydrogels also showed a pH-responsive release behavior. The ratio of the cumulative amounts of Rh-B released from P(AA-co-GelMA) hydrogels at pH 2.6 to pH 7.0 increased, when the AA content in the hydrogel decreased.

Secondary Dean flow characteristics of inelastic Bird-Carreau fluids in curved microchannels

Abstract: To effectively control the mixing of target materials inside microfluidic devices, the Dean flow features of generalized-Newtonian Bird-Carreau (BC) fluids in curved rectangular channels are theoretically investigated, as a passive technique. Governing equations coupled with the Cauchy momentum equation and the BC model are solved using the finite volume scheme with a semi-implicit method for pressure-linked equations-revised (SIMPLER) algorithm. The effects of the rheological parameters of BC model, such as viscosity ratio, power-law index, and relaxation time constant, on the Dean flow are systematically examined in a wide range of Dean numbers (Dn), (very low to O(102)). The entire flow characteristics of BC fluids in curved microchannels with increasing Dn are quantified using flow skewness, DnRef/DnMES, and magnitude of vorticity, resulting in two main findings of a more outward-skewed streamwise velocity profile and a more enhanced secondary Dean vortex for non-Newtonian fluids in comparison to the Newtonian case at the same Dn.

Microfluidic-based effective monitoring of bloods by measuring RBC aggregation and blood viscosity under stepwise varying shear rates

Abstract: To effectively discriminate changes in blood samples, RBCs aggregation and blood viscosity should be evaluated independently and simultaneously. In this study, by setting two syringe pumps for delivering two fluids (blood sample and reference fluid) at stepwise varying flow rates, RBCs aggregation (AI) and viscosity (µBlood) are sequentially measured by quantifying image intensity of blood flows () and interface between blood sample and reference fluid (αBlood) in microfluidic channels. The () for measuring AI was obtained at lower shear rate (γ < 91.7 s-1). Additionally, αBlood for measuring µBlood was obtained at higher shear rates (γ < 91.7 s-1). As a demonstration, the proposed method is employed to measure AI and µBlood for various blood samples composed of different concentrations of dextran solution or different degrees in RBCs deformability. After then, the method is employed to measure AI and µBlood of blood samples with respect to storage time of 25 days. As a result, RBCs aggregation and blood viscosity varied continuously up to 15 days of storage time. In conclusion, this method can be used effectively to detect changes in blood samples by measuring µmood and AI of blood samples simultaneously.

Generation mechanism of gloss defect for high-glossy injection-molded surface

Abstract: The gloss defect on a glossy surface is one of the surface defects to be found on injection-molded products. In this paper, the effects of the filling and packing stages on surface gloss are investigated. Based on observations, a generation mechanism of gloss difference is proposed. The difference of the surface gloss can be explained by the replication of the shrinking polymer surface represented by a replication factor. The replication factor is the ratio of melt pressure to surface stiffness, which is influenced by the filling condition and the material properties. The melt pressure as a driving factor to the replication reflects the effect of the flow front speed and viscosity. The surface stiffness as a resisting factor reflects the effect of the flow front speed, mold temperature in the filling stage, and storage modulus. The replication factor shows a high correlation to the surface gloss over a wide range of filling conditions. The proposed mechanism recommends a uniform and high flow front speed and mold temperature to suppress gloss defects.

Study of EDL phenomenon in Peristaltic pumping of a Phan-Thien-Tanner Fluid through asymmetric channel

Abstract: This paper is mainly attained to examine the electric double layer (EDL) phenomena and rheological effects on peristaltic pumping through asymmetric microchannel in presence of Lorentz force. To examine the electroosmosis mechanism, Poisson-Boltzmann equation is considered. To describe the rheological behavior of the fluids, a Phan-Thien-Tanner model is taken into account. The governing equations are simplified by using scaling analysis with low Reynolds number and large wavelength approximations. The set of non-linear partial differential equations are solved by regular perturbation technique to find out the series solutions for stream function, axial velocity and pressure gradient. Furthermore, the shear stress at the channel wall is derived. The graphical results for velocity, pressure gradient, stream lines and shear stress are illustrated using the in-house code written in Mathematica software. It is revealed that velocity field, shear stress and trapping phenomenon are strongly altered with EDL thickness, electric and magnetic fields. It is further concluded that rheological parameter i.e. Weissenberg number significantly affects the physical mechanisms. This model can be applicable in various complex systems where the rheological fluids can be transported by novel microfluidics peristaltic pumps.

Determination of velocity profiles of Bird-Carreau fluids in curvilinear microchannels using random sample consensus

Abstract: Flow features of rheologically complex fluids inside curved microchannels should be meaningfully scrutinized for effective mixing, sorting, and manipulation of nano- and micro-sized colloids or particles. In this study, a particle streak velocimetry method with coordinate transformation is incorporated to depict experimentally the axial velocity profiles of Newtonian and non-Newtonian (Bird-Carreau, BC) fluids in a curvilinear microchannel under constant flow rate conditions. Theoretical velocity distributions for both fluids are favorably substantiated from experimental observations that employ a random sample consensus (RanSAC) algorithm under various channel geometric conditions, demonstrating the good agreement between experiments and simulations previously developed. It is confirmed that the BC fluid showed blunt and non-parabolic profiles in comparison to the Newtonian case at a low Dean number. The suggested algorithm and method for accurately observing microscale flow fields provide useful insights into the elaborate manipulation and processing of non-Newtonian fluids in curved channel devices.

Simulations for the flow of viscoplastic fluids in a cavity driven by the movement of walls by Lattice Boltzmann Method

Abstract: The current paper is focused on analyzing the flow of viscoplastic fluid in a cavity that is driven by the two walls. The Lattice Boltzmann method (LBM) is used to solve the discrete Boltzmann equation. To represent the stress-strain rate relationship of viscoplastic fluids, the Bingham Papanastasiou constitutive model is considered. Cavity flow filled with Bingham fluids is considered for validating the present LBM code. After successful validation of the code, the analysis is extended for three dissimilar wall motions-simultaneous and opposed movement of the parallel facing walls, and the simultaneous motion of non-facing walls. The flow dynamics of Bingham fluid is influenced by Reynolds and Bingham numbers which can be studied using velocity and streamline plots. Subsequently, the yielded and un-yielded zones in a cavity have been effectively tracked using the limiting condition of yield stress. Further, the effect of wall motion on the variation of those zones inside a cavity has been studied. Finally, the drag coefficient for considered wall motions is presented.

Drag reduction characterizations of turbulent channel flow with surfactant additive by proper orthogonal decomposition and wavelet transform

Abstract: To explore the drag-reducing characteristics of turbulent channel flows with surfactant additive at relatively high Reynolds number from the perspectives of energy spectrum and multi-scale resolution, the two-dimensional fluctuation velocity fields of turbulent channel flows with/without surfactant additive at Reynolds number of Reτ = 590 obtained by large eddy simulation are decomposed by two-dimensional proper orthogonal decomposition (POD) and wavelet transform (WT). POD results show that the low-order eigenmode occupying most energy can be used to capture large-scale vortex structures, and fewer eigenmodes can be employed to capture coherent structures (CSs) in surfactant solution case compared with that in the Newtonian fluid. The spatial structures depicted by POD eigenmode state that buffer layer has a tendency to move towards the center of the channel in surfactant solution. Through wavelet analysis of fluctuation velocity fields in the streamwise-wall-normal planes, it is found that CSs mainly distribute in the near-wall region and the amount of CSs is smaller in surfactant solution. The results of local Reynolds shear measure (LRM) show that local contribution of CSs to the intermittency in turbulent channel flow of surfactant solution decreases, indicating the inhibition of intermittency by surfactant additive. In order to investigate the drag-reducing characteristics at different locations along the wall-normal direction, the fluctuation velocity fields in the streamwise-spanwise planes at different wall-normal locations are decomposed by two-dimensional WT. The results show that surfactant additive mainly affects the flow in the near-wall region (especially in the buffer layer) and thus induces drag reduction effect.

Rheological estimation of aggregate size for a colloidal suspension of carbon black particles

Abstract: Colloidal aggregation is quantitatively characterized by a rheological analysis of the colloidal suspension at various particle concentrations. The rheological analysis is combined with fractal concept to estimate the compactness, size, and size variation with shear stress on colloidal aggregates. The rheological measurement is carried out for a colloidal suspension of 24 nm carbon black particles suspended in ethylene glycol. The particle concentration ranges from 6.0 to 8.5 percent in volume, which is non-dilute regime where colloidal gelation occurs. Elastic modulus behavior with the particle concentration provides fractal dimension of aggregates. With the fractal dimension, concentration-dependent shear stress behavior is used to estimate aggregate size and its variation with shear stress through a rheological modeling. The estimated fractal dimension of aggregate is 2.020 and the average aggregate size exponentially decreases with the shear rate in the range 1152.24 nm at 1 s−1 to 150.00 nm at 1000 s−1. These estimations are compared with those from optical measurement using static small-angle X-ray scattering (SAXS) technique. The SAXS analysis gives the fractal dimension of 2.495 and the average aggregate size is 320.56 nm. It is found that the optical measurement gives slightly higher fractal dimension and the aggregate size is numerically close to that predicted one around the shear rate 68.7 s−1 where steep size reduction turns into being slow.

Flow and mixing characteristics of a groove-embedded partitioned pipe mixer

Abstract: We propose a groove-embedded partitioned pipe mixer (GPPM) and conduct an in-depth numerical study on the flow and mixing characteristics of the GPPM in the creeping flow regime. The GPPM is a variant of a previously reported mixer, the barrier-embedded partitioned pipe mixer (BPPM), and is designed to achieve better energy-efficient mixing compared to the BPPM. In this paper, we first introduce the working principle of the GPPM and its mixing protocols. Then, the flow system affected by mixing protocols and geometrical parameters of the GPPM is investigated using Poincare sections. As for mixing characteristics, the flux-weighted intensity of segregation is employed for quantitative mixing analysis. It turns out that a GPPM with a proper set of design parameters can indeed lead to a globally chaotic mixing. More importantly, the best GPPM showed better mixing in terms of energy consumption compared to its counterpart, the best BPPM.

The rheological properties and gelation kinetics of corn starch/bovine serum albumin blend

Abstract: The rheological properties and gelation kinetics of corn starch (CS)/bovine serum albumin (BSA) blends were investigated using the mass ratio of [CS]/[BSA] of 0, 0.5, 1, 2, and 3. The results from the temperature ramp and time sweep tests within linear viscoelastic regime showed a decrease in the gelation temperature (Tgel) and gelation time (tgel) of the BSA with increasing the mass ratio of [CS]/[BSA]. A significant decrease in the Tgel and tgel occurred at 5 wt.% BSA with [CS]/[BSA] = 3, where about 12 °C and 11 min drops in the Tgel and tgel were recorded. At the molecular scale, the decrease in the Tgel and tgel with increasing the mass ratio of [CS]/[BSA] was attributed to the breaking down of hydrogen bonding sites in the CS molecules with increasing temperature, and thereby allowing the hydroxyl group to engage in intermolecular hydrogen bonding with the BSA polar amino acid residues. The hydrogen bonding in addition to other non-covalent forces appeared to strengthen the microstructure of the CS/BSA gel as confirmed by the dynamic frequency sweep test. The frequency sweep test also showed that the value of the storage modulus (G′) at [CS]/[BSA] = 3 was about 10 to 103 orders of magnitude larger than its value recorded for the pure BSA. Kinetically, the gelation process was well modelled by the temperature dependency of reaction rate constant as represented by the Arrhenius equation. If interpreted in terms of the energy required to attain the gel state, the addition of CS reduced the gel activation energy (Ea) to 153.90 kJ/-mol as against the original value of 268.55 kJ/mol that was observed for the BSA without CS.

High temperature extensional rheology of commercially available polycarbonate mixed with flame retardant salts

Abstract: In this paper, we present a study of the dripping properties of polycarbonate (PC) modified with combinations of earth metal salts of inorganic sulfur, potassium perfluorobutane sulfonate (Rimar); non-halogenated flame retardant additives, potassium diphenyl sulfone-3-Sulfonate (KSS); and block co-polymers-polytetrafluoroethylene encapsulated with styrene acrylonitrile resin (T-SAN). Measurements of the extensional rheology of polycarbonate with different concentration of each flame retardant additive were performed using a custom-built high temperature Capillary Breakup Extensional Rheometer (CaBER) at temperatures up to T=400°C. From these measurements, the evolution of the apparent transient extensional viscosity was monitored as a function of time and strain both in air and in an inert nitrogen environment. The evolution of extensional viscosity has been shown to be an excellent tool for predicting the dripping behavior of polymers exposed to heat and a valuable tool for understanding the mechanism of polymer degradation which is typically dominated by either crosslinking or charring. We show that extensional rheology measurements are significantly more sensitive to temperature-induced changes to the polymer microstructure than shear rheology measurements. We have also performed systematic concentration of specific flame retardant salts and through variation in extensional rheology and investigated the optimum concentration required to achieve a V0 rating. Finally, we will show that extensional rheology is a powerful method for predicting the effect of flame retardant modifiers and optimizing their use in new flame resistant materials.

Hydroelastic instability of viscoelastic fluids in developing flow through a compliant channel

Abstract: Linear stability of a viscoelastic fluid obeying the Walters’ B model is analytically and numerically investigated in the entrance region of a plane channel formed between two parallel plates. The plates are compliant and obey the two degree-of-freedom von Karman solid model. Having obtained the base-flow velocity profiles using the boundary-layer theory, their vulnerability to infinitesimally small varicose disturbances is investigated using the temporal, normal-mode, linear stability analysis. The results obtained show that a fluid’s elasticity has a stabilizing effect on the developing velocity profiles. The distance at which the flow becomes unstable shifts further downstream (i.e., towards the fully-developed section) of the channel when the Deborah number is increased. An increase in the flexural rigidity of the plates is shown to have a stabilizing effect on the short waves (i.e., the flutter modes) whereas an increase in its mass can dramatically destabilize such modes. The flow becomes more stable when the stiffness of the soft matter restraining the vertical movement of the plates is increased with the effect being more significant on the long waves (i.e., flow-induced modes). Boosting the dissipating effect of this material is predicted to have a stabilizing effect on the short waves.

Numerical investigation of a non-Newtonian fluid squeezed between two parallel disks

Abstract: The transient, axisymmetric squeeze flow of the non-Newtonian shear thinning material between finite disks is studied numerically. The fluid between disks is assumed to follow the Carreau-Bird model. Two disks approach each other at a constant velocity while no-slip boundary condition is assumed. The time dependent simulation shows the effect of fluid nonlinearity, flow parameters and geometric aspect ratio on the flow dynamic and evolution of squeeze force. Also, some physical phenomena are shown and are explained at the edge of the disks and out of them. The conservation and momentum equations containing inertia effects are solved using moving mesh scheme and finite volume method. The SIMPLE algorithm is used to solve the pressure-velocity coupling.

Fabrication of PBAT/polyethylene blends mulching film via blown film extrusion process

Abstract: To extend the period of decomposition and property degradation of polybutylene adipate terephthalate (PBAT) mulching film for long-term use, PBAT was compounded with polyethylene mixture (LDPE: LLDPE = 15:85), calcium carbonate, and illite. PBAT mulching films were prepared using a blown film extrusion process with 80 mm diameter blowing die. To improve mechanical properties and processability of the film, crosslinking agent di-(t-butylperoxy isopropyl) benzene (BIBP) was added to PBAT compound and the effect of BIBP content on compatibility enhancement of PBAT and PE was investigated. Rheological measurements through dynamic oscillatory frequency sweep showed that the storage modulus of PBAT compound increased with the addition of BIBP and the slope of it decreased in the low frequency region because of crosslinking increase of PBAT and PE. Melt flow index of PBAT compounds decreased with increasing BIBP content. In the blown film extrusion process, the temperature of the heating zones in the extruder increased and the flow rate of the cooling air decreased with increasing content of BIBP. The highest output rate and a uniform surface morphology of mulching film were obtained because of a stable bubble with a tubular shape even at high extrusion speed with 800 ppm of BIBP. In addition, tensile strengths of both machine and transverse directions were the highest among all PBAT compounds. When the content of BIBP was higher than 800 ppm, the surface inhomogeneity of the film increased, resulting in decrease of the tensile strength in machine and transverse directions due to increased chemical cross-linking of PE and PBAT.

Localization behavior of multiwalled carbon nanotubes in ternary blends of PC, SAN, and PMMA

Abstract: In this study, the localization behavior of multiwalled carbon nanotubes (MWCNTs) was investigated in the ternary blends based on PC/SAN/PMMA (60/30/10). The major interest was focused on the effects of two different SANs having AN contents of 24 and 32 wt.% (SAN24 and SAN32, respectively) and PMMA on the phase structure of the composites. It was found that the spreading of PMMA around SAN32 was pronounced and relatively higher amount of MWCNTs were located at the interface of PC-PMMA, while the spreading behavior of PMMA around SAN24 was rather weak and MWCNTs were mostly found in PC phase. The unique phase structure characterized by the spreading of PMMA on SAN32 and the confinement of MWCNTs at the PC-PMMA interface was beneficial for the construction of electrical conduction pathways, as confirmed by the volume resistivities of the composites. It was shown that the volume resistivity of the composite containing SAN32 is lower than that of SAN24 at the same content of MWCNTs. The observed phase morphology was also correlated with the rheological properties of the composites.

Coagulation kinetics of round-sided disk particles under simple shear flow

Abstract: In this study a theoretical study is carried out on the binary collision of round-sided disks (RSD) suspended in a Newtonian fluid under a simple shear flow. RSD is composed of a disk part and a half-torus part which circumscribes the side of the disk. The diameter of the disk is fixed at 2 µm while the thickness and the half-torus size are varied so that the aspect ratio varies from 0.13 to 0.288. The liquid viscosity is changed from 0.01 to 1 Pa·s. The hydrodynamic force and van der Waals force with the Hamaker constant of 1.06 × 10−20 J are considered in tracking the position and the orientation of each particle. The Brownian motion is considered to be negligible. The collision of two particles initially separated by sufficiently a long distance is considered and the kinetic constant of coagulation is obtained by considering the presence of collision, the orientations of two particles and the flux of liquid flow. The result shows that the kinetic constant of coagulation is reduced to approximately 1/4 of the kinetic constant of non-interacting particles by the hydrodynamic interaction when the viscosity is 1 Pa·s. As collision modes, side-side and side-edge are considered. Side-side mode is found to be the dominant mode of collision for differing aspect ratio and differing viscosity of the liquid. The dominance of the side-side collision mode implies the formation of two-dimensional flocs in the shear flow.

A simple method of correcting the parallel plate rim shear stress for non-Newtonian behavior

Abstract: In the rheological characterization of non-Newtonian fluids in a steady torsional shear flow using a parallel-plate geometry, there is a need to correct the shear stress at the rim for non-Newtonian behavior. Solving the governing torsional flow equation inversely by Tikhonov regularization using experimental torque vs. rim shear rate data is the most scientific method for obtaining the model independent shear stress. In this short communication, a simple empirical method based on using the derivative of the polynomial fit to torque vs. rim shear rate to correct for the shear stress is presented. This method can be applied to any fluid behavior. The accuracy of this method was confirmed by the outstanding agreement of between the shear stress results of the two methods of polynomial and the inverse problem for magnetorheological fluids. The performance of other correction methods was also discussed.

Rheological Properties and Phase Structure of Polypropylene/Polystyrene/Multiwalled Carbon Nanotube Composites

Abstract: In this work, nanocomposites of polypropylene (PP)/polystyrene (PS)/multi-walled carbon nanotube (MWCNT) were prepared by the sonochemical polymerization of styrene in the presence of PP and MWCNT. This procedure offered a composite (PP-g-PS/MWCNT) having a unique phase structure in which MWCNTs are distributed in both phases of PP and PS, which is contrary to the typical observations in the composites made by the melt blending process (PP/PS/MWCNT) where MWCNTs are preferentially located in PS phase. It is shown that the PP-g-PS/MWCNT samples exhibit a strong solid-like behavior and viscosity upturn at a low-frequency range whose extent increases with MWCNTs content, while such features are not significant in the case of PP/PS/MWCNT. In addition, it is demonstrated that the storage modulus measured from dynamic mechanical analysis and electrical conductivity of PP-g-PS/MWCNT are also superior to those of PP/PS/MWCNT at the same loadings of MWCNTs, which can be attributed to the state of MWCNTs dispersion and their localization behavior in the biphasic systems of PP and PS.

Shear viscosity calculation of water in nanochannel: molecular dynamics simulation

Abstract: Shear viscosity is one of the important transport properties which affects different phenomena in nanoconfined water. This study aims to investigate the effect of sub-Angstrom variations of nanochannel size on the shear viscosity of water confined in a silicon wall by employing equilibrium molecular dynamics (EMD) simulations. Simulation results demonstrate that water molecules confined in the slits are layered and for channels width less than 21 A, the number of layers varies from one to six. We show that if the capillary size becomes less than 18.5 A, the sub-Angstrom variations significantly affect the layered structure of the confined water. This causes the anomalous behavior of water viscosity and therefore, the flow resistance of nanoconfined water. According to the previous studies, the shear viscosity is greatly enhanced for subnanometer capillaries so that the shear viscosity increases dramatically by decreasing the channel size; however, we found that shear viscosity obeys an oscillatory behavior and has a complicated behavior which originates from the consistency between the channel size and the space required to embed one layer of water molecules. Results show that five minima and four maxima values for the viscosity are observed for channels width less than 18.5 A. Such unfamiliar behavior of viscosity and, consequently, the flow resistance, friction coefficient and slip length should be taken into account in investigation and design of such nanoconfined water.

A thermal based RBC Aggregation model for two-phase blood flow

Abstract: Creating a reliable and accurate Red Blood Cell (RBC) aggregation model for small and midsize arteries and veins is still an active research subject with more in focus with a multi-scale approach including mesoscale effects. Better understanding the RBC aggregation requires a multi-phase and multi-scale approach for simulating blood with Newtonian and non-Newtonian parts. In our proposed work, viscosity, shear rates, phase distributions and volume fractions with a range of hematocrit levels of RBC are calculated using the depletion interaction theory for two-phase blood flow simulation and compared with the numerical and experimental data in literature. In addition, thermal effects are modeled using energy equations and changes in RBC aggregation are studied with respect to thermal variations. Two-phase fluid-fluid model is used including inter-phase momentum exchange. A new shape factor is proposed for the coupling effects on drag and lift forces. Finally, total interaction energy of RBCs, hematocrit levels of blood at varying temperatures and effects of temperature on viscosity and relative apparent viscosity are computed at varying shear rates and compared with the existing data in literature.

Comparative numerical and experimental investigation of process viscometry for flows in an agitator with a flat blade turbine impeller

Abstract: This paper presents a method for measuring the viscosity of generalized Newtonian fluid directly in flows generated by flat-blade turbine impellers, which are commonly used for moderate mixing and dispersion. A flat-blade turbine with four blades is defined as a model system and analyzed through numerical simulations with experimental verification. Carbopol 940 solution, a high viscosity non-Newtonian fluid with a yield stress, and a bentonite based drilling mud solution were selected as test fluids. Numerical simulation techniques for flow in agitators with a yield stress was established using the rotating coordinate system and flow solutions were validated with experiments by comparing the torque on the impeller shaft. The Metzner-Otto constant and the energy dissipation rate constant were predicted by numerical simulations using the Metzner-Otto correlation and validated via experiments. The effective viscosity that reproduces total energy dissipation rate identical to that of a Newtonian fluid was obtained from both numerical and experimental methods at different impeller speeds, from which the material viscosity curve was established as a function of the shear rate. The accuracy of viscosity prediction was compared with a rheological measurement and the average relative error was below 12% and 7% in the experiment and simulation, respectively. This method has the advantage of being able to measure the in-situ viscosity, where a drilling mud needs to transport more and heavier cuttings and careful preparation of the mud is key issue to a successful drilling process.

Flow characteristic during injection molding of PC/MWNT nanocomposites

Abstract: This work investigates the flow characteristic during injection molding process of PC/MWNT nanocomposites especially focusing on jetting. The initial flow pattern while filling has been compared with that of neat and other particle-filled PCs. The experimental results show that the flow of PC/MWNT 5% is comparable to that of PC/GF 15% and PC/CF 10%. It has been found that small amount of filled MWNT causes significant filling difficulty. Based on rheological investigations, this is attributed to extraordinary shear thinning by MWNT fillers.

Supercritical bifurcation to periodic melt fracture as the 1st transition to 2D elastic flow instability

Abstract: This study, employing a numerical approximation, computationally describes 2D melt fracture as elastic instability in the flow along and outside a straight channel. In the preceding research (Kwon, 2018, Numerical modeling of two-dimensional melt fracture instability in viscoelastic flow, J. Fluid Mech. 855, 595–615) several types of unique instability and corresponding bifurcations such as subcritical and chaotic transitions have been illustrated with possible mechanism presumed. However, the 1st bifurcation from stable steady to unstable periodic state could not be accurately characterized even though its existence was proven evident. The analysis herein aims at verification of this 1st transition to temporally (and also spatially) periodic instability, utilizing the same numerical technique with attentive control of flow condition. As a result of scrutinizing the solutions, the steady elastic flow described by the Leonov rheological model passes through supercritical Hopf bifurcation at the Deborah number of 10.42 and then transforms to the state of the 1st weak periodic instability. It has also been confirmed that near this bifurcation point it takes extremely long to completely develop into either steady state (in the stable case) or periodic instability, which obstructed immediate characterization of the transition in the previous work.

A study on the boundary of linear viscoelasticity in simple shear flows: model calculations

Abstract: Linear viscoelastic region is important to investigate material properties. We investigated boundary of linear viscoelasticity for simple shear flows with various time functions of shear strain. We used Phan-Thien and Tanner (PTT) model and Giesekus model in order to extract the linearity criterion. We determined critical strain (γc) as a function of dimensionless number such as De or Wi and the nonlinear parameter of the models. We found a superposition of the critical strain at start-up shear flow and oscillatory shear flow. Replacing relaxation time by mean relaxation time ( $$\left({\overline {\rm{\lambda }} = {J_{\rm{e}}}{{\rm{\eta }}_{\rm{o}}}} \right)$$ ), we checked the validity of the equation with experimental data.

Study on binary collision of rod-like particles under simple shear flow

Abstract: In this study a theoretical study is carried out on the collision of two rod-like particles suspended in Newtonian fluid under a shear flow. The length of the particle is fixed at 2 µm while the diameter is varied so that the aspect ratio (length/diameter) varies from 1 to 20. Liquid viscosity is changed from 0.01 to 1 Pa·s. The Brownian motion is considered to be negligible. Both hydrodynamic and van der Waals interactions are included in tracking the position and the orientation of each particle. The Hamaker constant is fixed at 1.06 × 10−20 J. The result shows that the kinetic constant of coagulation is reduced to approximately 40% of the value for the non-interacting particles when the viscosity is 1 Pa·s. As collision modes, face-edge, side-side, side-edge and edge-edge are considered. The side-edge mode is most frequently observed in the given range of aspect ratio.