Flow‐induced particle orientation and rheological properties of suspensions of organoclays in thermoplastic resins

The influence of nano‐scale particles on the viscoelastic properties of polymer suspensions is investigated. We have developed a simulation technique for the particle orientation and polymer conformation tensors to study various features of the suspensions. The nano‐particles are modeled as thin rigid oblate spheroid particles and the polymers as FENE‐P type viscoelastic and Newtonian fluid. Both interparticle and polymer‐particle interactions have been taken into account in our numerical computations. The nonlinear viscoelastic properties of nanocomposites of layered silicate particles in non‐Newtonian fluids are examined at the start‐up of shear flow and are interpreted using the model to examine the effects of model parameters as well as flow conditions on particle orientation, viscosity, and first normal stress difference of the suspensions. We have studied the microstructure of polymer‐clay nanocomposites using X‐ray diffraction (XRD) scattering and transmission electron microscopy (TEM). The rheology of these nanocomposites in step‐shear is shown to be fairly well predicted by the model. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 2003–2011, 2010     

Rheological behavior of fiber suspensions in a turbulent channel flow

Rheological behavior of fiber suspensions in a turbulent channel flow was investigated theoretically and numerically. A model of turbulent fiber suspensions was proposed to predict the orientation distribution of fibers. The fluctuating equation for the orientation distribution function (ODF) of fibers was theoretically solved using the method of characteristics. The self-governed mean equation for orientation distribution function (ODF) was derived by relating the fluctuating ODF and angular velocities-correlated terms to the gradient of mean ODF. Then the ODF of fibers was predicted by numerically solving the mean equation for ODF. Finally the shear stress and first normal stress difference of suspensions were obtained. The results, some of which agree with the available relevant experimental data, show that the orientation distribution of fibers in the vicinity of the center of the flow is relatively broad in turbulent regime, and becomes broader with the increase of Reynolds number. The shear stress of fiber suspensions increases, while the first normal stress difference decreases, from the wall to the center of the flow for varying Reynolds number.     

Simulation of semidilute suspensions of non-Brownian fibers in shear flow

Particle-level simulations are performed to study semidilute suspensions of monodispersed non-Brownian fibers in shear flow, with a Newtonian fluid medium. The incompressible three-dimensional Navier-Stokes equations are used to describe the motion of the medium, while fibers are modeled as chains of fiber segments, interacting with the fluid through viscous drag forces. The two-way coupling between the solids and the fluid phase is taken into account by enforcing momentum conservation. The model includes long-range and short-range hydrodynamic fiber-fiber interactions, as well as mechanical interactions. The simulations rendered the time-dependent fiber orientation distribution, whose time average was found to agree with experimental data in the literature. The viscosity and first normal stress difference was calculated from the orientation distribution using the slender body theory of Batchelor [J. Fluid Mech. 46, 813 (1971)], with corrections for the finite fiber aspect ratios. The viscosity was also obtained from direct computation of the shear stresses of the suspension for comparison. These two types of predictions compared well in the semidilute regime. At higher concentrations, however, a discrepancy was seen, most likely due to mechanical interactions, which are only accounted for in the direct computation method. The simulated viscosity determined directly from shear stresses was in fair agreement with experimental data found in the literature. The first normal stress difference was found to be proportional to the square of the volume concentration of fibers in the semidilute regime. As concentrations approached the concentrated regime, the first normal stress difference became proportional to volume concentration. It was also found that the coefficient of friction has a strong influence on the tendency for flocculation as well as the apparent viscosity of the suspension in the semidilute regime.     

Rheology and Structure of Cornstarch Suspensions in Water-Poly(propylene glycol) Mixtures

Concentrated aqueous cornstarch (CS) suspensions are often used to demonstrate an extreme example of shear thickening rheological behavior. Here, we describe the increased rheological complexity that occurs on the addition of poly(propylene glycol) (PPG) to an aqueous CS suspension. The appearance of shear thickening/jamming, shear thinning, yield stress and near-Newtonian behaviors is dependent on the PPG:water ratio. Rheological measurements have been complemented by dielectric measurements and optical microscopy. The complex behavior is interpreted in terms of reduced electrostatic stabilization of the CS particles with increased poly(propylene glycol) concentration. The analysis also suggests why cornstarch suspensions in water exhibit particularly good shear thickening characteristics.     

Dynamics of shearing in flocculating fine sediment suspensions

The variability of rheological properties in cohesive sediment suspensions are studied and related to the flocculation process. It was found that the steady state shear stress increases exponentially with increasing sediment concentration and increases logarithmically by increasing the salinity of the suspension. The salinity dependence of the shear stress was correlated with changes in the floc-size. These results reflect the dominant influence of fluid-sediment interactions on the flocculation behavior of the suspension. An analytic model predicting shear stress as a function of the physical and electrochemical properties of the fine sediment suspension is developed.     

Influence of poly(ethylene oxide) brushes on the rheological properties of MgO colloidal suspensions in water

A combined experimental and multiscale simulation study of the influence of polymer brush modification on interactions of colloidal particles and rheological properties of dense colloidal suspensions has been conducted. Our colloidal suspension is comprised of polydisperse MgO colloidal particles modified with poly(ethylene oxide) (PEO) brushes in water. The shear stress as a function of shear rate was determined experimentally and from multiscale simulations for a suspension of 0.48 volume fraction colloids at room temperature for both bare and PEO-modified MgO colloids. Bare MgO particles exhibited strong shear thinning behavior and a yield stress on the order of several Pascals in both experiments and simulations. In contrast, simulations of PEO-modified colloids revealed no significant yielding or shear thinning and viscosity only a few times larger than solvent viscosity. This behavior is inconsistent with results obtained from experiments where modification of colloids with PEO brushes formed by adsorption of PEO-based comb-branched chains resulted in relatively little change in suspension rheology compared to bare colloids over the range of concentration of comb-branch additives investigated. We attribute this discrepancy in rheological properties between simulation and experiment for PEO-modified colloidal suspensions to heterogeneous adsorption of the comb-branch polymers.     

Rheological peculiarities of concentrated suspensions

The rheological properties of concentrated suspensions of metal oxides dispersed in transformer oil, which are used as electrorheological fluids, are systematically studied. Colloidal particles have intermediate sizes between nano- and microsized scales. Low-amplitude dynamic measurements show that the storage moduli of the examined suspensions are independent of frequency and these materials should be considered as solidlike elastic media. The storage modulus is proportional to the five-powdered particle volume concentration. At the same time, a transition through an apparent yield stress with a reduction in the viscosity by approximately six orders of magnitude is distinctly seen upon shear deformation. The character of the rheological behavior depends on the regime of suspension deformation. At very low shear rates, a steady flow is possible; however, upon an increase in the rate, an unsteady regime is realized with development of self-oscillations. When constant shear stresses are preset, in some range of stresses, thickening of the medium takes place, which can also be accompanied by self-oscillations. In order to gain insight into the nature of this effect, measurements are performed for samples with different volume/surface ratios, which show that, in some deformation regimes, suspension is separated into layers and slipping occurs along a low-viscosity layer with a thickness of several dozen microns. Direct observations show a distinct structural inhomogeneity of the flow. The separation and motion of layers with different compositions explain the transition to the flow with the lowest apparent Newtonian viscosity. Thus, the deformation of concentrated suspensions is associated with self-oscillations of stresses and slipping along a low-viscosity interlayer.     

Effects of Surface Properties on Rheological and Interfacial Properties of Pickering Emulsions Prepared by Fumed Silica Suspensions Pre-Adsorbed Poly(N-Isopropylacrylamide)

The hydrophobic fumed silica suspensions physically pre-adsorbed poly(N-isopropylacrylamide) (PNIPAM) in water could prepare oil dispersed in water (O/W) Pickering emulsion by mixing of silicone oil. The resulting Pickering emulsions were characterized by the measurements of volume factions of emulsified silicone oil, adsorbed amounts of the silica suspensions, oil droplet size, and some rheological responses, such as stress-strain sweep curve and dynamic viscoelastic moduli as a function of the added amount of PNIPAM. Moreover, their characteristics were compared with those of the O/W Pickering emulsions prepared by the hydrophilic fumed silica suspensions pre-adsorbed PNIPAM. For the emulsions prepared by the hydrophobic silica suspensions, an increase in the added amount of PNIPAM led to (1) a decrease in the volume fraction of the emulsified oil in the emulsified phase, (2) both the size of oil droplets and the adsorbed amount of the corresponding silica suspensions being almost constant, except for the higher added amounts, and (3) both the storage modulus (G′) and the yield shear strain being constant. The term of 1 is the same for the emulsions prepared by the hydrophilic silica suspensions, whereas both the adsorbed amount of the corresponding silica suspension and the G′ value increase and both the droplet size and the yield shear strain decrease with an increase in the added amount of PNIPAM. The differences between the rheological properties of the emulsions prepared by the hydrophilic silica suspensions and those by the hydrophobic ones are attributed to the hydrophobic interactions of the flocculated silica particles in the Pickering emulsions.     

Model filled rubber. II. Particle composition dependence of suspension rheology

Monodisperse size colloidal particles varying in chemical composition were synthesized by emulsifier‐free emulsion polymerization. Using a stress‐controlled rheometer, the rheological behavior of colloidal suspensions in a low molecular weight liquid polysulfide was investigated. All suspensions exhibited shear thinning behavior. The shear viscosity, dynamic moduli, and yield stress increased as interactions between particles and matrix increased. The rheological properties associated with network buildup in the suspensions were sensitively monitored by a kinetic recovery experiment. We propose that interfacial interactions by polar and hydrogen bonding between particles and matrix strongly promote affinity of matrix polymer to the filler particles, resulting in adsorption or entanglement of polymer chains on the filler surface. A network structure was formed consisting of particles with an immobilized polymer layer on the particle surface with each particle floc acting as a temporary physical crosslinking site. As the interfacial interaction increases, the adsorbed layer thickness on the filler particles, hence, the effective particle volume fraction, increases. As a result, the rheological properties were enhanced in the order PS < PMMA < PSVP. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 815–824, 1999     

Concentration effects on the isolation and dynamic rheological behavior of cellulose nanofibers via ultrasonic processing

Chemical pretreatment combined with high-intensity ultrasonication was performed to disintegrate cellulose nanofibers from poplar wood powders. The cellulose content in each suspension was treated as the control variable because the suspension concentration significantly influences the properties of the resultant cellulose nanofibers via ultrasonic processing. The as-obtained cellulose nanofibers were characterized by fiber diameter distribution, crystal structure, and rheological analysis. An increase of not more than 1.2 % of the cellulose content resulted in finer nanofibers. Both storage modulus and loss modulus of cellulose nanofiber suspensions rapidly increased with increasing concentration because of the gradual formation of a stronger network structure. In addition, the dynamic mechanical behavior of suspensions with fiber contents lower than 0.8 % was affected by the frequency and temperature alteration in contrast with the suspension with higher fiber contents. The sol–gel transformation and the visco-elastic transition depend on the hydroxyl bonding and the cross-linking extent of cellulose nanofibers in various concentration environments.     

Effects of the degree of surface modification on the rheological responses of precipitated silica suspensions in benzyl alcohol

Two types of precipitated silica powders modified by poly (dimethylsiloxane) (PDMS) were suspended in benzyl alcohol and their rheological properties were investigated as a function of silica volume fraction, φ. The suspensions were classified into sol, pre-gel, and gel states based on the increase in φ. An increase in the degree of surface modification by PDMS caused gelation at higher φ. Plots of apparent shear viscosity against shear rate in the sol and pre-gel states of highly modified silica suspensions showed weak shear thickening behavior, while the same plots for silica suspensions with a low modification level exhibited shear thinning behavior. The dynamic moduli of hydrophobic suspensions in the pre-gel and gel states were dependent on the surface modification: the storage modulus G′ was larger than the loss modulus G″ in the linear region and these moduli increased with increasing φ, irrespective of the silica powder. The linear region of the φ range for the precipitated silica suspensions was wider than that for the fumed silica powders modified by PDMS suspended in benzyl alcohol, while the G′ value in the linear region for the precipitated silica suspensions was less than those for the fumed silica suspensions.     

The effect of particle shape on the structure and rheological properties of carbon-based particle suspensions

The structure and rheological properties of carbon-based particle suspensions, i.e., carbon black(CB), multi-wall carbon nanotube(MWNT), graphene and hollow carbon sphere(HCS) suspended in polydimethylsiloxane(PDMS), are investigated. In order to study the effect of particle shape on the structure and rheological properties of suspensions, the content of surface oxygen-containing functional groups of carbon-based particles is controlled to be similar. Original spherical-like CB(fractal filler), rod-like MWNT and sheet-like graphene form large agglomerates in PDMS, while spherical HCS particles disperse relatively well in PDMS. The dispersion state of carbon-based particles affects the critical concentration of forming a rheological percolation network. Under weak shear, negative normal stress differences(ΔN) are observed in CB, MWNT and graphene suspensions, while ΔN is nearly zero for HCS suspensions. It is concluded that the vorticity alignment of CB, MWNT and graphene agglomerates under shear results in the negative ΔN. However, no obvious structural change is observed in HCS suspension under weak shear, and accordingly, the ΔN is almost zero.     

Rheology and UV protection properties of suspensions of fine titanium dioxides in a silicone oil

Ultrafine particles of titanium dioxide (TiO2) are very attractive as a UV protection ingredient in cosmetic products. The UV-scattering behavior of TiO2 suspensions in a silicone oil are studied in relation to rheological properties. To control the dispersion stability of suspensions, two types of polyether-modified silicones are used as dispersants. When the suspensions are prepared with branch-type dispersants in which the polyether groups are incorporated as side chains along the backbone, the flow is shear-thinning even at low shear rates. The appearance of plateaus in the frequency-dependence curves of storage modulus implies the solidlike responses. On the other hand, the suspensions prepared with linear conformation dispersants, in which the silicone group and polyether group are alternately repeated in one long chain, are Newtonian at low shear rates. The suspensions are regarded as liquids, because the storage modulus decreases rapidly in the low-frequency region. The suspension rheology is strongly associated with flocculated structures that are primarily controlled by the interparticle attractions. The differences in rheological behavior can be explained by the differences in the adsorbed conformation of dispersant silicones. From optical measurements, it is confirmed that UV scattering increases with decreasing flocculation degree. Therefore, good agreement is established between rheological properties and UV protection ability.     

The effect of wall depletion on the rheology of microfibrillated cellulose water suspensions by optical coherence tomography

Rheology of microfibrillated cellulose (MFC) water suspensions was characterized with a rotational rheometer, augmented with optical coherence tomography (OCT). To the best of the authors’ knowledge, this is the first time the behavior of MFC in the rheometer gap was characterized by this real-time imaging method. Two concentrations, 0.5 and 1 wt% were used, the latter also with 10^?3 and 10^?2 M NaCl. The aim was to follow the structure of the suspensions in a rotational rheometer during the measurements and observe wall depletion and other factors that can interfere with the rheological results. The stepped flow measurements were performed using a transparent cylindrical measuring system and combining the optical information to rheological parameters. OCT allows imaging in radial direction from the outer geometry boundary to the inner geometry boundary making both the shear rate profile and the structure of the suspension visible through the rheometer gap. Yield stress and maximum wall stress were determined by start-up of steady shear and logarithmic stress ramp methods and they both reflected in the stepped flow measurements. Above yield stress, floc size was inversely proportional to shear rate. Below the yield stress, flocs adhered to each other and the observed apparent constant shear stress was controlled by flow in the depleted boundary layer. With higher ionic strength (10^?2 M NaCl), the combination of yield stress and wall depletion favored the formation of vertical, cylindrical, rotating floc structures (rollers) coupled with a thicker water layer originating at the suspension—inner cylinder boundary at low shear rates.     

三种不同基体中玻璃微球悬浮体系的流变行为

三种不同基体中玻璃微球悬浮体系的流变行为吴大诚，陈军武，徐建军，杜鹏，顾群，李玉景（成都科技大学纺织工学院成都６１００６５）关键词高分子流变学，Ｂｏｇｅｒ流体，悬浮体，聚异戊二烯悬浮体的流体动力学研究始于本世纪初，Ｅｉｎｓｔｅｉｎ“对极稀悬浮体粘度的...     

Effect of homogenization (microfluidization) process parameters in mechanical production of micro- and nanofibrillated cellulose on its rheological and morphological properties

The rheological properties of microfibrillated cellulose (MFC)/nanofibrillated cellulose (NFC) suspensions have an important role during processing and mixing. In this work, the process parameters for MFC/NFC production within a microfluidizer (i.e., the size of interaction chamber and number of passes) were varied to investigate the influences on morphology, zeta potential, chemical properties and rheological features including viscosity, creep, strain recovery and yield stress behavior. The stability and appropriate viscosity of the fiber suspensions can be controlled by optimizing the processing conditions, resulting in a reduction in fiber diameter and most negative zeta potential value. The viscosity increased with higher amount of fibrillation by using a smaller chamber or higher number of passes, but intermediate plateau values are characteristic for temporary aggregation and breaking-up of the fiber network. The creep response and yield stress have been described by parameters of the Burger model and Herschel–Bulkley model, respectively, showing a more prominent effect on yield stress of chamber size than number of passes. The network formation leads to lower creep compliance and step-like strain recovery. The transition from gel-like to liquid-like behavior as characterized by the dynamic yield point at a specific strain, is almost independent of the processing conditions. Most important, the total number of passes applied in production can be directly related to the rotational Péclet number, which combines rheological and morphological data.     

Orientation effects and rheology of short glass fiber-reinforced thermoplastics

The rheology of polyolefines, polyamide 6, and polystyrene-acrylonitrile filled with glass fibers of different concentrations and aspect ratios have been investigated in simple shear flow, capillary rheometry, and uniaxial elongation. A comparison is made with unfilled and glass bead-filled melts. Fiber orientation was investigated by X-ray microradiography.Steady-state viscosities are obtained on fibers aligned parallel to the direction of flow. Entrance pressure losses, the shape of the viscosity function, and the appearance of a yield stress are depending on the fiber aspect ratio. The temperature dependence of the viscosity function is not significantly different from that of the unfilled melt.Transient shear stresses were measured on samples of different initial orientations of the fibers. The change of fiber orientation during shear flow gives rise to a pronounced overshoot of shear stress and normal stress difference. Elastic strains in shear are increased by the fibers but elastic elongational strains are reduced. Mechanisms are proposed to explain the experimental observations.Dedicated to Dr. D. Heinze on the occasion of his 60th birthday.     

Effect of Rotary Inertia on the Orientational Behavior of Dilute Brownian and Non-Brownian Fiber Suspensions

The orientational behavior of a dilute suspension of slender Brownian and non-Brownian fibers with rotary inertia in simple shear and turbulent channel flows is numerically investigated. The translational inertia of fibers is neglected. The equations describing the evolution of fibers orientation are integrated along the Lagrangian paths of the fluid elements. The fully developed turbulent channel flow at Re _τ = 180 is provided by a direct numerical simulation (DNS). The coupling between the flow field and the fiber dynamics is one way. The Brownian motion is modeled by a stochastic Wiener process. The results are compared with those of inertia-free particles. In simple shear flow, the inertial non-Brownian fibers align slower than the inertia-free fibers to the shear direction while they tend to the same steady state orientation. For Brownian fibers, the steady state orientation of inertial and inertia-free fibers differ. In turbulent channel flow, the second moment of the orientation distribution function shows an oscillatory behavior at high values of inertia for non-Brownian fibers while the oscillations disappear at lower inertia. For Brownian fibers, the oscillations are weaker due to the damping effect of the Brownian diffusivity.     

不同分子量可德胶水悬浮液的粘弹性研究

采用动态粘弹性测量研究了不同分子量的生物大分子可德胶 (Curdlan)水悬浮液 (ASC)的流变学特性 .室温下观察到ASC具有弹性 (Solid like)行为 ,储能模量G′在测量范围内轻微依赖频率 ,而损耗模量G″和损耗角正切tanδ存在最小值 .ASC粘弹性随可德胶分子量和浓度的增加而增强 .ASC的流动特性符合Herschel Bulkley模型 ;其弹性行为可以通过渗流理论的标度弹性模型来描述 .网络结构是由于可德胶颗粒聚集或絮凝而形成的 ,当可德胶含量超过临界浓度cs=0 3 %时 ,弹性模量G′与可德胶浓度存在标度关系G′=Goεt,其中标度指数t=2 5 4.     

Study of a shear thickening fluid: the suspensions of monodisperse polystyrene microspheres in polyethylene glycol

The monodisperse polystyrene (PS) microspheres were prepared by dispersion polymerization. The rheological properties of shear thickening fluid (STF) based on PS microspheres dispersing in polyethylene glycol with different concentrations were studied through the steady and oscillatory shear at different temperatures, respectively. All suspensions successively present the first shear thinning, the shear thickening, and the second shear thinning. The experimental results indicate that the shear thickening behavior of STF is controlled by the concentration of PS microspheres and temperature, as changed from continuous shear thickening (CST) to discontinuous shear thickening (DST) with increasing solid content or decreasing temperature. The STF is affected by shear rate, temperature, and the viscosity of the dispersed medium, and it is reversible absolutely and presents transient response ability. Both CST and DST behave as dilatancy. The PS microsphere aggregations formed under shear stress may result in the shear thickening in STFs.