Applied Shear Field Research Articles

Sepiolite nanofibers structure. From tridimensional networks to bidimensional layers

The aim of this work is to obtain tridimensional (3D) networks and bidimensional (2D) layered structures from hydrogels containing high aspect ratio sepiolite nanofibers. In this sense, it is proposed an appropriate methodology by applying an oscillating shear strain field to sepiolite hydrogels which were previously obtained by sonication. The gel stability is crucial in the final structure of the spatial arrangement. Then, the influence of sepiolite concentration and shear field on hydrogel stability is assessed by studying its rheological properties in order to determine the process window to obtain 3D or 2D final nanostructure. Three hydrogels with different concentrations (0.002; 0.02 and 0.2 g sepiolite/mL water) of high aspect ratio sepiolite were analyzed. A direct relationship among the hydrogel concentration and stability gives a tool to obtain 2D or 3D nanostructures. Hydrogels with low sepiolite concentration (0.002 and 0.02 g/mL) under shear field break, giving 2D layered structure. Meanwhile, high sepiolite concentration hydrogel (0.2 g/mL) preserves 3D networks since it does not break under the applied shear field. The comparison of final structures, before and after shear field application, was performed by analyzing the corresponding dried hydrogels by Scanning Electronic Microscopy. A versatile method was proposed to prepare 2D and 3D structures from high aspect ratio sepiolite nanofibers. It is possible to tailor the final nanofibers spatial arrangement from a combination of particle concentration and applied shear field. As a consequence, the processing window as shear field for each desired final arrangement can be assessed depending on particle concentration.

Highlighting Thermo-Elastic Effects in Confined Fluids.

The recent identification of a finite shear elasticity in mesoscopic fluids has motivated the search of other solid-like properties of liquids. We present an innovative thermal approach of liquids. We identify a dynamic thermo-elastic mesoscopic behavior by building the thermal image produced by different liquids upon applying a low frequency mechanical shear field. We selected three fluids: a low molecular weight polybutylacrylate (PBuA), polypropyleneglycol (PPG), and glycerol. We demonstrate that a part of the energy of the shear strain is converted in cold and hot shear bands varying synchronously with the applied shear field. This thermodynamic change suggests a coupling to shear elastic modes in agreement with the low frequency shear elasticity theoretically foreseen and experimentally demonstrated.

Surface-Active Lipid Linings under Shear Load--A Combined in-Situ Neutron Reflectivity and ATR-FTIR Study.

We study shear effects in solid-supported lipid membrane stacks by simultaneous combined in-situ neutron reflectivity (NR) and attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). The stacks mimic the terminal surface-active phospholipid (SAPL) coatings on cartilage in mammalian joints. Piles of 11 bilayer membranes of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) are immobilized at the interface of the solid silicon support and the liquid D2O backing phase. We replace the natural hyaluronic acid (HA) component of synovial fluid by a synthetic substitute, namely, poly(allylamine hydrochloride) (PAH), at identical concentration. We find the oligolamellar DMPC bilayer films strongly interacting with PAH resulting in a drastic increase of the membranes d spacing (by a factor of ∼5). Onset of shear causes a buckling-like deformation of the DMPC bilayers perpendicular to the applied shear field. With increasing shear rate we observe substantially enhanced water fractions in the membrane slabs which we attribute to increasing fragmentation caused by Kelvin-Helmholtz-like instabilities parallel to the applied shear field. Both effects are in line with recent theoretical predictions on shear-induced instabilities of lipid bilayer membranes in water (Hanasaki, I.; Walther, J. H.; Kawano, S.; Koumoutsakos, P. Phys. Rev. E 2010, 82, 051602). With the applied shear the interfacial lipid linings transform from their gel state Pβ' to their fluid state Lα. Although in chain-molten state with reduced bending rigidity the lipid layers do not detach from their solid support. We hold steric bridging of the fragmented lipid bilayer membranes by PAH molecules responsible for the unexpected mechanical stability of the DMPC linings.

Shaken but not stirred: The formation of reversible particle – polymer gels under shear

Abstract Some dispersions of clay and silica particles in water in the presence of relatively high molecular weight polyethyleneoxide (PEO), which are fluid when at rest, become solid-like after a quick shake to such an extent that they can be held in the hand. On leaving the dispersions for a certain period of time, minutes to days depending on the polymer molecular weight and concentration, the dispersions become liquid-like again. These dispersions have been called “shake gels”, and a number of physical variables are determinant for producing the gel and controlling its behavior. In this work, we have studied the effect of the shape and size of the particles and of the PEO molecular weights. To that aim, we have mapped the “phase” behavior of silica (Ludox TM-50), montmorillonite and laponite dispersions in presence of PEO of different molecular weight. Shake gels are formed under certain concentrations of particles and PEO. The necessary degree of particle coverage for shake gel formation seems to depend on the particle shape. Whereas in the case of disc-shaped particles this limit is around the saturation concentration, in the case of spherical particles the limit is around 2/3 of the particle surface saturation. On the other hand, we observe some differences between montmorillonite (micrometer-size particles) and laponite (nanometric particles) dispersions. When we shake the former we find in some cases an important and irreversible phase separation; on the contrary, in the case of the laponite dispersions, the phase separation is far less frequent and extensive. Finally, we have found that the nature of the applied shear field has a profound effect on the sample behavior. When we place the dispersions in a conventional rheometer and shear them at moderate shear rate, for many minutes the gel is not formed. However, shaking it by hand or extruding it through a syringe brings about this effect in a matter of seconds.

Formation kinetics and rheology of alginate fluid gels produced by in-situ calcium release

The formation and properties of alginate fluid gels produced using in-situ calcium release and a defined shear field are investigated.Results show that, while the rate of particle growth during formation increases significantly upon lowering the shear rate, the rate of inter-particle interaction post-production remains unaffected by the magnitude of shear.The longer polymer chain allows a greater number of possible sites for calcium crosslinking per chain which enhances the formation of a percolating network and increases the number of rheologically-effective network crosslinks. As a result, high Mw alginate fluid gels exhibit faster gelation kinetics and greater viscosities than those of low Mw alginate fluid gels.Particle stiffness increases with increasing CaCO3 concentration until a critical value is reached. Quiescent gels display the same CaCO3 critical value, suggesting that the number of crosslinks within an individual fluid gel particle is equivalent to that of their counterpart quiescent gel. This is due to the fast coil-dimer transition which, under the applied shear field, leads to kinetically trapped fluid gel structures. However, their textural response differs from that of a quiescent gel which is likely caused by the bridging between fluid gel particles.In addition to advancing the understanding of the production of fluid gels from alginates, this work shows for the first time how the ordering kinetics and the resulting particle properties can be manipulated by the choice of alginate Mw and CaCO3 concentration, potentially allowing the design and delivery of specific structures with desirable attributes.

Influence of molar mass and temperature on the dynamics of network formation in polycarbonate/carbon nanotubes composites in oscillatory shear flows

The influence of molar mass and temperature on the formation of networks of multiwalled carbon nanotubes (MWCNT) in oscillatory shear flows was investigated. Combined rheological and electrical investigations were performed using composites with 0.5–5.0 wt% MWCNT in a low- and a high-viscosity polycarbonate (PC) at 190 °C and 250 °C. The objective of this work was to study the dynamics of the simultaneous formation of electrical and rheological networks by taking into account the superposition of (i) network breakup by the applied shear field and (ii) the diffusion-controlled clustering of carbon nanotubes. The formation (i.e. the buildup) of electrical and rheological networks proceeds more rapidly for a lower-viscosity matrix and at higher temperatures, whereas breakup of the electrical network is more pronounced at lower temperatures because of a larger stress-transfer between polymer matrix and MWCNT network. Sinusoidal shear deformation results in an oscillatory electrical conductivity with decreasing average value at large shear amplitudes, indicating that the electrical nanotubes network consists of weakly bonded carbon nanotube clusters which can be easily released and reformed. These data also show that MWCNT–MWCNT bonds can be reversibly deformed up to a maximum deformation. The latter experimental result precises the current understanding of electrical carbon nanotubes networks.

Crystallization under shear in isotactic polypropylene containing nucleators

We report results from time-resolved small/wide angle X-ray scattering of well-characterized isotactic polypropylene (iPP), in its pure form as well as loaded with nucleating additives of different shapes (isotropic, elongated, platelet-like). Isothermal crystallization of these systems has been studied both under quiescent conditions and after application of a shear pulse. A quantitative analysis is presented that couples the applied shear field and the specific additive to the nucleation of iPP and the subsequent kinetics of crystallization. The effect of flow is only relevant if its contribution to the nucleation density is at least of the same order of magnitude as the nucleation density under quiescent conditions. Anisotropic additives show strong interactions with the flow field leading to extra nucleating sites.

Rheology and birefringence of Fomblin YR at very high shear rates

The rheological and structural properties of perfluoropolyether (PFPE) lubricant films including viscosity, shear stress, and birefringence were measured at relatively low to extremely high shear rates using a rotational optical rheometer. The viscosity of various films with different thicknesses exhibit Newtonian behavior up to a shear rate 1 × 104 s−1, with a transition to shear-thinning behavior obvious at higher shear rates. Birefringence of these films was also measured for the first time, and these results indicate chain alignment with shear in the shear-thinning regime. The shear rate at which alignment occurs is similar to that of the onset of shear thinning. This correlation between chain alignment and shear thinning provides direct evidence that the ability of PFPEs to lubricate hard drives at high shear rates is a direct consequence of the ability of the applied shear field to align the molecules on a molecular level.

Study of Starch Gelatinization in a Flow Field Using Simultaneous Rheometric Data Collection and Microscopic Observation

Abstract Gelatinisation of aqueous cornstarch dispersions with different amylose/amylopectin ratios were studied with respect to their response to the simultaneous application of thermal and mechanical stresses. The study was conducted using a combined controlled stress (CS) rheometer and microscope with camera, allowing variations of viscosity and microstructure to be recorded concurrently. The microscopic images were recorded by the camera, one image for each rheological data point. It is the first time a direct relationship between viscosity and physical variation of starch granules in dispersion has been established and directly optically observed for starch gelatinisation in the presence of an applied shear field. It was found that there are two steps for starch gelatinisation under these conditions with regard to the physical transition of the starch granules: (1) starch swelling and gradual dissolving and leaching of starch components; (2) rapid dissolution of a high proportion of the granule. In the first step the viscosity increaseed gradually, as the starch granule was slowly dissolved and materials were leached into the water carrier. The granules became almost transparent in the presence of the applied thermal and mechanical fields and dimensional changes in the starch granules (swelling) during the latter stages of gelatinisation can not be clearly observed. In the second step, the starch granule disappeared in a short time and viscosity of the aqueous material increased sharply. The gelatinisation temperature under shear stress, as measured by the CS rheometer/microscope combination is lower than that measured by Differential Scanning Calorimetry (DSC) under shear-less conditions.

The effect of temperature and shear rate upon the aggregation of whey protein and its implications for milk fouling

In milk fouling, the rate of production and composition of the fouled layer is controlled by reactions in the bulk fluid which lead to the creation of aggregates which are deposited on heated surfaces. The influences of temperature (70 °C–90 °C) and shear rate (111 s −1–625 s −1) upon the rate of growth and size of aggregates in whey protein concentrate solutions (WPC35) have been investigated using a Couette apparatus with and without the addition of mineral Calcium and Phosphorous. At temperatures below 75 °C, the aggregates formed are small and weakly bonded, whilst aggregates formed at higher temperatures (and higher shear rates) were denser and more rigid. This was attributed to weak van der Waals bonding between the particles at low temperatures and the formation of stronger covalent disulphide bonds at higher temperatures. Growth of the aggregates is due to both protein denaturation and aggregation; temperature step-change experiments have verified that denaturation is a strong function of temperature whilst aggregation is a function of the applied shear field and the strength of the particles. Step-changes in shear rate have shown that both denaturation and particle growth rate are enhanced by increasing shear rate due to an increase in the number of particle collisions, yet the final particle size showed a complex behaviour with the increase in shear rate. The addition of minerals to the WPC solution resulted in the formation of much smaller aggregates and increased deposition onto the surface of the Couette apparatus. This was attributed to interactions between Ca and β-lactoglobulin both in the bulk and at the heated surface.

Development of highly oriented polymer crystals from row assemblies

The development of organized structures from sheared isotactic polypropylene melts with different amounts of dibenzylidene sorbitol (DBS) as a nucleating agent has been followed in real time using small angle X-ray scattering in conjunction with synchrotron radiation. The results show that, above a certain DBS concentration threshold, the lamellar crystals are highly oriented perpendicular to the direction of the prior applied shear field. The role of the directing agent in nucleating the crystallization and templating the crystal anisotropy is discussed.

Aging under shear: Structural relaxation of a non-Newtonian fluid

The influence of an applied shear field on the dynamics of an aging colloidal suspension has been investigated by the dynamic light-scattering determination of the density autocorrelation function. Though a stationary state is never observed, the slow dynamics crosses between two different nonequilibrium regimes as soon as the structural relaxation time tau(s) approaches the inverse shear rate gamma (-1) . In the shear-dominated regime (at high gamma values) the structural relaxation time is found to be strongly sensitive to the shear rate ( tau(s) approximately gamma (-alpha) , with alpha approximately 1 ) while aging proceeds at a very slow rate. The effect of the shear on the detailed shape of the density autocorrelation function is quantitatively described, assuming that the structural relaxation process arises from the heterogeneous superposition of many relaxing units, each one independently coupled to shear with a parallel composition rule for time scales: 1/tau-->1/tau+A gamma .

Dipolar and chain-linking effects on the rheology of grafted chains in a nanopore under shear at different grafting densities.

Nonequilibrium molecular dynamics simulations are applied to investigate the rheological properties of coplanar nanopore systems of amphiphilic chain molecules with the tails grafted to the walls of the nanopore and with the head-group ends immersed in a solvent inside the nanopore. In particular, the effects of modifying the interaction between the amphiphilic head-groups by repulsive dipolar interactions or directly covalently linking pairs of chains at the head-groups have been studied. Different grafting densities are considered. The chains are modeled by a harmonic bead-spring model, and all particles interact through the repulsive part of a shifted Lennard-Jones potential. Head-group linking is also governed by a bead-spring potential. A harmonic potential models the lattice vibrations of the atomic boundaries. The rheological properties are studied by a shearing process in which the heat generated is conducted away from the system through the walls by applying a Nosé-Hoover thermostat. Computed geometric parameters such as average chain length and average tilt angle indicate reduction in chain flexibility at large dipole moments. Dipolar repulsion is found to broaden the density profiles of the solvent. This effect is opposed by chain linking. For increasing head-group repulsion, the amphiphile-solvent interfaces become less diffusive that leads to systematic variations in viscosities with increasing dipole moments. Friction forces become stronger at large grafting density and for larger dipole moments. The changes in rheological properties for fixed grafting density are essentially governed by the change in the response of the normal pressure to the applied shear field. The velocity gradients depend strongly on the degree of grafting density but appear to be less sensitive to the strength of the interactions between the head groups.

Microstructures and Rheological Responses of Aqueous CTAB Solutions in the Presence of Benzyl Additives

In this paper, we investigated the effects of various benzyl additives on the microstructure of a cationic surfactant, cetyltrimethylammonium bromide (CTAB), in aqueous medium. Benzyl derivatives considered here were sodium salicylate (NaSal), sodium benzoate (NaBen), and p-methylsalicylic acid (PMSA). The CTAB concentration ranged from 1 to 20 mM, and the molar ratio of an additive to CTAB was varied from 0.1 to 10.0. The experimental results showed that the CTAB solution exhibited a variety of rheological responses, such as shear thinning, shear thickening, and long time relaxation, under an applied shear field. In particular, the shear viscosity of a dilute CTAB solution, which showed pronounced shear thinning at low shear rates, exhibited shear thickening at high shear rates. As the shear rate was increased even more, the viscosity shear thinned again. The rheological response associated with microstructural transition was closely dependent on the concentrations and molecular structures of constituent...

Enhanced Polymer Segment Exchange Kinetics Due to an Applied Shear Field

Introduction. Fundamental knowledge about the behavior of polymer melts near solid surfaces is important to many technologies such as electronics packaging, adhesion, coatings, and polymer processing. When polymers are placed under flow, surface effects can be critical in widely used polymer processing methods such as die extrusion and injection molding. Changes in the extrusion die material can suppress or induce instabilities that lead to unsatisfactory products.1 These results show that the short-ranged interactions between the polymer and the substrate (coupled with the general viscoelastic properties of polymers) have macroscopic impacts. These processing instabilities have been correlated with “wall slip” at the polymer/solid interface where the apparent fluid velocity near the solid surface is no longer zero. Much work has been performed to determine the important physical parameters for wall slip and to understand the underlying mechanisms of the behavior of polymers at solid boundaries with the goal of better designing processing routes.2-12 However, the effect of flow on the microscopic behavior of polymer chains directly at a solid surface is not well understood. Few experimental data are available for the behavior of polymer melt chains at the solid interface because of the inherent difficulty of distinguishing between surfacebound chains and the melt matrix.13-15 In this communication, neutron reflectometry was used to study the effect of a shear deformation on the adsorption/desorption kinetics of an entangled polymer melt at an attractive solid substrate. The high spatial resolution of neutron reflectivity, O (A), provides information directly at the polymer/solid interface. A labeled (deuterated) polymer layer was formed at the surface of a silicon wafer and was welded with a hydrogenated polymer matrix. The fraction of deuterated polymer segments at the silicon surface was monitored with different annealing times with no applied shear and with varying shear rates after a set annealing time. Increases in the shear rate resulted in an increased rate of desorption of the surface-bound polymer chains (or adsorption of the matrix polymers). To the best of our knowledge, this shear-induced adsorption/desorption of an entangled polymer melt at an attractive solid surface was directly observed for the first time. These results provide important experimental evidence of molecular mechanisms which should be incorporated into future models of flow effects on polymer dynamics at surfaces.

Effects of dispersant on rheological behavior of silicon nitride aqueous suspension

Colloidal processing techniques such as slip casting, centrifugal slip casting or pressure slip casting are considered as a powerful route to fabricate silicon nitride ceramic components with complex shapes [1, 2]. For this purpose, Si3N4 powder should be dispersed efficiently in liquid medium thus forming a desirable suspension with the lowest viscosity and highest solid content. Colloidal behavior of Si3N4 suspension with either aqueous or organic medium has been studied widely [3–7]. The same Si3N4 powder behaves differently depending on the various processing parameters applied. In order to achieve promising powder suspensions, dispersants are frequently used [7–10]. After that the particle surface has been modified with dispersant by either changing the surface charge, i.e. by electrolytes, or by coating with a large amount of organic barrier, i.e. by polymers, or by a combined action of both factors, i.e. by polyelectrolytes, Si3N4 particles can be stabilized to a promising level in liquid media. The introduction of dispersant to suspension generally has important influence on the electrokinetics of suspended particles and the rheological behavior of suspension. In this letter, some experimental results about the effects of dispersant on the rheological behavior of Si3N4 aqueous suspension will be discussed. Silicon nitride powder (H.C. Starck, Germany) with an average particle size of 0.60 μm and a BET specific surface area of 21.0 m2 g−1 was used for the present study. The dispersant used was an inorganic compound which stabilized the suspension by changing the surface charge of Si3N4 particles with its negatively charged anion group in electrostatic mechanism. The zeta potential of Si3N4 particles was measured with diluted Si3N4 dispersion (volume fraction, φ < 0.005) at 25 ◦C on Zetasizer 4 (Malvern, UK). The rheological behavior of Si3N4 suspension was characterized in steady shear mode with a couette (cup radius: 34.0 mm, bob radius: 32.0 mm, bob length: 33.3 mm) on Rheometric Fluid Spectrometer II (RFS II). The zeta potential of Si3N4 particles as a function of pH of suspension is shown in Fig. 1. As can be seen the isoelectric point (IEP) of Si3N4 particles is at the pH value of 4.2 and the particles have a maximum zeta potential at about pH = 11. The introduction of dispersant increases efficiently the zeta potential of Si3N4 particles in magnitude and shifts the IEP to acid region. Meanwhile, the pH values corresponding to the maximum of zeta potential for Si3N4 particles with different dispersant concentration remain constant. Therefore, in this work, all suspensions investigated are kept at pH = 11 expecting a good fluidity obtained. It seems that the changes of zeta potential and IEP of Si3N4 particles result from the increasing of the charging density of the particles caused by the adsorption of negatively charged anion groups of dispersant on the surface of Si3N4 particles. Fig. 2 illustrates the flow curves of 23.9 vol % Si3N4 aqueous slurry containing various amounts of dispersant. As is shown, the behavior of slurry transfers from an anti-thixotropy via near Bingham fluid to thixotropy with increasing amount of dispersant. It is clear that the Si3N4 suspension studied is a non-Newtonian time-dependent fluid which exhibits a pronounced thixotropy. When the amount of dispersant is below 0.8 wt % (weight percent, on a powder basis) the suspension exhibits an anti-thixotropic behavior and the anti-thixotropy decreases with increasing the dispersant amount. The anti-thixotropic behavior of the suspension is considered as due to the irregular morphology of Si3N4 particles. At appropriate dispersant range from 0.8–1.2 wt %, the particles are well dispersed in media so that the anti-thixotropic behavior disappeared and a near Bingham behavior is exhibited. With further increasing of the dispersant, the excessive dispersant moleculars are bridged together to produce some particle networks in suspension which could be broken down in the high applied shear field, so the suspension exhibits a thixotropic behavior at higher dispersant amounts.

Profile unbiased thermostat with dynamical streaming velocities

We have devised a reversible set of nonequilibrium molecular dynamics equations of motion to study viscous flow under an applied shear field utilizing a new profile unbiased thermostat (PUT). Based on a modification of the original equations presented by Evans and Morriss [Phys. Rev. Lett. 56, 2172 (1986)], we have extended the phase space to include dynamically evolving streaming velocities. The new method is advantageous because it allows control of the rate at which the streaming velocities evolve and thereby removes the effects of high frequency thermal fluctuations. Further, in tests on a Weeks–Chandler–Andersen fluid, we find that a shear-induced microstructure, the so-called ‘‘string phase,’’ was not destabilized by the new PUT scheme.

Non-equilibrium molecular dynamics simulations of structured molecules

To investigate the inter-relationship of structure and viscosity, non-equilibrium molecular dynamics simulations of Lennard-Jones site-site models representing selected simple molecules were performed. The simulation conditions nearly span the liquid density range over which experimental data are currently avalaible. The relationship between fluid structuer and the applied shear field are discussed

Small-angle neutron scattering from colloidal dispersions

A survey is given of recent work on the use of small-angle neutron scattering to examine colloidal dispersions. Particular attention is given to the determination of particle size and polydispersity, the determination of particle morphology and the behaviour of concentrated colloidal dispersions, both at rest and under the influence of an applied shear field.

DYNAMICS OF AGGLOMERATE SIZE DISTRIBUTION IN LINEAR SHEAR FLOW FIELDS

ABSTRACT A dispersive mixing model was developed and analyzed for the case when separation of the fragments following rupture of agglomerates primarily determines the dynamics of the mixing process. The model incorporates the effects of van der Waals forces within agglomerates and hydrodynamic interactions between fragments as they are convected apart from each other by the applied shear field. The essential features of the model are illustrated by examining the effect of shear on the breakage of parent agglomerates and their first five generations of fragments for a particular test case. In addition, the dynamics of the agglomerate size distribution produced by linear shear flow fields in gaps of various aspect ratios was analyzed for two types of initial conditions. These results provide insight to the design of the high shear zones in actual mixing equipment.