Influence Of Shear Flow Research Articles

Anisotropy and Nanomechanics of Cellulose Nanocrystals/Polyethylene Glycol Composite Films.

The influence of shear flow on the nanomechanical properties of cellulose nanocrystal (CNC)/polyethylene glycol (PEG) composite films and the distribution of anisotropic phases are investigated at various CNC/PEG ratios. Here, the drying process of CNC/PEG mixed suspensions is systematically traced by rheology, followed by the spatial mapping of local mechanical properties of CNC/PEG films by nanoindentation. The detailed study of the morphology of CNC/PEG films by polarized optical microscopy (POM) and image analysis revealed the link between the mechanical properties and the influence of shear flow. A comparison of the data obtained for shear-dried films with nonsheared films showed the improved reduced Young's modulus (Er) and hardness (H), and suppression of microphase separation in the shear-dried films. Based on this experimental evidence, a mechanism is proposed to explain the microstructural transition during the shear-drying process leading to the generation of the anisotropic domains containing the shear-induced assembled structure of CNC particles coexisting with the elongated PEG microphases.

Shear-Induced Migration of a Transmembrane Protein within a Vesicle

Biomembranes feature phospholipid bilayers and serve as the interface between cells or organelles and the extracellular and/or cellular environment. Lipids can move freely throughout the membrane; the lipid bilayer behaves like a fluid. Such fluidity is important in terms of the actions of membrane transport proteins, which often mediate biological functions; membrane protein motion has attracted a great deal of attention. Because the proteins are small, diffusion phenomena are often in play, but flow-induced transport has rarely been addressed. Here, we used a dissipative particle dynamics approach to investigate flow-induced membrane protein transport. We analyzed the drift of a membrane protein located within a vesicle. Under the influence of shear flow, the protein gradually migrated toward the vorticity axis via a random walk, and the probability of retention around the axis was high. To understand the mechanism of protein migration, we varied both shear strength and protein size. Protein migration was induced by the balance between the drag and thermodynamic diffusion forces and could be represented by the Péclet number. These results improve our understanding of flow-induced membrane protein transport.

Magnetic relaxation of magnetic nanoparticles under the influence of shear flow

We investigate the magnetic relaxation behavior of magnetic nanoparticles (MNP) under the influence of shear flow. To this end, we employed magnetorelaxometry (MRX), which is a very sensitive technique to measure the magnetization decay (magnetic relaxation) of MNP after polarizing their magnetic moments in a magnetic field. For the measurements, we developed a fluidic setup to measure the magnetic relaxation of MNP dispersions passing at specific flow rates through a capillary tube inside the magnetizing coil of the MRX device. We observed a decrease of the two characteristic MRX parameters, the relaxation time (up to 60%) and relaxation amplitude (up to 45%), with increasing flow rate (up to 16 ml min−1) compared to the corresponding values measured at zero shear rate. Further, two different regimes of magnetic relaxation behavior under shear flow could be distinguished: for low MNP concentrations the relaxation time was concentration independent, while for higher MNP concentrations an additional increase of relaxation values by about 13% was observed. This is attributed to MNP interparticle interactions. Since dipole–dipole interactions could be ruled out, we concluded that hydrodynamic interactions are the cause for the shear flow MRX behavior in the high MNP concentration regime.

Gravity Wave‐Induced Ionospheric Irregularities in the Postsunset Equatorial Valley Region

AbstractPlasma irregularities in the postsunset equatorial valley region ionosphere are investigated experimentally and through numerical simulation. Coherent radar backscatter observed at the Jicamarca Radio Observatory shows two classes of irregularities in different altitude bands—one mainly below about 125 km and the other mainly above. Irregularities in both bands are organized into wavefronts with wavelengths of a few kilometers. However, only the irregularities in the high‐altitude band exhibit consistent propagation speeds and directions. Some previous observations of irregularities in the nighttime electrojet suggest that gravity waves may sometimes influence their morphology. The possibility that the valley region irregularities are also related to gravity waves (GWs) is therefore investigated numerically. A model of a GW packet propagating through a tidal wind field is used to drive another model which predicts the resulting ionospheric electrodynamics. The combined simulation shows that GWs can induce field‐aligned currents and excite resistive drift waves which could be responsible for the valley region irregularities in the high‐altitude band. The GWs also induce irregularities in the upper E region directly through simple dynamo action which subsequently deform under the influence of shear flow. This may explain the irregularities in the low‐altitude band.

Synthesis and temperature-induced self-assembly of a positively charged symmetrical pentablock terpolymer in aqueous solutions

A novel linear cationic ABCBA pentablock terpolymer composed of a positively charged poly (3-acrylamidopropyl) trimethyl ammonium chloride)) (PAMPTMA (+)) block at both ends and two thermoresponsive poly (N-isopropylacrylamide) (PNIPAAM) blocks separated by a hydrophilic poly(ethylene glycol) (PEG) block was synthesized via a “one-pot” atom transfer radical polymerization procedure (ATRP). The chemical composition of the pentablock terpolymer was confirmed by nuclear magnetic resonance (NMR) and asymmetric flow field-flow fractionation (AFFFF). Depending on the polymer concentration in aqueous solution, this terpolymer forms unimers and self-assembled structures at elevated temperatures. The effect of concentration and temperature-induced self-assembling behavior of the pentablock terpolymer in aqueous solution was examined by using turbidimetry, shear viscosity, rheo-small angle light scattering (rheo-SALS), dynamic light scattering (DLS), and small angle neutron scattering (SANS). The turbidity measurements demonstrated that the formation of intermicellar structures and compaction of the complexes are function of both polymer concentration and temperature. The viscosity and rheo-SALS experiments elucidated the intricate interplay between building-up and breaking-up of interchain complexes under the influence of shear flow. The DLS experiments show the coexistence of small entities and interchain complexes at low temperatures and the evolution of large intermicellar structures at higher temperatures. At the highest temperatures, compaction of the complexes occurred. The results from SANS revealed significant temperature-induced changes of the copolymer structure on a semi-local dimensional scale.

Concentration fluctuations in polymer solutions under mixed flow

In this work, we extend the classical analysis of concentration fluctuations in polymer solutions under shear flow to consider the same phenomenology under mixed (shear + extensional) flows. To investigate this phenomenon, we couple stress and concentration using a two-fluid model with fluctuations driven by thermal noise incorporated through a canonical Langevin approach. The polymer stress is governed by the Rolie-Poly model augmented with finite extensibility to account for large stretching of chains at high Weissenberg numbers. Perturbing the equations about homogeneous flow for weak amplitude inhomogeneities, but arbitrary flow strength, we solve for the steady state structure factor (Fourier transformed pair correlation function) under general linear flows using a unique method of characteristics solver. Under shear flow, the model predicts butterfly patterns in accord with previous experimental and theoretical work, including a full rotation of peaks past the flow axis. In addition, the magnitude of the structure factor initially grows with the Weissenberg number until reaching a maximum at intermediate shear rates and decaying thereafter. Under mixed flow, the butterfly patterns as well as the location and magnitude of the peak structure factor are strongly tied to both the flow type parameter and the Weissenberg number (the characteristic strain rate). As expected, for flows characterized as strong, the scattering patterns typically appear like a rotated version of pure extension. However, as the flow type approaches the pure shear limit, the influence of shear flow on the butterfly patterns becomes more pronounced. In particular, for large Weissenberg numbers, contrary to expectations, the flow type need not be very near shear flow in order for the scattering patterns to no longer be simply rotated versions of extensional flow.

Mechanistic Insights into the Shear-Inducedβ-Form Crystal Formation of iPP

The conformation of a helical type polymer chain in the crystalline phase prefers the state with lower energy. The molecular simulation of the melting behaviors of α and β isotactic poly(propylene) (iPP) crystals with ultrafast heating shows that the ratio of right-handed and left-handed helix stays at 50% in α-iPP and changes from 100% to 50% in β-iPP before and after melting. Till date, the formation of β form crystal in iPP has not been understood well together with the mechanism of the crystallization process. One of the approaches to produce β-iPP is to shear the melt. The influence of shear flow on the crystallization behavior of iPP was investigated via ex situ and in situ X-ray measurements. It was found that preceding or simultaneously initiated formation of β-iPP together with α-iPP occurs under proper shear conditions. The β-rich iPP sample could no longer form β crystal at 135 °C after heating to the β-melting temperature with the melting of β-iPP and the retaining of oriented α-iPP, which is contrary to the accepted formation process of β-iPP. A mechanism was proposed to understand the shear-induced β crystal in iPP, considering the mesophase and stereocomplex type helical conformation of the polymer chains.

Characterization and influence of shear flow on the surface resistivity and mixing condition on the dispersion quality of multi-walled carbon nanotube/polycarbonate nanocomposites

Multi-walled carbon nanotube (MWCNT)/polycarbonate (PC) nanocomposite was prepared by direct melt mixing to investigate the effect of the shear rate on the surface resistivity of the nanocomposites. In this study, an experiment was carried out to observe the shear induced orientation of the MWCNT in the polymer matrix using a very simple melt flow indexer with various loads. The compression-molded, should be eliminated. MWCNT/PC nanocomposite sample exhibited lower percolation thresholds (at 0.8 vol%) and higher electrical conductivity values than those of samples extruded by capillary and injection molding. Shear induced orientation of MWCNT was observed via scanning electron microscopy, in the direction of flow in a PC matrix during the extrusion process. The surface resistivity rose with increasing shear rate, because of the breakdown of the network junctions between MWCNTs. For real applications such as injection molding and the extrusion process, the amount of the MWCNT in the composite should be carefully selected to adjust the electrical conductivity.

Acute Mechanical Stimulation Activates the Chemotactic Signaling Network

Cells migrate in a directional manner in a variety of conditions, including chemoattractant gradients, electric fields, or under the influence of shear flow. Chemotaxis, or directed migration up a chemical gradient, is the best understood of these processes, and involves activation of multiple parallel signal transduction pathways that transmit the input from the chemoattractant receptor to the cytoskeletal network, leading to biased pseudopod projection in the direction of the gradient. Many components of the chemotactic signaling network display a polarized distribution in migrating cells. Although some of these markers have also been observed in cells migrating in an electric field or under shear flow, it is unclear whether these processes involve activation of a similar signaling network, or how they trigger cell migration. We found that acute mechanical stimulation of Dictyostelium cells leads to phosphorylation and activation of multiple components of the chemotactic signaling network, including PKB and ERK. Furthermore, using a microfluidic device, we demonstrated that application of shear flow for two seconds triggers translocation of leading and lagging edge markers to and from the cortex, respectively, similarly to global stimulation with a chemoattractant. Remarkably, the signaling network activated by acute mechanical stimulation displayed many behaviors characteristic of the chemotactic signaling network, including a refractory period, which is indicative of the system's excitatory nature. Simultaneous inhibition of multiple signaling pathways, including PI3K, PLA2 and TORC2, did not block the response to acute mechanical stimulation. In contrast, the response depended on the presence of an intact actin cytoskeleton, as well as Ca2+ flux mediated by the IP3 receptor homolog. Overall, these findings provide insights into the mechanism of shear stress induced migration, as well as a novel approach for studying the properties of the chemotactic signaling network without input from a chemoattractant.

Synthesis and characterization of graft copolymers able to form polymersomes and worm-like aggregates

Poly(glycerol adipate)-graft-(poly(e-caprolactone)-block-poly(ethylene oxide)) (PGA-g-(PCL-b-PEO)) is synthesized by ring opening polymerization of e-caprolactone initiated by the hydroxyl groups of PGA. This is followed by grafting of poly(ethylene oxide) monomethylether mPEO-N3 onto the PCL blocks using copper-catalyzed azide–alkyne cycloaddition (CuAAC, “click” reaction). All polymers form micelles of radii in the range of 10 nm after dissolving in acetone and dialysis against water. Micelles formed by PGA-g-(PCL-b-PEO) show smaller critical micelle concentration (cmc) and higher stability against temperature increase compared to micelles formed by PCL-b-PEO with an identical chemical composition to the grafted segments. Additionally, PGA17-g-(PCL24-b-PEO44) forms worm-like aggregates prepared by the cosolvent/evaporation method. The resulting worm-like aggregates are visualized by transmission electron and confocal laser scanning microscopy and show shape persistent behaviour over their entire contour length. It is suggested that these worm-like aggregates are formed by partially fused polymersomes under the influence of shear flow. They have the potential for simultaneous delivery of hydrophobic and hydrophilic drugs.

Influence of shear flow on the linear response of a nematic liquid crystal to external electric fields

We have investigated the linear response of shear stress to ac electric fields under shear flow in a nematic liquid crystal. The experimental results were compared with the theoretical results derived from the Ericksen-Leslie theory. Although close agreement was obtained at low shear rates, discrepancies were observed at high shear rates. By introducing a two-mode coupling model the experimental results were well reproduced for the entire range of shear rates, and nonconservative forces were found to play an important role in determining the fluctuation dynamics, which is a characteristic of nonequilibrium steady states.

Effect of shear on vesicle and lamellar phases of DDAB/lecithin ternary systems

The influence of shear flow on bilayer structures (vesicle and planar lamellar phases, L α), formed in DDAB/lecithin ternary systems, is studied by means of conventional rheology, Rheo NMR, and optical microscopy. The vesicles in the diluted (Lam 1) phase are polydisperse multilamellae which turn into smaller monodisperse vesicles under shear. The concentrated (Lam 2) phase is formed by non-oriented lamellae that do not surprisingly exhibit any pronounced shear-induced alignment prior to the transition into giant multilamellar vesicles. The biphasic region (Lam 1 + Lam 2) shows a mosaic texture with a powder pattern indicating the prevalence of lamellae that transform into onions under shear.

Characterization of temperature-induced association in aqueous solutions of charged ABCBA-type pentablock tercopolymers

In this work, two novel linear ABCBA pentablock tercopolymers, which are composed of an anionic poly(4-styrenesulfonic acid sodium) (PSSS) block on both ends and two thermosensitive poly(N-isopropylacrylamide) (PNIPAAM) blocks that are separated by a hydrophilic poly(ethylene glycol) (PEG) block have been synthesized via a simple “one-pot” ATRP procedure. The two studied samples have the following compositions: (P(SSS)12-b-P(NIPAAM)63-b-P(EG)34-b-P(NIPAAM)63-b-P(SSS)12 (C34) and P(SSS)14-b-P(NIPAAM)65-b-P(EG)77-b-P(NIPAAM)65-b-P(SSS)14 (C77). The main difference between the copolymers is the length of the hydrophilic block, which plays a crucial role for the physical properties of the copolymers in aqueous solution. A solution of the C34 sample becomes turbid at elevated temperatures and compact interchain complexes are formed with a behavior that is reminiscent of a suspension of particles. A solution of C77 becomes turbid at intermediate temperatures but it is transformed to a transparent solution at high temperatures. Depending on the polymer concentration, this pentablock copolymer forms a fragmented or interconnected network at elevated temperatures. At sufficiently high polymer concentration, the viscosity measurements indicate that a reversible hydrogel is formed. The interchain complexes or networks are readily broken up under the influence of shear flow. The results from the small angle neutron scattering experiments reveal significant structural differences between the pentablock copolymers at different temperatures.

The influence of shear flow on the α- and γ-effects in helical MHD turbulence

We investigate the α-effect and the turbulent transport of magnetic flux (γ-effect) by considering a turbulence driven by an isotropic helical forcing in the presence of a background large-scale shear flow. Due to shear flow, the turbulence is highly anisotropic. The effect of this shear-induced anisotropy on the structure of the electromotive force is computed self-consistently. We show that for weak shear, magnetic pumping (γ-effect) appears, and the α-effect becomes highly anisotropic: the component of the α-effect in the direction of the shear is smaller than the one in the direction perpendicular to the shear with non-vanishing non-diagonal components. However, a sufficiently strong shear leads to severe quenching of the γ- and α-effects due to shear stabilisation.

Influence of shear flow on polymorphic behavior and microstructural development during palm oil crystallization

AbstractThe influence of shear on the crystallization of palm oil was studied at four different crystallization temperatures (18, 20, 22 and 25 °C). Time‐resolved X‐ray analyses were carried out to study the effect of continuous shear on the crystallization kinetics of the fat. Rheological measurements were used to assess the effect of a shear step on crystallization, and finally polarized light microscopy was used to follow changes in microstructure due to the applied initial shear step. It was shown that shear enhanced the primary crystallization, even when low shear rates were applied for a short period. Furthermore, a shear step prior to crystallization without shear has a marked influence on the microstructural development.

Chemical pattern formation induced by a shear flow in a two-layer model

We study chemical patterns arising from instabilities in reaction-diffusion-advection systems under the influence of shear flow. Turing pattern formation without shear flow can occur in an activator-inhibitor system as long as the diffusivity of the inhibitor is larger than the diffusivity of the activator. In the presence of shear flow, a homogeneous steady state can become unstable even if this condition is not satisfied. Chemical patterns arise as a result of this instability. We study this instability in a simple system consisting of two layers moving relative to each other. We carry out a linear stability analysis showing the onset of the instability as a function of the relative speed between the layers. We solve numerically the nonlinear reaction-diffusion-advection equations to obtain these patterns. We find stationary, oscillatory, and drifting patterns extending along each layer. We also find regions of bistability that allow the formation of localized structures. The instability is analyzed in terms of Taylor dispersion.

Temperature-induced intermicellization and contraction in aqueous mixtures of sodium dodecyl sulfate and an amphiphilic diblock copolymer

Aqueous solutions of a thermoresponsive amphiphilic diblock copolymer, containing poly( N-isopropylacrylamide), in the presence of the anionic sodium dodecyl sulfate (SDS) surfactant can undergo a temperature-induced transition from loose intermicellar clusters to collapsed core–shell nanostructures. The polymer–surfactant mixtures have been characterized with the aid of turbidity, small-angle neutron scattering (SANS), intensity light scattering (ILS), dynamic light scattering (DLS), shear viscosity, and rheo-small angle light scattering (rheo-SALS). In the absence of SDS, compressed intermicellar structures are formed at intermediate temperatures, and at higher temperatures further aggregation is detected. The SANS results disclose a structure peak in the scattered intensity profile at the highest measured temperature. This peak is ascribed to the formation of ordered structures (crystallites). In the presence of a low amount of SDS, a strong collapse of the intermicellar clusters is observed at moderate temperatures, and only a slight renewed interpolymer association is found at higher temperatures because of repulsive electrostatic interactions. Finally, at moderate surfactant concentrations, temperature-induced loose intermicellar clusters are detected but no shrinking was registered in the considered temperature range. At a high level of SDS addition, large polymer–surfactant complexes appear at low temperatures, and these species are compressed at elevated temperatures. The rheo-SALS results show that the transition structures are rather fragile under the influence of shear flow.

Intramolecular and Intermolecular Association during Chemical Cross-Linking of Dilute Solutions of Different Polysaccharides under the Influence of Shear Flow

Effects of shear flow on intramolecular and intermolecular associations of dilute aqueous alkali solutions of dextran, hydroxyethylcellulose (HEC), and a hydrophobically modified analogue (HM-HEC) in the presence of a chemical cross-linker agent were characterized with the aid of viscometry and rheo-small-angle light scattering (rheo-SALS) methods. The picture that emerges at short times in the course of cross-linking of the polymer solutions under the influence of a constant shear rate is that HEC coils contract because of intramolecular cross-linking, whereas the HM-HEC species show an incipient association and the dextran molecules are unaffected. At longer times, interchain cross-linking of the polymers promoted the growth of large flocs, which were disrupted by shear forces when they were sufficiently large. These findings are novel, and both the building up of aggregates and disaggregation are well substantiated by the SALS results.

Anomalous Transition in Aqueous Solutions of a Thermoresponsive Amphiphilic Diblock Copolymer

The influence of shear flow on aggregation and disaggregation in aqueous solutions of the thermoresponsive methoxy-poly(ethylene glycol)-block-poly(N-isopropylacrylamide) (MPEG53-b-PNIPAAM113) copolymer that exhibits a lower critical solution temperature was investigated with the aid of turbidity, shear viscosity, and rheo small angle light scattering (rheo-SALS) methods. The turbidity results at quiescent conditions revealed a novel transition peak in the turbidity curve at intermediate temperatures, which reflects the delicate interplay between temperature-induced aggregation and shrinking of the species. A similar anomalous transition peak (located at the same temperature) was observed in the steady shear viscosity measurements at intermediate temperatures, and the amplitude of the peak was reduced with increasing shear rate as a consequence of breakup of interaggregate chains. At low temperatures (low sticking probability), enhanced shear rate generated interpolymer aggregates; whereas in the high-temperature domain (high sticking probability) association structures were broken up as the shear rate was increased. The rheo-SALS experiments disclosed growth of aggregates at low temperatures and destruction of association complexes at high temperatures. An increase of the cloud point temperature with rising shear rate is reported, which is interpreted as being a disruption of clusters under the influence of shear stresses.

Shear flow effect on the crystalline forms in polyamide 6/montmorillonite nanocomposites

The influence of shear flow on the crystallization of polyamide 6/MMT nanocomposites prepared by melt intercalation process was investigated in detail by differential scanning calorimetry. The melted nanocomposites were controlled sheared in the steady and oscillatory shear flow, using a rotational rheometer, and cooled in an inert atmosphere. The effects of shear rate or frequency, clay concentration, and crystallization conditions on PA 6 crystalline phase development were studied. As expected, an opposite impact of shearing on γ-phase formation in the nanocomposites and neat matrix was found. Surprisingly, a critical shear frequency for the onset of γ-form crystallinity formation in the nanocomposites, increasing with the filler content as a consequence of polymer chains confinement within oriented clay platelets was found. At higher shear frequencies, the proportion of γ-form in the nanocomposites increased dramatically with the clay concentration and reached 30–40%. The shear flow effects were influenced by cooling conditions, and more significant effect for rapidly cooled samples was observed. The isothermal crystallization at the solidification temperature 205°C reduced the γ-form content. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007