Strong Shear Field Research Articles

Influence of Strong Shear Field on Structure and Performance of HDPE/PA6 In Situ Microfibril Composites.

As one of the most widely applied general-purpose plastics, high-density polyethylene (HDPE) exhibits good comprehensive performance. However, mechanical strength limits its wider application. In this work, we introduced the engineering plastic PA6 as a dispersed phase to modify the HDPE matrix and applied multiple shears generated by vibration to the polymer melt during the packing stage of injection molding. SEM, 2D-WXRD and 2D-SAXS were used to characterize the morphology and structure of the samples. The results show that under the effect of a strong shear field, the dispersed phase in the composites can form in situ microfibers and numerous high-strength shish-kebab and hybrid shish-kebab structures are formed. Additionally, the distribution of fibers and high-strength oriented structures in the composites expands to the core region with the increase in vibration times. As a result, the tensile strength, tensile modulus and surface hardness of VIM-6 can reach a high level of 66.5 MPa, 981.4 MPa and 72, respectively. Therefore, a high-performance HDPE product is successfully prepared in this study, which is of great importance for expanding the application range of HDPE products.

Preparation of PLA/PBAT blends with high performance via the synergistic effect of high mold temperature and strong shear field

As a bio-based polymer with excellent mechanical properties, good biodegradability and biocompatibility, polylactic acid (PLA) displays considerable potential to replace traditional petrochemical-based polymers for application. However, poor toughness and heat resistance restrict its wide application. In this work, PLA based PLA/PBAT blends with excellent comprehensive performance were prepared through the synergistic effect of high mold temperature as well as strong shear field. SEM, DSC and WAXD were conducted to characterize the crystal and micro-morphology of the samples. The results show that highly oriented shish-kebab and hybrid shish-kebab structures are observed in VIM-H sample. The crystallinity increases dramatically, which can reach 48%. Besides, the toughness, strength and heat resistance of VIM-H all markedly improve. The Vicat softening temperature (VST) reaches up to 149.2 °C, while the tensile strength and impact strength increase to 89.07 MPa and 10.36 kJ/m2, respectively. This study proves the synergistic enhancement of the properties for PLA/PBAT blends by high mold temperature as well as strong shear field, which provides a new method to obtain high-performance PLA materials and is beneficial for the wider application of PLA products.

Novel Strategy to Improve the Performance of Poly(l-lactide): The Synergistic Effect of Disentanglement and Strong Shear Field

As a biobased and biodegradable material that can replace traditional petrochemical-based polymers, poly(l-lactide) (PLLA) shows great application potential. However, its widespread application still faces some obstacles, mainly associated with the poor thermal resistance, because the molecular chains of PLLA are semirigid and of poor regularity. Therefore, the crystallization rate is low, making it difficult to crystallize during the molding process, and the finally obtained PLLA is almost amorphous. In this work, the entanglement density of molecular chains was reduced to promote the molecular chain mobility. Then, PLLA was processed in combination with a strong shear field provided by the vibration injection molding technology. The results show that the synergistic effect of disentanglement and strong shear field can significantly accelerate the crystallization kinetics of PLLA. In addition, due to the better mobility, molecular chains of PLLA tend to be more regularly arranged along the flow direction under strong shear field, resulting in thicker shear layers as well as the generation of denser and more perfect shish-kebab structures. As a result, PLLA material with enhanced performance is obtained, which favors to broaden the application range of PLLA.

The coupling effect of cellulose nanocrystal and strong shear field achieved the strength and toughness balance of Polylactide

Up to now, unbalanced mechanical properties and poor heat resistance have become two major problems of polylactic acid (PLA). In this study, the coupling between Cellulose nanocrystal (CNC) and strong shearing field formed a unique hierarchical structure. Compared with pure PLA, the tensile strength of DPIM PLA/CNC increased from 57.9 MPa to 79.6 MPa without sacrificing the toughness of PLA, and the vicat softening temperature of DPIM PLA/CNC increased from 60 °C to 155 °C. The microstructure of PLA/CNC composites was analyzed by SEM, SAXS and WAXD, and it was found that the coupling effect of CNC and strong shear flow field could significantly change the crystallization behavior of PLA. CNC could increase PLA shish length from 251 nm to 889 nm under the action of shear field. At the same time, due to this coupling effect, more PLA shish-kebab structures were induced at the interface. This special hierarchical structure composed of CNC and PLA Shish-Kebab is of great significance and can provide important guidance for achieving the balance of strength and toughness of polymer materials.

Influence of Injection Application on the Sol-Gel Phase Transition Conditions of Polysaccharide-Based Hydrogels.

Polysaccharide matrices formed via thermoinduced sol–gel phase transition are promising systems used as drug carriers and minimally invasiveness scaffolds in tissue engineering. The strong shear field generated during injection may lead to changes in the conformation of polymer molecules and, consequently, affect the gelation conditions that have not been studied so far. Chitosan (CS) and hydroxypropyl cellulose (HPC) sols were injected through injection needles (14 G–25 G) or sheared directly in the rheometer measuring system. Then the sol–gel phase transition conditions were determined at 37 °C using rheometric, turbidimetric, and rheo-optical techniques. It was found that the use of low, respecting injection, shear rates accelerate the gelation, its increase extends the gelation time; applying the highest shear rates may significantly slow down (HPC) or accelerate gelation (CS) depending on thixotropic properties. From a practical point of view, the conducted research indicates that the use of thin needles without preliminary tests may lead to an extension of the gelation time and consequently the spilling of the polymeric carrier before gelation. Finally, an interpretation of the influence of an intensive shear field on the conformation of the molecules on a molecular scale was proposed.

Recent Progress on the Confinement, Assembly, and Relaxation of Inorganic Functional Fillers in Polymer Matrix during Processing.

Functional polymer composites have gained extensive interest due to their wide range of applications. Various functionalities are mainly provided by functional fillers and their networks in polymer matrix. Therefore, morphology control on filler network has important influence on the final functionalities. The confinement, assembly, and relaxation of inorganic functional fillers in polymers are thought as the basic issues related to such control. Processing as the means to achieve the desired shape and properties for materials often involves strong temperature and shear field, among other factors-not to mention the complex interaction between polymer and filler. These processing fields and interactions during processing are reported to have profound influence on the confinement, assembly, and relaxation of inorganic fillers in polymer matrix. Therefore, various functionalities could be significantly affected by processing. This paper is a review of recent developments in this area.

Achieving high-efficiency and robust 3D thermally conductive while electrically insulating hybrid filler network with high orientation and ordered distribution

Continuous and robust filler network is very critical in improving thermal conductivity of the thermally conductive yet electrically insulating composite. Regrettably, such network is commonly constructed by large amount of thermally conductive while electrically insulating fillers with sacrificing mechanical properties of the final composite. In the present work, to address this issue and further improve the thermally conductive property, the robust 3D thermally conductive while electrically insulating hybrid filler network with high orientation and ordered distribution was constructed successfully through strong shear and extension flow field in confined space during the multilayer co-extrusion. In the hybrid filler network with the 32-layer alternating structure, along the extrusion direction, graphite (Gt) flakes were highly oriented; meanwhile, along the thickness direction, the local Gt-MWCNTs network was confined between two adjacent SiC-filled layers. As a result, thermal conductivity of the final composite reached as high as 2.05 W/(m × K). Furthermore, significantly anisotropic electrical resistivities were also obtained, which endowed the final composite with excellent electrical insulation, great anti-static and electromagnetic interference shielding properties. Moreover, compared with commonly compounded composites, mechanical properties of the multilayered composite were also enhanced significantly. As a consequence, the multifunctional composite with robust thermally conductive while electrically insulating filler network and excellent mechanical properties can be obtained based on this strategy, thus it plays a critical role in facilitating the development of the thermal management materials.

Simulations of behavior of magnetic particles in magnetic functional fluids using a hybrid method of lattice Boltzmann method, immersed boundary method and discrete particle method

Abstract The microstructure formation of magnetic particles in magnetorheological (MR) fluids has an important role on the macroscopic properties such as viscosity. To simulate the behavior and microstructure formation of magnetic particles, hybrid simulations were conducted by combining the lattice Boltzmann, immersed boundary and discrete particle methods, and by taking account of magnetic interactions between magnetic particles under an external magnetic field. The microstructure formation of magnetic particles in an MR fluid between two parallel plates, where one plate moves with constant velocity and the plate remains stationary, is calculated. A sheet-like structure appears when relatively strong shear and strong magnetic field are applied. In contrast, a fibrous-like structure moves in the flow when the shear is small.

Exfoliation of organic montmorillonite in iPP free of compatibilizer through the multistage stretching extrusion

Polypropylene (PP)/organic montmorillonite (OMMT) nanocomposites were first prepared through twin-screw extruder and then subjected to multistage stretching extrusion with an assembly of laminating-multiplying elements (LMEs, which divide and recombine polymer melts). The exfoliated efficiency of LMEs on OMMT dispersed in PP matrix was investigated by optical microscopy, scanning electron microscope, transmission electron microscopy and X-ray diffraction. Because of the absence of compatibilizer, molecular chains of PP were not intercalated into the galleries of OMMT during the multistage stretching extrusion. The exfoliation of OMMT was induced by the strong force occurred in LMEs, which can destruct van der Waal’s interaction between the laminate OMMT platelets. The exfoliation degree of OMMT has been improved with the increase of number of LMEs used. The dispersion morphology of OMMT was thermodynamically stable after secondary melt processing. As a result, the mechanical properties of composites have been enhanced with increasing LME number. We realized the exfoliation of OMMT by the function of strong shear field without the incorporation of compatibilizer.

Effect of multistage tensile extrusion induced fiber orientation on fracture characteristics of high density polyethylene/short glass fiber composites

Abstract The high density polyethylene (HDPE)/short glass fiber (SGF) composites were prepared by multistage tensile extrusion with an assembly of laminating-multiplying elements (LMEs) connecting with an extruder. The strong shear and elongational force field in LME could make the fibers orientated along the melt flow direction. The degree of fiber orientation could be controlled by the number of LMEs. The SEM morphological observations illustrated that the fiber orientation degree was gradually increased by increasing the number of LMEs. However, the fiber length distribution and crystallinity of composites did not change much with increasing the LMEs, and would not affect the mechanical properties. The essential work of fracture (EWF) analysis at quasi-static rate of loading showed that the fracture toughness along the melt flow direction increased with the number of LMEs increasing because of the change of the fiber related damage mechanism. An opposite trend was found if samples were stretched perpendicular to the melt flow direction.

Enhanced thermally conductivity and mechanical properties of polyethylene (PE)/boron nitride (BN) composites through multistage stretching extrusion

Abstract Thermally conductive polyethylene (PE)/boron nitride (BN) composites were prepared by multistage stretching extrusion without any surface treatment of BN. Effects of the intensity of the shear field on the distribution of the BN particles were characterized by scanning electron microscope (SEM). The thermally conductive behaviors, rheological and mechanical properties of the PE/BN composites were also investigated. The results showed that the strong shear field in 8 LMEs was helpful to improve the dispersion of the BN particles in PE matrix so that much more thermally conductive paths can be built in form of the filler–crystal–filler in PE matrix at large concentration of the BN particles. To well understand the thermal conduction mechanism of the PE/BN composites, the heat conductive model was proposed. According to this model, a large BN aggregates existed in PE matrix. However, the BN aggregates were damaged due to the strong shear field so that much more thermally conductive paths (connection via filler–crystal–filler) can be built in the PE/BN composites. Therefore, the thermal conductivity of the PE/BN composites was improved after multistage stretching extrusion processing. Meanwhile, it was found that the strong shear field was helpful to enhance the storage modulus, complex viscosity, tensile and impact strength of the PE/BN composites. Therefore, the processing method of multistage stretching extrusion can not only enhance the thermal conductivity of the PE/BN composites but also improve their mechanical properties.

Suppressing of γ-Crystal Formation in Metallocene-Based Isotactic Polypropylene during Isothermal Crystallization under Shear Flow

The effect of shear flow on isothermal crystallization behavior of γ-crystals in metallocene-based isotactic polypropylene melt was investigated by in situ synchrotron wide-angle X-ray diffraction (WAXD). In the sample under weak shear (at strain of 300% for 30 s duration), simultaneous evolution of α- and γ-crystals occurred, and the final fraction of γ-crystals (fγ) was 0.66, which was identical to the undeformed sample (PP-Static). In this scenario, α-crystals probably served as effective seeds for nucleation of γ-crystals. In the samples under strong shear (at strain of 500% for 30 s duration or long-time continuous shear at strains of 100% and 500%), the sequential emergence of α- and γ-crystals was observed. In this case, molten polymer chains were probably constrained by the surrounding crystals after intense short-time shear and/or maintained their extended chain conformation after long-time shear. These oriented chains had little chance to form the γ-crystals directly, behaving very differently from the relaxed chains. Under strong shear fields, the emergence of γ-crystals was delayed or inhibited, whereas the fγ value was also decreased rapidly. A simple model for the possible pathway of γ-crystal formation in the strong shear environment was proposed.

Morphological Distribution in Micro-Injected Polypropylene Parts in the Presence of β-Nucleating Agent

Micro-injection molding is attracting much attention nowadays. Characterization of the morphological distribution in parts prepared by micro-injection molding is thus of growing importance. The morphological features of micro-parts may strongly differ from those of the macro-parts prepared by conventional injection molding, resulting in specific physical properties. In the present study, β-nucleated isotactic polypropylene micro-parts (μPPB) with 200 μm thickness, as well as macro-parts (PPB) with 2000 μm thickness, were prepared. Polarized light microscopy (PLM), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and wide-angle X-ray diffraction (WAXD) were used to investigate their morphological features. The results show that the morphology distribution in μPPB had many differences from that of the PPB. The one-dimensional WAXD and DSC analysis showed similar results; the degree of crystallinity of the μPPB was higher than that of the PPB. However, the content of β-crystals of μPPB was lower than that of the PPB. This can be explained by the restraining effect for the formation of β-crystals in β-nucleated isotactic polypropylene (iPP) under the strong shear field. The through-the thickness-morphology of both μPPB and PPB exhibited a “skin-core” structure from PLM observations, but the former had a large fraction of shear layer in comparison to the latter implied. The SEM observations showed that the shear layer of μPPB consisted of a highly oriented shish-kebab structure, while that of the core layer consisted of deformed spherulites structure. The two-dimensional WAXD pattern of the core layer of PPB, showing full Debye rings, indicated an overall random orientation of the iPP chains, while the arcing indicated a pronounced orientation in the shear layer. The more pronounced arcing of the μPPB indicated a more pronounced orientation.

3D Reconstruction of a Rotating Erupting Prominence

A bright prominence associated with a coronal mass ejection (CME) was seen erupting from the Sun on 9 April 2008. This prominence was tracked by both the Solar Terrestrial Relations Observatory (STEREO) EUVI and COR1 telescopes, and was seen to rotate about the line of sight as it erupted; therefore, the event has been nicknamed the "Cartwheel CME." The threads of the prominence in the core of the CME quite clearly indicate the structure of a weakly to moderately twisted flux rope throughout the field of view, up to heliocentric heights of 4 solar radii. Although the STEREO separation was 48 degrees, it was possible to match some sharp features in the later part of the eruption as seen in the 304 {\AA} line in EUVI and in the H\alpha-sensitive bandpass of COR1 by both STEREO Ahead and Behind. These features could then be traced out in three-dimensional space, and reprojected into a view in which the eruption is directed towards the observer. The reconstructed view shows that the alignment of the prominence to the vertical axis rotates as it rises up to a leading-edge height of \approx 2.5 solar radii, and then remains approximately constant. The alignment at 2.5 solar radii differs by about 115 degrees from the original filament orientation inferred from H{\alpha} and EUV data, and the height profile of the rotation, obtained here for the first time, shows that two thirds of the total rotation is reached within \approx 0.5 solar radii above the photosphere. These features are well reproduced by numerical simulations of an unstable moderately twisted flux rope embedded in external flux with a relatively strong shear field component.

Flow-Directed Block Copolymer Micelle Morphologies via Microfluidic Self-Assembly

The self-assembly of amphiphilic block copolymers in a gas-liquid microfluidic reactor produces variable, flow-directed micellar morphologies entirely different from off-chip equilibrium structures. A polystyrene-block-poly(acrylic acid) copolymer, which forms exclusively spheres off-chip, generates kinetic cylinders, Y-junctions, bilayers, and networks by a mechanism of collision-coalescence enabled by strong and localized on-chip shear fields. Variation in the size and relative amount of flow-directed nanostructures is achieved by changing the water content and flow rate. These results demonstrate on-chip processing routes to specific functional colloidal nanostructures.

Suspension flow modelling in particle migration and microfiltration

We review existing mixture models for shear-induced migration (SIM) in flowing viscous, concentrated particle suspensions via an analysis of the models from the perspective of a two-fluid formulation. Our analysis shows that particle suspensions in strong non-linear shear fields are a prime example of a driven soft matter system. The driving forces for particle migration can be expressed in terms of non-equilibrium osmotic pressure and chemical potential. Using the linear scaling of the effective temperature with the shear stress, we show that the osmotic pressure and shear-induced diffusion coefficients can be written in identical equations. This is similar to the equations for Brownian motion - with the temperature replaced by the effective temperature. As a guiding application we have taken crossflow microfiltration, where the driving is very strong and there is formation of a jammed state, cake layer, coexisting with the fluid state. The question whether the SIM mixture models holds for this aplication is investigated. Another questions is how SIM models can be extended for bidisperse suspensions, which is relevant for microfiltration applications involving particle fractionation. Analysis of existing closures of SIM mixture models from the two-fluid perspective learns us that the theory seems to be extendable towards bidisperse suspensions by means of the effective medium theory.

Is Interleaving in the Agulhas Current Driven by Near-Inertial Velocity Perturbations?

Abstract Recent observations taken at a number of latitudes in the Agulhas Current reveal that the water mass structure on either side of its dynamical core is distinctly different. Moreover, interleaving of these distinct water masses is observed at over 80% of the stations occupied in the current, particularly within the subsurface density layer between tropical surface water and subtropical surface water masses, and within the intermediate layer between the Antarctic Intermediate Water and Red Sea water masses. Direct velocity measurements allow for a comparison between the characteristic vertical length scales of the Agulhas intrusions and those of velocity perturbations found throughout the current. It is found that the interleaving scales match those of the velocity perturbations, which are manifest as high-wavenumber vertical shear layers and are identified as near-inertial oscillations. Furthermore, the properties of the intrusions indicate that double diffusion is not an important process in their development: they are generally not associated with a density anomaly, their slope and thickness fall outside the predicted maxima for instability, and a strong horizontal shear field acts to separate water parcels more quickly than intrusions would be able to grow by double-diffusive processes. Instead, the position, thickness, and slope of Agulhas intrusions relative to the background salinity and density field suggest that they are forced by rotating inertial velocities, with subsequent growth possibly driven by small-scale baroclinic instabilities. However, not all the evidence points conclusively toward advectively driven intrusions. For instance, there is a discrepancy between the observed salinity anomaly amplitude and the predicted inertial displacement given the background salinity gradient, which deserves further examination. Hence, there is a future need for more pointed observations and perhaps the development of an analytical or numerical model to understand the exact nature of Agulhas intrusions.

Shear-thickening flow of suspensions of carbon nanofibers in aqueous PVA solutions

The suspensions of carbon nanofibers in aqueous poly(vinyl alcohol) solutions were prepared in the presence of spherical carbon black particles, and the steady-shear viscosity and dynamic viscoelasticity were measured for complex suspensions. Although the single suspensions of carbon black are highly stable, the flocculation of carbon nanofibers is promoted by the addition of carbon black particles. The complex suspensions show remarkable shear thickening in the steady-flow and strain hardening in oscillatory shear with large amplitude. The nonlinear responses strongly depend on the carbon black concentration, whereas the dynamic viscoelasticity at low strains in the linear ranges is not significantly influenced. As the highly elastic effects arise from the long-range motion of particles, the possible mechanism may be the orientation of nanofibers in strong shear fields. The suspensions show the time-dependent behavior of viscosity when the time-scale of measurements is shorter than that of structural recovery to the isotropic states.

Comparison of the effects of dimethyl and dichloro benzoate counterions on drag reduction, rheological behaviors, and microstructures of a cationic surfactant

Arquad 16–50 (commercial CTAC, cetyltrimethylammonium chloride) (5 mM) with the counterions 3, 4-dichlorobenzoate (5 and 10 mM), 3, 4-dimethylbenzoate (5 and 10 mM) and 3, 5-dichlorosalicylate (5 mM) were studied to compare the effect of concentration of the counterion and its ratio to surfactant concentration (ξ) on drag reduction, rheological behavior, and microstructures. The first four solutions are good drag reducers at different temperature ranges. The 3, 4-dimethylbenzoate system (ξ=1) is effective at 5–40 °C and the 3, 4-dichlorobenzoate system (ξ=1) at 20–70 °C. Increasing the concentration ratio to ξ=2 increased the upper temperature limit 10 °C for each system. The counterion concentration changes affect microstructures in the quiescent state in different ways. The viscoelastic ξ=1 solution of 3, 4-dimethylbenzoate has a microstructure of both vesicles and threads in the quiescent state which probably transform to a threadlike micellar network under shear. However, its ratio of apparent extensional viscosity to shear viscosity is very low, unusual for a surfactant drag reducer. Its ξ=2 solution has typical surfactant drag reducer properties, viscoelastic, high extensional viscosity, and threadlike micellar networks. The ξ=1 solution of 3, 4-Cl-benzoate is like the ξ=2 solution of 3, 4-CH3-benzoate while its ξ=2 solution is nonviscoelastic with vesicles and spherical micelle microstructures in the quiescent state which also probably transform to a network structure in strong shear fields. The only system tested with a counterion having four-substituent groups, 3, 5-dichlorosalicylate, is not effective as a drag reducer, has water-like behavior, and contains very large vesicles.Arquad 16–50 (commercial CTAC, cetyltrimethylammonium chloride) (5 mM) with the counterions 3, 4-dichlorobenzoate (5 and 10 mM), 3, 4-dimethylbenzoate (5 and 10 mM) and 3, 5-dichlorosalicylate (5 mM) were studied to compare the effect of concentration of the counterion and its ratio to surfactant concentration (ξ) on drag reduction, rheological behavior, and microstructures. The first four solutions are good drag reducers at different temperature ranges. The 3, 4-dimethylbenzoate system (ξ=1) is effective at 5–40 °C and the 3, 4-dichlorobenzoate system (ξ=1) at 20–70 °C. Increasing the concentration ratio to ξ=2 increased the upper temperature limit 10 °C for each system. The counterion concentration changes affect microstructures in the quiescent state in different ways. The viscoelastic ξ=1 solution of 3, 4-dimethylbenzoate has a microstructure of both vesicles and threads in the quiescent state which probably transform to a threadlike micellar network under shear. However, its ratio of apparent extensi...

Self-diffusion of rodlike molecules in strong shear fields.

We apply recently derived Green-Kubo relations for the various elements of the self-diffusion tensor (SDT) of shearing fluids to three different model systems of rodlike molecules: the Tildesley-Madden model for carbon disulfide, the Gay-Berne fluid, and the Ryckaert-Bellemans model for decane. We study how the various elements of the SDT depend on the shear rate. This dependence is explained in greater detail by examining the various velocity autocorrelation functions, the order parameter, and the shear induced rotation rate. We find that two different mechanisms are responsible for the behavior of the diagonal elements of the SDT. The first is the elliptical distortion of the nearest-neighbor shell and enhances the diagonal elements of the SDT. The second is the shear alignment of the molecules. This facilitates diffusion in the alignment direction and suppresses diffusion in the perpendicular direction. We also suggest ``collision'' sequences that are responsible for the sign of the off-diagonal elements of the SDT.