Strong Shear Stress Research Articles

Sediment remobilization over subaqueous sand waves: In-situ observation in the northern South China Sea

High-resolution tripod observations were conducted in a subaqueous sand-wave field in the northern South China Sea. The objective was to gain insight into the dynamic mechanism by which sandy sediments are remobilized by oceanic dynamic processes, in particular internal solitary waves and internal tides. Daily-recurring high suspended sediment concentrations were observed, likely due to lateral transport of sediments resuspended by critical reflection of diurnal internal tides at the shelf break. The passage of extreme internal solitary waves results in resuspension of sediments from the seabed and ejection of these sediments out of the boundary layer. The resuspension of sediments from the seabed is controlled by current-induced strong bed shear stress, while the ejection of sediments out of the bottom boundary layer is driven by the compression and subsequent expansion of the mixing layer, together with the developments of the separation bubbles behind the internal solitary waves. These findings provide new insights into understanding the dynamic mechanism of sediment remobilization by internal solitary waves. Moreover, they contribute to our understanding of the migration of subaqueous sand waves.

Fabricating high-performance biomedical PLLA/PVDF blend micro bone screws through in situ structuring of oriented PVDF submicron fibers in microinjection molding

Poly(l-lactide) (PLLA) is regarded as a polymer with excellent biocompatibility, but its inherent brittleness property greatly restricts its application in the biomedical engineering field. Blending PLLA with other polymers is one of the sufficiently viable methods of property improvement. In this paper, the Poly(l-lactide) (PLLA)/Polyvinylidene fluoride (PVDF) blend was first prepared by melt-compounding, and then microinjection molded into the blend micro bone screw with high strength and toughness. The experimental results show that the PVDF dispersed phases could in situ form the highly oriented fibers with shish-kebab structures under the effect of strong shear stress field of microinjection molding along flow direction. These parallelly aligning PVDF fibers could simultaneously realize the significant enhancement in both toughness and strength of the micro bone screw. Specifically, in the bending test the blend micro bone screw with oriented PVDF fibers can withstand the compression displacement of nearly 2 mm, while the neat PLLA one would break within only 0.5 mm displacement. In addition, a bending recovery phenomenon was observed in the load-displacement curve of the blend, demonstrating the mechanical transition from brittle fracture to ductile fracture. Moreover, with addition of 30 % PVDF, the elongation at break increases from 7.8 % to 57.8 %, while the tensile strength increases from 60.9 MPa to 74.3 MPa. The PLLA/PVDF micro screws possess better toughness and excellent biocompatibility compared to pure PLLA products. They also exhibit great potential for stimulating the proliferation of cells through piezoelectric output, opening up new possibilities for the development of next-generation fracture fixation materials.

Preparation of Polyoxymethylene/Exfoliated Molybdenum Disulfide Nanocomposite through Solid-State Shear Milling.

In this paper, the solid-state shear milling (S3M) strategy featuring a very strong three-dimensional shear stress field was adopted to prepare the high-performance polyoxymethylene (POM)/molybdenum disulfide (MoS2) functional nanocomposite. The transmission electron microscope and Raman measurement results confirmed that the bulk MoS2 particle was successfully exfoliated into few-layer MoS2 nanoplatelets by the above simple S3M physical method. The polarized optical microscope (PLM) observation indicated the pan-milled nanoscale MoS2 particles presented a better dispersion performance in the POM matrix. The results of the tribological test indicated that the incorporation of MoS2 could substantially improve the wear resistance performance of POM. Moreover, the pan-milled exfoliated MoS2 nanosheets could further substantially decrease the friction coefficient of POM. Scanning electron microscope observations on the worn scar revealed the tribological mechanism of the POM/MoS2 nanocomposite prepared by solid-state shear milling. The tensile test results showed that the pan-milled POM/MoS2 nanocomposite has much higher elongation at break than the conventionally melt-compounded material. The solid-state shear milling strategy shows a promising prospect in the preparation of functional nanocomposite with excellent comprehensive performance at a large scale.

A comparison between the FENE-P and sPTT constitutive models in large-amplitude oscillatory shear

The finitely extensible nonlinear elastic with Peterlin closure (FENE-P) and simplified Phan-Thien–Tanner (sPTT) viscoelastic models are used widely for modelling of complex fluids. Although they are derived from distinct micro-structural theories, these models can become mathematically identical in steady and homogeneous flows with a particular choice of the values of the model parameters. However, even with this choice of parameter values, the model responses are known to differ from each other in transient flows. In this work, we investigate the responses of the FENE-P and sPTT constitutive models in large-amplitude oscillatory shear (LAOS). In steady shear, the shear stress scales with the non-dimensional group $Wi/(aL)$ ( $Wi\,\sqrt {\epsilon }$ ) for the FENE-P (sPTT) model, where $Wi$ is the Weissenberg number, $L^2$ is the limit of extensibility in the FENE-P model ( $a$ being $L^2/(L^2-3)$ ), and $\epsilon$ is the extensibility parameter in the sPTT model. Our numerical and analytical results show that in LAOS, the FENE-P model shows this universality only for large values of $L^2$ , whereas the sPTT model shows it for all values of $\epsilon$ . In the strongly nonlinear region, there is a drastic difference between the responses of the two models, with the FENE-P model exhibiting strong shear stress overshoots that manifest as self-intersecting secondary loops in the viscous Lissajous curves. We quantify the nonlinearity exhibited by each constitutive model using the sequence of physical processes framework. Despite the high degree of nonlinearity exhibited by the FENE-P model, we also show using fully nonlinear one-dimensional simulations that it does not shear band in LAOS within the range of conditions studied.

Modelling biofilm development: The importance of considering the link between EPS distribution, detachment mechanisms and physical properties

In industry, treatments against biofilms need to be optimized and, in the wastewater treatment field, biofilm composition needs to be controlled. Therefore, describing the biochemical and physical structures of biofilms is now required to better understand the influence of operating parameters and treatment on biofilms. The present study aims to investigate how growth conditions influence EPS composition, biofilm physical properties and volume detachment using a 1D biofilm model. Two types of EPS are considered in the present model, proteins and polysaccharides. The main hypotheses are that: (i) the production of polysaccharides occurs mainly under strong nutrient limitation(s) while the production of proteins is coupled to both the substrate uptake rate and the lysis process; (ii) the local biofilm porosity depends on the local biofilm composition. Both volume and surface detachment occur in biofilms and volume detachment extent depends on the biofilm local cohesion and thus on the local composition of biofilms for a given shear stress.The model is based on experimental trends and aims to represent these observations on the basis of biochemical and physical processes. Four case studies covering a wide range of contrasting growth conditions such as different COD/N ratios, applied SOLR and shear stresses are investigated. The model predicts how the biochemical and physical biofilm structures change as a result of contrasting growth conditions. More precisely simulation results are in good agreement with the main experimental observations reported in the literature, such as: (i) a strong nitrogen limitation of growth induces an important accumulation of polysaccharides leading to a more porous and homogenous biofilm, (ii) a high applied surface organic loading load allows to obtain a high biofilm thickness, (iii) a strong shear stress applied during the biofilm growth leads to a reduction of the biofilm thickness and to a consolidation of the biofilm structure. Overall, this model represents a relevant decision tool for the selection of appropriate enzymatic treatments in the context of negative biofilm control. From our results, it appears that protease based treatments should be more appropriate for biofilms developed under low COD/N ratios (about 20 gCOD/gN) whereas both glucosidases and proteases based treatments should be more appropriate for biofilms developed under high COD/N ratio (about 70 gCOD/gN). In addition, the model could be useful for other applications such as resource recovery in biofilms or granules, and help to better understand biological membrane fouling.

Rapid oscillation of sediment transport between the Bohai Sea and the Yellow Sea induced by Typhoon Lekima (2019)

Amidst the continued northward migration of tropical cyclones in the context of global warming, mid and high latitudes marginal seas, like the East China Seas in the northwest Pacific Ocean, are increasingly attacked by tropical cyclones. As such, it is crucial to pay more attention to the impact of tropical cyclones on these marginal seas. To address this issue, we utilized a well-calibrated FVCOM model to study the effects of Typhoon Lekima (2019) on sediment transport between the Bohai Sea and the Yellow Sea. The results showed that during Typhoon Lekima's passage, sediment dynamics in the Bohai and Yellow seas were significantly altered. During the first two days of Typhoon Lekima's arrival, the water and sediment transport pattern rapidly changed from normal summer conditions to a strong inflow into the Bohai Sea, which increased water levels in the southwest Bohai Sea. As Typhoon Lekima weakened, the barotropic effect drove the water outflow from the Bohai Sea to the Yellow Sea. Strong bottom shear stress induced by the high-energy waves introduced strong sediment resuspension, which led to massive sediment escaping out of the Bohai Sea via the outflow during the weakening period of Typhoon Lekima. After two days following Typhoon Lekima's passage, the transport pattern gradually returned to normal summer conditions. The study revealed that the net sediment load from the Bohai Sea to the Yellow Sea during Typhoon Lekima was approximately 0.84 Mt., which is equivalent to 5–33% of the annual flux. The study highlights the critical role of extreme weather events in sediment dynamics and provides important insights into the impacts of typhoon on source-to-sink process in mid and high latitude marginal seas.

The effect of hydrostatic pressure on grain refinement and deformation behaviors of Mg–Gd–Y–Zn–Zr alloy under complex shear stress

The effects of hydrostatic pressure on grain refinement and deformation behaviors of Mg–13Gd–4Y–2Zn–0.5Zr alloy were investigated. With the increase of hydrostatic pressure, the equivalent stress and forming load increase, and stress triaxiality also increases. The hydrostatic pressure has little effect on dynamic recrystallization (DRX) fraction and grain size, and the DRX fraction difference is not greater than 10%. This is because DRX critical strain and grain nucleation are almost unaffected by hydrostatic pressure. The slight increase in DRX fraction under higher hydrostatic pressure is attributed to the enhanced <a> dislocation activity near the lamellar long-period stacking ordered (LPSO) phase and significant local temperature elevation in the deformation zone. The high hydrostatic pressure is mainly borne by the LPSO phase. The bending deformation energy of the lamellar LPSO phase increases with the increase of hydrostatic pressure. The area fraction of samples is decreased from 18.5% of one direction compression & simple shear (1-CS) sample to 13.6% of three directions compression & simple shear (3-CS) sample. The solute diffusion and deconstruction of the bulk LPSO phase occur at higher hydrostatic pressure. The hydrostatic pressure has little effect on the basal texture strength of α-Mg phase under strong shear stress. The activation of <c+a> dislocation, as the primary source of non-basal slip, shows the same distribution characteristics.

Turbulent flow simulations in a simplified channel bend model of Dragon style rivers with constant curvature and different single-short-branch characteristics

This study investigates the turbulent flows in simplified Dragon style river channels with a constant critical curvature and different single-short-branch characteristics, using an effective numerical method based on real geometry statistics. First, when the branch outlet is closed, in the main channel, strong bed shear stresses are concentrated at the inner bend apex and downstream; as the branch widens, more flow enters the branch, reducing the downstream velocities, increasing the helical flow strength (I) and turbulent kinetic energy (TKE); however, when the branch radius is about 3.5 times the main channel width, the opposite occurs. Second, in this non-free-flowing branch, upstream of a critical section, the cross-sectional I and TKE increase with increasing width, but both decrease with increasing radius or decreasing orientation angle (relative to the direction tangent to the main channel). Third, when the branch outlet is open, in the main channel, the bifurcation location primarily affects the main channel flow, moving it upstream creates a more regular low-velocity zone and recirculation zone, and lower shear stresses downstream, and meanwhile, the cross-sectional I, TKE, and shear stresses decrease upstream but increase downstream of the bifurcation location, respectively. Finally, in this free-flowing branch, shear stresses occupy the inner branch; the recirculation zone grows with increasing branch width, reaches a maximum around a relative branch radius of 3.0 (scaled with main-channel width), and disappears at a branch orientation angle of about 67°; both the cross-sectional I and TKE become negative, and the graphical cross-sectional I (along the flow direction) curves downward with increasing width, upward with increasing radius, and straightens with decreasing orientation angle. Findings provide fundamental conditions for harmonious vegetative integration and gradual development of Dragon style river regulation patterns, especially to mitigate crises like the ‘suspended river’ issues in the lower Yellow River.

Gradient alternating deformation mechanism of two metals and interface bonding mechanism of cu/Al cold rolling composite process

Cu/Al clad plates, with a reduction of 51%, were prepared by cold roll bonding (CRB). The bonding process, microstructural evolution, and bonding mechanism, in the rolling deformation zone of the clad plate, were studied. The interface of the deformation zone was analyzed qualitatively by experimental and numerical simulation methods. The results show that the two metal layers underwent gradient alternating deformation after the metal components enter the roll gap, grain refinement, grain boundary strengthening, and dislocation strengthening occur successively in the metal layers. Strong frictional shear stress at the interface has been produced by the gradient alternating deformation of the metal layers, which accelerated the fracture of the brittle layers of the interface. From the microscopic morphology of the shear sections, many fresh metal areas have been exposed on the surface of the metal layers before the interface began to bond. It can be seen that the cracking of the brittle layers of the metal surface and the bonding process of the interface were not simultaneous. During the cold roll bonding process, the interface undergoes the following stages: (1) brittle layer fracture; (2) fresh metals are squeezed into cracks; (3) fresh metals begin to contact with each other; (4) cracks propagation, sufficient and effective contact between fresh metals; (5) a large number of atomic vacancies are generated and the interface is firmly bonded.

Performance Enhancement of PLA-Based Blend Microneedle Arrays through Shish-Kebab Structuring Strategy in Microinjection Molding.

Poly(lactic acid) (PLA) microneedles have been explored extensively, but the current regular fabrication strategy, such as thermoforming, is inefficient and poorly conformable. In addition, PLA needs to be modified as the application of microneedle arrays made of pure PLA is limited because of their easy tip fracture and poor skin adhesion. For this purpose, in this article, we reported a facile and scalable strategy to fabricate the microneedle arrays of the blend of PLA matrix and poly(p-dioxanone) (PPDO) dispersed phase with complementary mechanical properties through microinjection molding technology. The results showed that the PPDO dispersed phase could be in situ fibrillated under the effect of the strong shear stress field generated in micro-injection molding. These in situ fibrillated PPDO dispersed phases could hence induce the formation of the shish-kebab structures in the PLA matrix. Particularly for PLA/PPDO (90/10) blend, there are the densest and most perfect shish-kebab structures formed. The above microscopic structure evolution could be also advantageous to the enhancement in the mechanical properties of microparts of PLA/PPDO blend (tensile microparts and microneedle arrays), e.g., the elongation at break of the blend is almost double that of pure PLA while still maintaining the high stiffness (Young's modulus of 2.7 GPa) and the high strength (tensile strength of 68.3 MPa) in the tensile test, and relative to pure PLA, there is 100% or more increase in the load and displacement of microneedle in the compression test. This could open up new spaces for expanding the industrial application of the fabricated microneedle arrays.

Microscopic Study of Shale Anisotropy with SEM In Situ Compression and Three-Point Bending Experiments

The microscopic anisotropy of shale has an important impact on its mechanical properties and crack behavior, so it is essential to understand the microscopic origin of anisotropic growth with a more effective laboratory work scheme. Uniaxial compression test and three-point bending test are considered to be efficient means to study the elastic properties and crack behavior of rocks. In this paper, uniaxial compression experiments and three-point bending experiments were conducted on shale outcrops in the Changqing area using field emission scanning electron microscopy (SEM) and in situ tensile testing, and the microscopic deformation and crack processes were quantitatively characterized by the digital image correlation (DIC) method. For the compression experiments, the observation of the first principal strain fields indicated that the microscopic anisotropy of shale was related to the bedding planes, and the microscopic deformations were mainly concentrated in the clay mineral accumulation area and at the microcracks. Elastic moduli and compressive strengths of specimens with different bedding angles were affected by the strong shear stress effects. The specimens with a bedding angle of 30° showed lower peak loads and compressive strengths, and the specimens with a bedding angle of 60° had lower elastic moduli. Three-point bending experiments were conducted for studying the effects of crack-bedding orientation relationships on cracking processes, and four critical fracturing mechanical properties were calculated. The short transverse-type cases were prone to break and had a lower peak load, tensile strength, fracture toughness and elastic-bending modulus. The divider-type cases were more difficult to break, formed a more tortuous crack and had a higher tensile strength, fracture toughness and elastic-bending modulus. The arrester-type cases had a middle range of mechanical parameters but developed the longest cracks. This study provides a feasible experimental and analysis method for understanding the microscopic anisotropy of shale samples. The small specimen size also makes the requirements of core samples easier to be satisfied considering the field application. Furthermore, the anisotropy of cracking processes can be understood better by building the connections between microstructural characteristics and mechanical performances.

Scalable microfluidic fabrication of vertically aligned two-dimensional nanosheets for superior thermal management.

Two-dimensional (2D) nanosheets have been assembled into various macroscopic structures for wide engineering applications. To fully explore their exceptional thermal, mechanical, and electrical properties, 2D nanosheets must be aligned into highly ordered structures due to their strong structural anisotropy. Structures stacked layer by layer such as films and fibers have been readily assembled from 2D nanosheets due to their planar geometry. However, scalable manufacturing of macroscopic structures with vertically aligned 2D nanosheets remains challenging, given their large lateral size with a thickness of only a few nanometers. Herein, we report a scalable and efficient microfluidics-enabled sheet-aligning process to assemble 2D nanosheets into a large-area film with a highly ordered vertical alignment. By applying microchannels with a high aspect ratio, 2D nanosheets were well aligned vertically under strong channel size confinement and high flow shear stress. A vertically aligned graphene sheet film was obtained and applied to effectively improve the heat transfer of thermal interfacial materials (TIMs). Superior through-plane thermal conductivity of 82.7 W m-1 K-1 at a low graphene content of 11.8 vol% was measured for vertically aligned TIMs. Thus, they demonstrate exceptional thermal management performance for switching power supplies with high reliability.

CFD simulation of the hydrodynamics within an RVF module in laminar regime

The dynamic filtration module called Rotating and Vibrating Filtration (RVF) is a performant device for membrane fouling control and limiting the formation of cake layer. Indeed, it can generate strong shear stress at the membrane surface decoupled from the feeding flowrate. Moreover, the pressure fluctuation that takes place in the device is also beneficial to control membrane fouling. A CFD model has been validated in laminar regime from experimental measurements of velocity fields and pressure distribution inside the RVF. The boundary layer development at the walls were carefully studied to better explain the effects of the rotation speed and the viscosity on the spatial distribution and time variations of pressure and shear stress at the membrane surface. It appears that shear stress is more efficient than pressure to control membrane performances in laminar regime.

Study on Overburden Structure Characteristics and Induced Scour Mechanisms of Horizontal Sublevel Mining in Steep and Extra-Thick Coal Seams

In order to study the space–time evolution law and the induced impact mechanism of overburden breaking in the tangential horizontal sublevel during the fully mechanized mining of extra-thick and steep coal seams, we took the Yaojie No. 3 mine as an example. Through the establishment of an overburden breaking mechanical model, the structural characteristics of hinged rock beams after overburden breaking and the space–time evolution law of overburden structure instability were analyzed, the static and dynamic load conditions that induce rockbursts were analyzed, and the induced impact mechanism of dynamic and static load superposition was revealed. Our research showed that, due to the asymmetry of the roof and floor, the coal body in the working face is in the strong shear stress zone at the end of the air inlet roadway, which easily produces shear failure. The lateral support pressure and the shear stress of the coal body in the goaf are the static load sources of the rockburst in the steep coal seam; after the roof overburden is broken, a hinge-bearing structure is formed under the support of the sliding force of the fault block and the floating gangue in the goaf. When the coal is mined in the lower section, the strong dynamic load formed by the impact of the fault block on the topmost coal is the main dynamic load source of the impact on the working face. Under the superpositions of the dynamic load and static load, the coal and rock lose stability and release energy in a large range, generating dynamic and static superimposed rockbursts. Furthermore, the internal mechanism of the occurrence of rockbursts during the mining of steep and extra-thick coal seams in the Yaojie No. 3 coal mine was revealed. The static load of the coal body comes from the clamping actions of the roof and floor, and the dynamic impact load comes from the clamping structure’s instability. The reason for the occurrence of rockbursts in the mining of steep and extra-thick coal seams in the Yaojie No. 3 coal mine was reasonably explained.

Spray Entrainment Coefficient Modeling for High Injection Pressure Based on Entrainment Velocity and Force Analysis

Abstract The entrainment coefficient reflects the air–fuel mixing quality, which is one of the foremost concerns for the development of cleaner and higher power-density internal combustion engines. Previous prediction models constructed by the change of axial mass flow rate have lower accuracy under high injection pressure conditions. During modeling in the work, a new construction method based on local entrainment velocity and local entrainment area is developed, and the influences of dilution effect and forces such as flow resistance, lateral pressure, etc. on the local entrainment velocity are considered. With the modified model, its prediction accuracy can be effectively extended to high injection pressure and detailed information about entrainment can be provided for analysis. It is found that, with the increase of injection pressure, the entrainment coefficient rises in the whole flow field. When increasing to high injection pressure, the entrainment coefficient constantly decreases with distance in the far-field, which is consistent with the experiments, but not a constant value predicted by previous models. Besides, the decreased rate of entrainment coefficient rises with the increase of injection pressure. Meanwhile, the increase of ambient pressure also makes the entrainment coefficient rise, but barely influences the decreased rate in the far-field. The large decrease of local entrainment velocity in the far-field caused by strong shear stress and flow resistance can explain the decrease of entrainment coefficient with distance. Overall, the modified model is able to rapidly predict the spray mixing quality over a wider range of operational conditions and provide more detailed entrainment information for analysis.

Effect of extreme mechanical densification on the electrical properties of carbon nanotube micro-yarns

We have explored the effect of high pressure post-treatment in optimizing the properties of carbon nanotube yarns and found that the application of dry hydrostatic pressure reduces porosity and enhances electrical properties. The CNT yarns were prepared by the dry-spinning method directly from CNT arrays made by the hot filament chemical vapour deposition (HF-CVD) process. Mechanical hydrostatic pressure up to 360 MPa induces a decrease in yarn resistivity between 3% and 35%, associated with the sample’s permanent densification, with CNT yarn diameter reduction of 10%–25%. However, when increasing the pressure in the 1–3 GPa domain in non-hydrostatic conditions, the recovered samples show lower electrical conductivity. This might be due to concomitant macroscopic effects such as increased twists and damage to the yarn shown by SEM imaging (caused by strong shear stresses and friction) or by the collapse of the CNTs indicated by in situ high pressure Raman spectroscopy data.

New Equipment and Method for Refining the Solidified Grain Structure

The mechanical properties of solidified alloys strongly depend on the grain size. In many practical cases at the given solidification parameters (temperature gradient and solid/liquid interface velocity), the solidified microstructure is columnar, meaning that the mechanical properties differ depending on the direction, which results in the material being unsuitable for application. The microstructure can be changed from columnar to equiaxed through the inclusion of grain refinement material. This strategy is well known in the literature as the columnar/equiaxed transition (CET). In some cases, it is beneficial if the CET can be produced without using grain refinement material; for example, it may detrimentally affect the mechanical properties (such as when the Al alloy ingot is used in pressing). The stirring of the melt as an alternative for the use of grain refinement material could solve this problem as intensive melt flow can break some particles from growing dendrites. This paper demonstrates a new type of traveling magnetic field inductor that is used to produce strong shearing stress in the flow perpendicular to the solidification front by causing part of the metallic melt layers touching each other to flow in an opposite direction. Through some examples, we demonstrate the effect of stirring by the new inductor on the solidified grain structure.

Investigation on magnetic force and buoyancy force on flow and heat transfer of Al2O3-water nanofluid at various temperature-dependent thermophysical models by a novel PLBM

A novel preconditioned lattice Boltzmann method (PLBM) is applied to investigate the thermo-hydrodynamic characteristics of Al2O3-water nanofluid with various temperature-dependent thermophysical properties and slip boundary conditions in a microchannel. Also, the effects of a magnetic field force, buoyancy force, and volume fraction of nanoparticles are analyzed, respectively. The theoretical level confirms that the convergence performance of the PLBM is greatly improved at large Reynolds numbers compared to the original LBM. Numerical results show that the Koo & Kleinstreuer model has the largest average shear stress and average Nusselt number. Among all the physical models considered, these two values are approximately 1.05 times and 1.2 times of the values by the Maxwell & Brinkmann model, respectively. Once the magnetic field is considered, the average shear stress is increased twice and the average Nusselt number increases by about 10% and further by about 20% at higher buoyancy forces. The growth of these two values gradually slows down when the volume fraction of nanoparticles exceeds 2%. The heat transfer process makes the largest contribution to entropy generation when the magnetic field force is not considered. Accordingly, it will dominate the irreversible loss due to the strong shear stress once the magnetic field force is involved in the heat transfer process.

One step fabrication of aligned carbon nanotubes using gas rectifier

We report the one-step fabrication of aligned and high-quality carbon nanotubes (CNTs) using floating-catalyst chemical vapor deposition (FCCVD) with controlled fluidic properties assisted by a gas rectifier. The gas rectifier consists of one-dimensional straight channels for regulating the Reynolds number of the reaction gas. Our computational fluid dynamics simulation reveals that the narrow channels of the gas rectifier provide steady and accelerated laminar flow of the reaction gas. In addition, strong shear stress is induced near the side wall of the channels, resulting in the spontaneous formation of macroscopic CNT bundles aligned along the direction of the gas flow. After a wet-process using chlorosulfonic acid, the inter-tube voids inherently observed in as-grown CNT bundles are reduced from 16 to 0.3%. The resulting CNT fiber exhibits a tensile strength of 2.1 ± 0.1 N tex−1 with a Young’s modulus of 39 ± 4 N tex−1 and an elongation of 6.3 ± 0.6%. FCCVD coupled with the strong shear stress of the reaction gas is an important pre-processing route for the fabrication of high-performance CNT fibers.

Basal shear zones of recurrent mass transport deposits serve as potential reservoirs for gas hydrates in the Central Canyon area, South China Sea

Recurrent mass transport deposits (MTD) not only act as competent seals trapping fluids, but their basal shear zones connected by pipes also serve as potential reservoirs for gas hydrate within the gas hydrate stability zone. However, this has seldom been addressed in previous studies. Using conventional 3D seismic reflection data and well data collected from the Central Canyon area of the northwestern South China Sea, we find that gas hydrates mainly lie in the fractures within MTDs. These fractures can be subdivided into two groups. The first group is composed of the horizontal and low-angle oblique fractures characterized by pseudo-SC structures, the majority of which occurred within the basal shear zones of MTDs. They probably resulted from the strong shear stresses during mass transport. The second group comprises the nearly vertical fractures within pipes 1 and 2. They are attributed to the result of hydraulic fracturing during the generation of pipes. It is important to note that the basal shear zones of MTDs 2–4 were connected by the pipes in the vertical direction and have good lateral continuities (with the area of hundreds of km2), and thus they probably provide larger reservoir spaces for gas hydrates, compared with the hydraulic fractures merely confined within pipes 1 and 2 (with the area less than 1 km2). Therefore, in the future gas hydrate exploration, much more attention should be paid to basal shear zones of recurrent MTDs that are connected by pipes in the vertical direction. Hydrocarbon gas that formed gas hydrates with water molecular, were mainly supplied by three large-scale gas chimneys above the basement highs. We suggest that the single overpressure configuration and effective seals provided by the deep-seated MTD1 were responsible for the continuous gas accumulations above the basement highs. Hence, hydraulic fracturing took place and these gas chimneys were formed. We also propose a 3D conceptual model to show how MTDs influence fluid migration and accumulation, and prevent methane seepages.