Bending Instability Research Articles

Study on Deposition of Coaxial Electrospinning Fibers by Coaxial Auxiliary Flow Field

Gas-assisted coaxial electrospinning (GACES) is a simple and general method for the mass preparation of coaxial nanofiber membranes, which has great industrial potential. However, in the manufacturing process, due to the bending instability of the jet in the electric field and the pulling effect of the gas flow field, the deposition uniformity of the fiber is still a big problem. Through finite element simulation analysis of the flow field in the manufacturing process and the construction of the jet mechanics model after adding the flow field, the influence mechanism of coaxial auxiliary flow on the fiber deposition area and its uniformity was successfully revealed in this research. Finally, the deposition area and thickness uniformity of coaxial fibers are increased by 3 times (the deposition area: 19.63 cm2 → 78.50 cm2) and 2.34 times (the standard variance: 3 μm2 → 10 μm2) by gas-assisted coaxial electrospinning. At the same time, the coaxial auxiliary gas flow also reduces the coaxial fiber diameter by 36.9% (the average fiber diameter: 241 nm ± 5 nm → 152 nm ± 23 nm) and the distribution range by 66% (the standard variance: 1.5 × 102 nm2 → 51 nm2). This research provides a reliable idea and experimental basis for homogeneous preparation of coaxial nanofiber membranes.

Asymmetric fluctuations and self-folding of active interfaces

We study the structure and dynamics of the interface separating a passive fluid from a microtubule-based active fluid. Turbulent-like active flows power giant interfacial fluctuations, which exhibit pronounced asymmetry between regions of positive and negative curvature. Experiments, numerical simulations, and theoretical arguments reveal how the interface breaks up the spatial symmetry of the fundamental bend instability to generate local vortical flows that lead to asymmetric interface fluctuations. The magnitude of interface deformations increases with activity: In the high activity limit, the interface self-folds invaginating passive droplets and generating a foam-like phase, where active fluid is perforated with passive droplets. These results demonstrate how active stresses control the structure, dynamics, and break-up of soft, deformable, and reconfigurable liquid-liquid interfaces.

Experimental mechanical properties, nonlinear bending and instability analysis of 3D-printed auxetic tubular metastructures using multiscale finite element and Ritz methods

In the field of medical technology, specialized equipment utilizes tubular metastructures with a negative Poisson’s ratio in vascular stents to reduce the risk of embolism. The deliberate incorporation of auxetic structures into stent design offers several benefits over traditional stents. This study examines the nonlinear instability and bending responses of reentrant perfect and imperfect 3D-printed tubular metastructures. First, the governing equations for the auxetic tube with geometric imperfections under transverse and axial mechanical loads are derived using the von-Kármán nonlinear assumption and Timoshenko theory. Then, the equations are derived using the principle of virtual displacement. The physical characteristics of the reentrant structure are derived using Malek-Gibson relations, while tensile tests with digital image correlation (DIC) are used to determine the physical properties of polylactic acid (PLA). Scanning electron microscopy (SEM) images help investigate variations in modulus of elasticity and ultimate tensile strength in dogbone specimens, and the Ritz method with Chebyshev polynomials is employed to discretize the nonlinear equations. Second, two numerical algorithms are used to analyze the static behavior of the metatube within the nonlinear framework. The validation study is conducted for the auxetic tube and the representative volume element (RVE) of the unit cell based on the literature and finite element software Abaqus. Following the validation of the mathematical model, an extensive investigation is conducted to assess how differing parameters impact the mechanical bending and nonlinear instability analysis of the reentrant tube.

An experimental and numerical study on the behavior of finite-length column vortex

This paper explores experimental and numerical investigation of the spatiotemporal dynamics of a finite-length vortex column in three dimensions using particle image velocimetry and large eddy simulation. The research examines the combined impact of bending, buckling, and core-splitting on a finite-length vortex column. More precisely, the work centers on the fundamental motions, evolutions in flow along the axis, how the shape of the vortex core changes over time, the instability caused by the long waves on the vortex column, and the division of the vortex core. A novel prototype is developed that utilizes piston-driven inflow and produces a vortex column through the principles of flow separation and Biot–Savart induction, which is studied for predicting an ex situ cyclonic line vortex. The present study discusses the complete three-dimensional overview of the same columnar vortex. The meridional swirl and the axial transport of vorticity deform the shape of the core cross sections in different lengths of the column. The jump in the axial velocity at the core boundary allows for the occurrence of bending instabilities with a left-handed helical structure. These instabilities have a long wavelength, similar to the length of the vortex column, and belong to the m = +1 mode. The vortex column experiences a non-uniform curvature and torsion caused by the intricate shape of the laboratory model and varying flow speeds at different heights of the vortex column. The results offer valuable insights into the role of inertia, Coriolis, and viscous forces on the dynamics of the vortex column.

Dynamic Failure Mode Analysis for a Transmission Tower-Line System Induced by Strong Winds

The prevailing approach to the wind resistance design of transmission towers is rooted in the quasi-static method. However, this methodology faces criticism for neglecting tower-line coupling dynamics. Despite efforts to boost structural wind resilience, the research on tower failure mechanisms, especially under extreme winds considering tower-line coupling, is limited. To address this gap, the wind-induced dynamic failure modes of the transmission tower-line system are investigated in this paper. The consistent discrete random flow generation method is employed to simulate the fluctuating wind field for transmission lines. Incorporating the compressive buckling mode of angle steel, the plastic hinge model of the frame element is employed to simulate mechanical nonlinearity. A typical transmission tower-line system is concerned, with a finite element model established for a three-tower, four-line coupled configuration. The findings reveal that the wind-induced collapse of the transmission tower is directly triggered by the buckling failure of the compressed main members, with the vulnerable section located beneath the lower cross-arm. The transmission tower experiences bidirectional bending and compression instability under an unfavorable wind direction. In contrast, the traditional pushover collapse modes of the transmission tower cannot fully capture the characteristics of the collapse failure, mainly due to the ignorance of the transverse wind force action induced by the coupling effect. This research underscores the importance of incorporating lateral dynamic considerations into transmission tower designs and advocates for optimizing strategies to mitigate wind-induced collapse modes.

Analysis of Bending Deformation and Stress of 6063-T5 Aluminum Alloy Multi-Cavity Tube Filled with Liquid.

The production of aluminum alloy multi-lumen tubes primarily involves hot bending formation, a process where controlling thermal deformation quality is difficult. Specifically, the inner cavity wall of the tube is prone to bending instability defects under the bending stress field. To address these challenges in the bending deformation of aluminum alloy multi-lumen tubes, a multi-lumen liquid-filled bypass forming method is proposed in this paper. This study focuses on the 6063-T5 aluminum alloy double-lumen tube as the research object. The liquid-filled bending deformation behavior of the aluminum alloy double-lumen tube was investigated, and the deformation theory of the aluminum alloy double-lumen tube was studied. Through experimental and numerical simulation methods, the influence of support internal pressure, bending radius, and tube wall thickness on the liquid-filled bending deformation behavior of the double-lumen tube was examined. The results indicate that when the value of internal pressure was 7.5 MPa, the straightening of the outer wall was improved by 2.51%, the thinning rate of wall thickness was minimized, and the internal concave defect was effectively suppressed. The liquid-filled bending method provides a promising new approach for the integrated bending and forming of multi-lumen tubes.

Microswimmers Knead Nematics into Cholesterics.

The hydrodynamic stresses created by active particles can destabilize orientational order present in the system. This is manifested, for example, by the appearance of a bend instability in active nematics or in quasi-two-dimensional living liquid crystals consisting of swimming bacteria in thin nematic films. Using large-scale hydrodynamics simulations, we study a system consisting of spherical microswimmers within a three-dimensional nematic liquid crystal. We observe a spontaneous chiral symmetry breaking, where the uniform nematic state is kneaded into a continuously twisting state, corresponding to a helical director configuration akin to a cholesteric liquid crystal. The transition arises from the hydrodynamic coupling between the liquid crystalline elasticity and the swimmer flow fields, leading to a twist-bend instability of the nematic order. It is observed for both pusher (extensile) and puller (contractile) swimmers. Further, we show that the liquid crystal director and particle trajectories are connected: in the cholesteric state the particle trajectories become helicoidal.

Study on uniformity of multi‐needle electrostatic spinning by auxiliary flow field

AbstractMulti‐needle electrospinning is a simple and general method for mass preparation of nanofiber membrane, which has great industrial potential. However, the bending instability produced in the electrospinning process makes that the deposition uniformity of the nanofiber is still a big concern, resulting in non‐uniform nanofiber membrane, which seriously affects the application of electrospun membrane in environmental filtration, new energy and medical fields. In order to improve the uniformity of nanofiber deposition in multi‐needle electrospinning, an auxiliary flow field system (AFF) is proposed, which can effectively improve the uniformity of nanofiber deposition. After image processing, the uniformity of nanofiber deposition is quantified with the index of grey distribution, and the effectiveness of this method is verified. Combined with the multi‐physical field analysis, the influence mechanism of cross‐wind field on the uniformity of fibre deposition was revealed. By optimizing the experimental parameters, the non‐uniformity of nanofiber deposition was reduced by 49.19%. Based on multi‐needle electrospinning technology, a reliable idea (AFF) and experimental basis are provided for the uniform preparation of nanofiber membrane.

Spiral Structured Cellulose Acetate Membrane Fabricated by One-Step Electrospinning Technique with High Water Permeation Flux

A functionally graded membrane (FGM) with a special spiral-structured cellulose acetate (CA) membrane was prepared by electrospinning under different collection distances. The membrane morphology was analyzed by scanning electron microscopy (SEM). FESEM images revealed that the high concentration shows the formation of fibers with an irregular diameter, with a large diameter distribution range. The fiber collected at a short distance of 10 cm experiences the strong electrostatic force, resulting in the short flight time for the polymer jet. This causes the bending instability of the polymer jet forming the comparatively thick fiber diameters, whereas the fiber collected at 15 cm shows the presence of a smooth, homogeneous diameter. Furthermore, the water flux of the membrane was determined using 50 mL of Amicon stirred cells. The fiber collected at different distances showed diameter variation, which is used to design a special spiral structure on the membrane by auto-moving the collector between the fixed distances of 10–20 cm. This technique will reveal a new approach for the fabrication of a special spiral structure on the nanofibrous membrane for different biomedical applications from different polymers. Meanwhile, the fabricated FGM with a special spiral-structure CA membrane demonstrates high water permeation flux.

Theoretical analysis of inflated tube wrinkling behavior under pure bending

Inflatable tubes, owing to their lightweight and foldable characteristics, have important applications in soft robots and space expandable structures. The application of a bending load to the inflated tube might induce local instability on its compressed side, leading to the formation of wrinkles that could potentially compromise the structural strength. Therefore, it is necessary to have a comprehensive understanding of the wrinkling behavior exhibited by inflated tubes under bending load. Initially, a theoretical solution is developed for the critical wrinkling behavior of inflated tubes under pure bending, drawing upon the principle of minimum potential energy. According to the energy expression of the system, the critical wrinkling behavior of inflated tubes depends on dimensionless geometrical and internal pressure parameters. The critical wrinkling load and wrinkle pattern of the system are influenced by the thickness-radius ratio and the ratio of internal air pressure to Young's modulus. Subsequently, the theoretical solutions are validated through several finite element analysis examples and a systematic investigation is conducted into the influence of dimensionless geometrical and internal pressure parameters. Finally, the evolution of morphology in post-buckling is investigated through numerical results. The findings suggest that during post-buckling, the wrinkles may undergo a secondary bifurcation and evolve into a non-axisymmetric diamond-like pattern. These results hold significant implications for understanding the bending instability and failure mechanisms of inflated tubes.

The vertical structure of galactic discs: non-local gravity versus dark matter

ABSTRACT Recent isolated galactic simulations show that the morphology of galactic discs in modified gravity differs from that of the standard dark matter model. In this study, we focused on the vertical structure of galactic discs and compared the bending instability in the vertical direction for both paradigms. To achieve this, we utilized high-resolution N-body simulations to construct two models in a specific non-local gravity theory (NLG) and the standard dark matter model and compared their stability against the bending perturbations. Our numerical results demonstrate that the outer regions of the disc are more susceptible to the instability in NLG, whereas the disc embedded in the dark matter halo is more unstable in the central regions. We then interpret these results based on the dispersion relation of the bending waves. To do so, we presented an analytical study to derive the dispersion relation in NLG. Our numerical results align with the predictions of our analytical models. Consequently, we conclude that the analysis of bending instability in galactic discs offers an explanation for the distinct vertical structures observed in simulated galactic discs under these two theories. These findings represent a significant step towards distinguishing between the modified gravity and dark matter models.

Crashworthiness analysis of variable thickness CFRP/Al hybrid multi-cell tube

In order to fully utilize the energy absorption potential of hybrid structure composed of multiple materials, a variable thickness Carbon Fiber Reinforced Plastic/ aluminum (CFRP/Al) hybrid multi-cell tube is proposed, with a focus on the design strategies of variable thickness filament winding and multi-cell layout. First, based on the validated finite element (FE) model, a comparative study of different tube configurations is implemented to clarify the advantages and disadvantages of multi-cell hybrid tube. By analyzing the energy absorption contribution of two component materials and local structures, the bending instability mechanism of multi-cell hybrid tube at large angles is revealed to emphasize the necessity of variable thickness filament winding. It is found that multi-cell Al tube is the key factor to induce structural instability, and CFRP with positive gradient thickness distribution is beneficial to restrain the bending instability of hybrid multi-cell tube. Second, in order to better describe the influence mechanism of the design parameters, two comprehensive crashworthiness performance indicators for multi-angle compression conditions are proposed, namely comprehensive specific energy absorption and energy absorption stability. Then, a detailed parametric study is performed to explore the influence mechanism of the variable thickness winding parameters and multi-cell cross-section parameters on the comprehensive energy absorption capacity of the hybrid tube under multi-angle compression conditions. Finally, an application recommendation for the variable thickness CFRP/Al hybrid multi-cell tube is given to provide a reasonable design strategy. Furthermore, a comparative analysis of crashworthiness between the suggested CFRP/Al hybrid tube and reported typical energy-absorbing structures is carried out to reveal the superiority of the variable thickness CFRP/Al hybrid multi-cell tube in the energy absorption stability. This research provides valuable guidelines and suggestions for the crashworthiness design of multi-material hybrid structures facing complex loading conditions.

Measuring the Thickness of Fiber Mats Using Light Transmittance via Image Processing

Fiber materials possess unique properties, including a large active surface area, high surface-to-volume ratio, high porosity, high mechanical performance, and low density. Consequently, they serve as key components in various applications such as energy production and storage cells, batteries, wastewater treatment membranes, sensors, drug-releasing band-aids, and protective clothing. Electrospinning is a simple and versatile method for producing fiber materials. Despite its simplicity in terms of working principles, it faces challenges in achieving homogeneous thickness (evenly on the entire surface) throughout fiber mats due to inherent bending instability during the process. Non-uniform thickness not only diminishes the efficiency of these mats in applications but also adversely impacts their mechanical functionality. In this study, the aim is to develop a thickness measurement system based on image processing and the principle of light transmittance as a solution to the issues encountered in achieving uniform thickness and thickness control of fiber mats produced by electrospinning devices. To accomplish this, a closed mechanism with a light-impermeable encapsulation method was established. Various fiber mats, produced at different times were placed on the acetate floor built on LED lighting on the base of the mechanism. Using a camera with the adjusted settings positioned at the focal point on top of the mechanism, images of the fiber mats were captured, and image processing techniques were employed to determine threshold value ranges for the mat thicknesses. The actual thicknesses of the fiber mats were verified using an optical microscope, revealing that regions defined with the same color in different samples exhibited similar thicknesses.

Research on the impact and control techniques of gangue rib in hard roof

Blocking gangue support is a key link in gob-side entry retaining by roof cutting and pressure release (GERRC), and its stability is related to the success or failure of the retained roadways. Due to the severe dynamic pressure and high hardness of the gangue during hard roof collapse, serious bending and toppling instability of the section steel can easily occur. Since potential safety hazards exist in gangue ribs, the 1105 panel of the Qiuji Coal Mine is used as the engineering background, a mechanical model of gangue impact is constructed based on four forms of gangue motions, and the failure of a blocking gangue support structure under hard roof conditions is explored with ABAQUS software. The results indicate the following: (1) Section steel experiences transient bending by gangue impact, and will further bending failure or toppling instability occurs under roadway roof subsidence and gangue compression. (2) This impact causes the steel bar mesh to generate a significant transient lateral force, which couples with gangue compression, resulting in instability and slip of the section steel. To this end, three optimized components have been proposed, sliding yield I-steel, reinforced bolts and link components, and the optimal parameters are obtained. The results of the numerical simulation and field tests suggest that the new blocking gangue support structure has a good support effect, and the field deformation of the gangue rib is reduced by 86%. No failure of I-steel occurs with the new blocking gangue support structure. This study can provide a reference for the control of gangue ribs in GERRC under similar geological conditions.

Nonlocal strain gradient-based isogeometric analysis of graphene platelets-reinforced functionally graded triply periodic minimal surface nanoplates

Recent developments in additive manufacturing (AM) technologies have empowered the design and fabrication of intricate bioinspired engineering structures at the nano/micro scale. However, mathematical modeling and computation of these structures are still challenging. The main target of this study is to address an efficient computational approach for predicting the mechanical behavior of graphene platelets (GPLs)-reinforced functionally graded triply periodic minimal surface (FG-TPMS) nanoplates. The computational model integrates both nonlocal elasticity and strain gradient effects into the NURBS-based isogeometric analysis of these small-scale structures. We establish advanced nanoplate models by combining three sheet-based TPMS architectures with two new porosity distribution patterns and three distribution patterns of GPLs across the thickness direction. Moreover, the present work makes a pioneering attempt to elucidate how the stiffness-hardening and stiffness-softening mechanisms influence FG-TPMS nanoplates reinforced with GPLs. Compared with two common cellular solids, the superiority of TPMS architectures' mechanical performance is demonstrated. Among all, P and IWP TPMS types along with symmetric porosity and GPLs distributions exhibit outstanding behaviors under static bending, free vibration, and dynamic instability. Furthermore, we conducted a performance analysis for the first time, showcasing the superior capabilities of TPMS architectures under dynamic in-plane compressive loads, especially when compared to isotropic plates of equal weight. The findings of this study greatly enhance our understanding of the intricate mechanical responses of GPLs-reinforced TPMS architectures at the small-scale level, contributing to future interdisciplinary applications.

Bending instability of a stiff lamella embedded in soft matrix

Sandwiched composite structures consisting of stiff lamella embedded in soft matrix are ubiquitous in nature and engineering, in which the instability of lamella is generally observed. Instead of the previously well-explored in-plane compression, a novel bending-induced instability in such sandwiched structure is investigated theoretically, numerically and experimentally in this manuscript. The theoretical model considers the non-linearity of both geometry and material to describe the finite bending of the sandwiched structure and makes use of the linear incremental theory to investigate the triggered instability. The high consistency among theoretical predictions, numerical simulations and experimental measurements of the wrinkling characteristics provides quantitative verification for the theoretical model. On this basis, the effects of the film position in the whole structure on the critical bending curvature and wrinkling characteristics are researched systemically. And finally, a phase diagram is established, enabling simple and intuitionistic prediction of bending instability characteristics from the geometric parameters of the system. These results might shed light on the underlying physical mechanisms of bending instability and provide design guidelines in some practical applications.

Electrospinning of Cyclic Olefin Copolymer for the Production of Highly Aligned Dye‐Doped Optical Fibers

AbstractCyclic olefin copolymer (COC, Topas 5013, with 75% norbornene) has special properties like high glass transition temperature, low moisture uptake, chemical inertness, and a high degree of transmission in UV range, making it competitive with other materials such as poly (methyl methacrylate) and polycarbonate for the production of optical devices. Electrospinning provides a facile way to produce fibers. In order to generate highly aligned fibers fulfilling the requirements for optical applications, stable‐jet electrospinning can be used, in which the bending instabilities are suppressed. However, electrospinning of COC fibers by is still a major challenge due to the lack of suitable solvents. The influence of various solvents on the fiber formation is investigated. The addition of salts enhances the spinnability of the COC solution. By utilizing binary‐solvent systems with the ratio of 2:8 (v/v), morphology and alignment of fibers are significantly improved, as visualized via scanning electron microscopy. The binary‐solvent systems that content nonpolar and polar solvents also provide dispersibility for both organic dyes and inorganic nanoplatelets with hydrophobic ligand shells. By adding such dyes and nanoplatelets during fiber fabrication process, dye‐doped COC fibers can be produced without compromising the optical performance of the dyes and nanoplatelets.

Bending instabilities of m = 1 mode in disc galaxies: interplay between dark matter halo and vertical pressure

ABSTRACT A self-gravitating, differentially rotating galactic disc under vertical hydrostatic equilibrium is supported by the vertical pressure gradient force against the gravitational collapse. Such discs are known to support various bending modes, for example warps, corrugation, or scalloping (typically, higher order bending modes) of which m = 1 bending modes (warps) are the most prevalent ones in galactic discs. Here, we present a detailed theoretical analysis of the bending instability in realistic models of disc galaxies in which an exponential stellar disc is under vertical equilibrium and residing in a cold rigid dark matter halo. A quadratic eigenvalue equation describing the bending modes is formulated and solved for the complete eigen spectrum for a set of model disc galaxies by varying their physical properties such as disc scale-height, and dark matter halo mass. It is shown that the vertical pressure gradient force can excite unstable bending modes in such a disc as well as large scale discrete modes. Further, it is shown that the unstable eigen modes in a thinner disc grow faster than those in a thicker disc. The bending instabilities are found to be suppressed in discs dominated by massive dark matter halo. We estimate the growth time-scales and corresponding wavelength of the m = 1 unstable bending modes in Milky Way like galaxies and discuss its implication.

Establishment of a one-dimensional physical model for electrospinning multiple jets of spinnerets

Electrospinning is a technique used to prepare nanofiber material where an electrically charged jet of polymer solution is accelerated and stretched by electrostatic forces. There are two stages in electrospinning, the stable jet and bending instability. Almost all electrospinning research involves the theory and model of a single jet’s movement. Many devices for the mass production of nanofibers have been developed and studied to date. The multiple jets of spinnerets can form throughput nanofibers. However, a greater understanding is needed on the formation mechanism of these multiple jets, which would allow researchers to control nanofiber productivity and instruct spinneret design. In this work, we investigated the formation mechanism of multiple jets based on an annular spinneret. A one-dimensional physical model was established and multiple equations were combined; the wavelength [Formula: see text] with respect to the applied voltage [Formula: see text] was obtained based on the derivation of the theoretical formulation. The theory order relationship was verified with experiments using multiple jets containing the annular spinneret.

Preparation of high positioning accuracy lattice patterns with polymeric microfibers derived from near‐field electrospinning

AbstractElectrospinning is a straightforward and versatile technology that has been broadly applied to the preparation of microfibers. However, the fabrication of complicated patterns with high positioning accuracy is still a challenge. The enhancement of the positioning accuracy is restricted by the bending instability of the jet and the motion stage. Here, an effective method to improve the positioning accuracy of ordered fibers by optimizing printing parameter and printing order is proposed. Highly oriented and uniformly aligned microfiber arrays composed of polyethylene oxide (PEO) fibers with a diameter of 2.8 ± 0.13 μm are deposited by adjusting the process parameters. Moreover, the accumulative return error of the printed pattern was reduced by 30.54 μm in X‐axis and 55.59 μm in Y‐axis using an optimized printing order. The lattice patterns with a high positioning accuracy of 0.98 ± 0.06 μm were obtained, which is nearly two orders of magnitude higher than that of the structure with unoptimized printing order. Results indicate that the method can significantly improve the positioning accuracy of the lattice patterns of polymeric microfibers, which has a promising prospect in the field of high‐precision micro manufacturing.