Spring-like Structure Research Articles

Preparation and characterization of ultralight glass fiber wool/phenolic resin aerogels with a spring-like structure

Spring-like ultralight glass fiber wool enhanced phenolic resin aerogels was produced by a simple sol-gel polymerization in a slurry containing ultralight glass fiber wool (UGFW), phenolic resin (PR), hexamethylenetetramine (HMTA) as well as ethylene glycol (EG). UGFW was hierarchically structured and covered with a filmy layer of PR binder between the contact points of fibers to fabricate a stabile three-dimensional (3D) enhancement skeleton before PR aerogel attached. PR aerogels was distributed evenly and randomly on the UGFW skeleton. Compared with other fiber reinforced aerogels (FRAs), ultralight glass fiber wool enhanced phenolic resin aerogels (UGFW/PRAs) exhibited excellent resiliency, low density and structurally adaptive performances. The elastic modulus for the aerogels were ranged from 0.81 to 3.92 kPa at the densities from 7.17 to 37.1 mg/cm3, respectively. The degrees of uniformity (DU) of both samples were under 15.19%. Furthermore, the sound transmission loss (STL) of UGFW/PRAs was significantly increased by 13.16 dB at 4 kHz, 3 times of the calculated value in accordance with mass law. The thermal conductivity (λ) for the composites was ranged from 0.031 to 0.0366 W m−1 K−1 with the density decreased from 36.1 to 13.95 mg/cm3. Moreover, the basic mechanisms of the resilient and the reinforced principles of UGFW/PRAs were analyzed. Accordingly, the synthesis of ultra-lightweight flexible fiber reinforced organic aerogels is expected to involve various fabrics with stable structure, which provides a rich way to develop various new fiber reinforced functional composites.

Effect of different bending shapes on thermal properties of flexible light-emitting diode filament**Project supported by the National Natural Science Foundation of China (Grant No. 51302171), Science and Technology Commission of Shanghai Municipality, China (Grant No. 14500503300), Shanghai Municipal Alliance Program, China (Grant No. Lm201547), Shanghai Cooperative Project, China (Grant No. ShanghaiCXY-2013-61), and Jiashan County Technology Program, China (Grant No. 20141316).

Heat dissipation is an important part of light-emitting diode (LED) filament research and has aroused constant concern. In this paper, we studied the thermal performance of flexible LED filament by numerical simulation and through experiment. The heat dissipation characteristics of spring-like structure flexible LED filament were computed by finite volume method, and it was found that the chip junction temperature was closely related to the pitch and the bending radius. The effect of inclination angle of lighting LED filament was discussed because it is relevant to the spring-like structure flexible LED filament in geometry. The results demonstrated that the temperature of the filament increases as the inclination angle improves.

Highly Compressible Wood Sponges with a Spring-like Lamellar Structure as Effective and Reusable Oil Absorbents.

Aerogels derived from nanocellulose have emerged as attractive absorbents for cleaning up oil spills and organic pollutants due to their lightweight, exceptional absorption capacity, and sustainability. However, the majority of the nanocellulose aerogels based on the bottom-up fabrication process still lack sufficient mechanical robustness because of their disordered architecture with randomly assembled cellulose nanofibrils, which is an obstacle to their practical application as oil absorbents. Herein, we report an effective strategy to create anisotropic cellulose-based wood sponges with a special spring-like lamellar structure directly from natural balsa wood. The selective removal of lignin and hemicelluloses via chemical treatment broke the thin cell walls of natural wood, leading to a lamellar structure with wave-like stacked layers upon freeze-drying. A subsequent silylation reaction allowed the growth of polysiloxane coatings on the skeleton surface. The resulting silylated wood sponge exhibited high mechanical compressibility (reversible compression of 60%) and elastic recovery (∼99% height retention after 100 cycles at 40% strain). The wood sponge showed excellent oil/water absorption selectivity with a high oil absorption capacity of 41 g g-1. Moreover, the absorbed oils can be recovered by simple mechanical squeezing, and the porous sponge maintained a high oil-absorption capacity upon multiple squeezing-absorption cycles, displaying excellent recyclability. Taking advantage of the unidirectional liquid transport of the porous sponge, an oil-collecting device was successfully designed to continuously separate contaminants from water. Such an easy, low-cost, and scalable top-down approach holds great potential for developing effective and reusable oil absorbents for oil/water separation.

Effect of polystyrene grafted graphene nanoplatelets on the physical and chemical properties of asphalt binder

Polystyrene (PS) grafted graphene nanoplatelets (GNPs) composite (PS-GNPs) was prepared by in-situ polymerization and applied as modifier to Styrene-butadiene-styrene (SBS) for modified asphalt preparation. Specific amount of PS-GNPs incorporation improved ductility, softening point and penetration while DSR, MSCR, BBR and LAS tests showed enhancement in viscoelastic, high temperature rutting resistance performance, anti-fatigue and low temperature performance of the parent asphalt. FT-IR, XRD, SEM and AFM techniques confirmed the successful grafting of PS onto GNPs surface, which increased the interplanar spacing of GNPs with a spring-like macromolecular structure. It was deduced that PS grafted onto GNPs surface dissolved the SBS network in modified asphalt, thus greatly improving its dispersions. This study could be of great help for highway and construction industries.

Soft Tendril-Inspired Grippers: Shape Morphing of Programmable Polymer-Paper Bilayer Composites.

Nastic movements in plants that occur in response to environmental stimuli have inspired many man-made shape-morphing systems. Tendril is an exemplification serving as a parasitic grasping component for the climbing plants by transforming from a straight shape into a coiled configuration via the asymmetric contraction of internal stratiform plant tissues. Inspired by tendrils, this study using a three-dimensional (3D) printing approach developed a class of soft grippers with preprogrammed deformations being capable of imitating the general motions of plant tendrils, including bending, spiral, and helical distortions for grasping. These grippers initially in flat configurations were tailored from a polymer-paper bilayer composite sheet fabricated via 3D printing a polymer on the paper substrate with different patterns. The rough and porous paper surface provides a printed polymer that is well-adhered to the paper substrate which in turn serves as a passive strain-limiting layer. During printing, the melted polymer filament is stretched, enabling the internal strain to be stored in the printed polymer as memory, and then it can be thermally released, which will be concurrently resisted by the paper layer, resulting in various transformations based on the different printed geometries. These obtained transformations were then used for designing grippers to grasp objects with corresponding motions. Furthermore, a fully equipped robotic tendril with three segments was reproduced, where one segment was used for grasping the object and the other two segments were used for forming a tendril-like twistless spring-like structure. This study further helps in the development of soft robots using active polymer materials for engineered systems.

Direct imaging of cross-sectional magnetization reversal in an exchange-biased CoFeB/IrMn bilayer

The exchange coupling between ferromagnetic (FM) and antiferromagnetic materials has been intensely studied for fundamental physics and technological applications in various devices. However, the experimental reported magnitudes of exchange coupling are often smaller than that predicted theoretically, and for which the formation of springlike spin structure in the FM layer has been suggested as the cause. However, investigating the spin structure around the interface of exchange-coupled systems is challenging. Here we report the direct imaging of the cross-sectional magnetization reversal and spin structure at the interface of a model exchange-biased CoFeB/IrMn bilayer by a Lorentz transmission electron microscope with electron holography techniques. Through imaging of in situ magnetization reversal and spin structure at the remanent state, the springlike spin structure (either Bloch-wall-like or N\'eel-wall-like) in the CoFeB layer has been deduced within subnanometer region of the interface. This result puts a strong constraint on the theories of exchange coupling in inhomogeneous magnetic systems.

Prediction of Stiffness and Damping of Gas Foil Journal Bearing for High-Speed Rotor

Contamination-free bearings such as gas bearings are an alternate solution in various high-speed turbomachinery such as turboexpander and turbocharger. The major limitation of gas bearing is its low dynamic characteristics such as stiffness and damping. Gas foil bearings (GFBs) are the bearings with elastic spring-like structure in between the runner and the bearing base. These elastic structures in GFB help to tailor the dynamic coefficients of bearing by modifying foil material and its geometry. A bump type GFB is quite popular among tribologists due to its ease of fabrication and assembly compared to other types of GFB. In a bump type GFBs, the overall bearing stiffness is due to stiffness the bump foil, top foil, and the pressurized bearing gas. The overall damping inside the bearing is due to coulombs friction between (i) top and bump foil and (ii) bump foil and the bearing base. A number of numerical techniques have been successfully developed to predict the static characteristic, unlike the dynamic characteristics. This paper discusses a perturbation technique to predict the dynamic characteristic such as overall stiffness and damping of a bump type gas foil bearing. Finite difference formulation is used to determine four stiffness and four damping coefficients. The four coefficients include two direct and two cross-coupled coefficients. The influence of foil material and its geometry over the dynamic characteristics are also discussed in the paper.

Flame Synthesis of Spring-Like Nanocarbon and Its Application in Flexible Free-Standing Mattress-Like Supercapacitor Electrode

Nanocarbons in a unique spring-like helical structure were prepared by a simple and low-cost flame combustion method to act as active and conductive nano-springs in constructing a mattress-like graphene-based paper as a flexible freestanding electrode. As the electrode is applied in a supercapacitor, it simultaneously exhibits high activity, cycling and mechanical stability owing to its unique structure. At a working current density of 0.2 mA cm−2, the areal capacitance of this electrode is as high as 1.07 F cm−2, which is 228% higher than that of the electrode without the helical interlayer. In addition, the capacitive retention of the electrode reaches 93.02% after 1000 charge-discharge cycles at a current density of 2 mA cm−2, and it can remain at 81.22% even if the electrodes are bent 120° in a symmetrical supercapacitor. Such a preparation route is simple and effective, which makes it a promising method for fabricating general spring-like nanomaterials for high-performance wearable electronic devices.

A computational study of a phenolic based polymer with a spring-like structure

We report the stretching potentials for a helical phenolic-based polymer with high symmetry and a spring-like structure that can be stretched by a factor of 4 along the spring direction and still return to its original structure. We hope that synthetic polymer chemists assess if this polymer or a similar one can be synthesized and tested.

Mechanical tuning of conductance and thermopower in helicene molecular junctions.

Helicenes are inherently chiral polyaromatic molecules composed of all-ortho fused benzene rings possessing a spring-like structure. Here, using a combination of density functional theory and tight-binding calculations, it is demonstrated that controlling the length of the helicene molecule by mechanically stretching or compressing the molecular junction can dramatically change the electronic properties of the helicene, leading to a tunable switching behavior of the conductance and thermopower of the junction with on/off ratios of several orders of magnitude. Furthermore, control over the helicene length and number of rings is shown to lead to more than an order of magnitude increase in the thermopower and thermoelectric figure-of-merit over typical molecular junctions, presenting new possibilities of making efficient thermoelectric molecular devices. The physical origin of the strong dependence of the transport properties of the junction is investigated, and found to be related to a shift in the position of the molecular orbitals.

Fabrication and Characterization of Double Helix Structures for Compliant and Reworkable Electrical Interconnects

Microelectromechanical systems (MEMS)-type double helix chip-level electrical interconnect structures are fabricated and characterized in this paper. Due to their springlike structure, double helix interconnects have the potential to provide large mechanical compliance to compensate for nonidealities, such as nonplanarity and thermal expansion mismatch between silicon chips and substrates. A double helix configuration provides for structures with a high volumetric density of conductor for enhanced current carrying capability. The fabrication process is compatible with wafer-level fabrication and packaging. Instead of using solder to form semipermanent interconnections, the double helix interconnects use pressure to make electrical connection and provide sufficiently low resistance (~35 ± 15 mΩ). Large arrays of double helix structures have been fabricated and characterized with excellent yield. The mechanical and electrical models of the structures are presented. Reworkability tests were performed and the structures show a consistent resistance over 50 remating cycles.

Dissecting the molecular mechanism underlying the intimate relationship between cellulose microfibrils and cortical microtubules

A central question in plant cell development is how the cell wall determines directional cell expansion and therefore the final shape of the cell. As the major load-bearing component of the cell wall, cellulose microfibrils are laid down transversely to the axis of elongation, thus forming a spring-like structure that reinforces the cell laterally and while favoring longitudinal expansion in most growing cells. Mounting evidence suggests that cortical microtubules organize the deposition of cellulose microfibrils, but the precise molecular mechanisms linking microtubules to cellulose organization have remained unclear until the recent discovery of cellulose synthase interactive protein 1 , a linker protein between the cortical microtubules and the cellulose biosynthesizing machinery. In this review, we will focus on the intimate relationship between cellulose microfibrils and cortical microtubules, in particular, we will discuss microtubule arrangement and cell wall architecture, the linkage between cellulose synthase complexes and microtubules, and the feedback mechanisms between cell wall and microtubules.

The Formation of a Potential Spring in the Ribosome

Time-dependent chemical modification and cleavage results have provided intriguing insights into structural changes that occur in the distal loop of helix 11 in 16S ribosomal RNA (rRNA). Located distant from the decoding region, between proteins S17 and S20, the results of this study suggest that this region of rRNA may act as a buffer or a spring between these two proteins during protein biosynthesis. During the assembly process, protein S17 apparently produces the major structural deformations in this region, causing it to be folded in a spring-like structure. Base C264 in this region shows erratic behavior during assembly and also shows time-dependent enhancement when elongation factor G with GTP is added to 70S ribosomes. Evidence is presented to suggest that this region of rRNA may be used to allow relative motion to occur between proteins S17 and S20 during translocation.

Numerical investigation of elastic mechanical properties of graphene structures

The computation of the elastic mechanical properties of graphene sheets, nanoribbons and graphite flakes using spring based finite element models is the aim of this paper. Interatomic bonded interactions as well as van der Waals forces between carbon atoms are simulated via the use of appropriate spring elements expressing corresponding potential energies provided by molecular theory. Each layer is idealized as a spring-like structure with carbon atoms represented by nodes while interatomic forces are simulated by translational and torsional springs with linear behavior. The non-bonded van der Waals interactions among atoms which are responsible for keeping the graphene layers together are simulated with the Lennard-Jones potential using appropriate spring elements. Numerical results concerning the Young’s modulus, shear modulus and Poisson’s ratio for graphene structures are derived in terms of their chilarity, width, length and number of layers. The numerical results from finite element simulations show good agreement with existing numerical values in the open literature.

Sensilla on the antennae, legs and ovipositor of Spathius agrili Yang (Hymenoptera: Braconidae), a parasitoid of the emerald ash borer Agrilus planipennis Fairmaire (Coleoptera: Buprestidae)

To investigate the host-finding mechanisms of Spathius agrili Yang, a parasitoid of the emerald ash borer, Agrilus planipennis Fairmaire, a highly concealed insect pest of Fraxinus spp. The morphology and ultrastructure of sensilla on the antennae and legs of both sexes and on the ovipositors and ovipositor sheaths of females were observed using SEM. Results showed that abundant sensilla trichodea (ST) are distributed on the antennae of both females and males, with the sharp trichoid with a socket at the base being the most numerous sensillum. There are 6-7 sensilla placodea on each segment of the flagellum, arranged lengthways in parallel rows. There are very few sensilla styloconica on the female antennae. The tibiae have mostly sensilla chaetica, whereas three types of ST and the relatively plentiful sensilla chaetica are present on the tarsi. Besides ST, a mass of sensilla squamiformia and small short spines are distributed on the pulvilli. Basically there are no sensilla on the female ovipositor, only steering mechanisms for drilling bark and a variety of v-shaped grooves. However, a large numbers of short sensilla chaetica and microtrichia are found on the internal surface of ovipositor sheath. The outer surface of the ovipositor sheath also has ST. In addition, a clear spring-like extension structure can be observed on the fore parts of ovipositor sheath. These findings suggest that the main function of sensilla on the antennae is olfaction, while the sensilla on legs and ovipositor sheaths are mechanoreceptors and tactile sensors.

Soft microflow sensors

We present a rapid prototyping method for integrating functional components in conventional PDMS microfluidic devices. We take advantage of stop-flow lithography (D. Dendukuri, S. S. Gu, D. C. Pregibon, T. A. Hatton and P. S. Doyle, Lab Chip, 2007, 7, 818)(1) to achieve the in situ fabrication of mobile and deformable elements with a controlled mechanical response. This strategy is applied to the fabrication of microflow sensors based on a deformable spring-like structure. We show that these sensors have a large dynamic range, typically 3 to 4 orders of magnitude, and that they can be scaled down to measure flows in the nl per min range. We prepared sensors with different geometries, and their flow-elongation characteristics were modeled with a simple hydrodynamic model, with good agreement between model and experiments.

Improved long‐term recording of nerve signal by modified intrafascicular electrodes in rabbits

Methods for long-term recording of peripheral nerve activity via intrafascicular electrodes have not been optimized. We compared the long-term functionality of custom-made 95%Pt/5%Ir intrafascicular electrodes containing a proximal spring-like structure to that of conventional straight electrodes. The modified electrode was implanted into the sciatic nerve fascicle of a random hind limb in 14 rabbits for 9 months. A conventional electrode was implanted in the opposite hind limb as a control. Orthodromic and antidromic nerve potentials were sampled and analyzed monthly. Latency, amplitude, and nerve conduction velocity of electrical signals were generally similar within the modified group and straight control group at different time intervals (P > 0.05). However, at the conclusion of the study period, the modified electrode group had a greater number of functioning electrodes (P < 0.05) and a greater total functioning electrode time (P = 0.006). Intrafascicular electrodes with a spring-like structure demonstrated superior potential for long-term electrophysiological monitoring over straight electrodes.

What is normal black African hair? a light and scanning electron-microscopic study

Background: The hair of normal black Africans forms a mat of tightly interwoven hair shafts. The effect of this on the structure of the hair shaft and the response to grooming is unknown. Objective: Our purpose was to use light and scanning electron microscopy (SEM) to examine the structure of Negroid-type hairs and effects of combing in black African volunteers. Methods: Hair samples were collected, by combing, from Africans and compared with those from Caucasian and Asian volunteers. The volunteers had never used chemical treatments. Their hair had not been cut for at least 1 year and grooming had been limited to shampooing, drying, and combing. Results: More than 2000 hairs in 12 African volunteers were examined by light microscopy. The hairs appear as a tight coiled springlike structure. Many shafts exhibited knots (10%-16% vs 0.15%) and appear broken compared with hair shafts from other ethnic groups. SEM of African hairs showed features consistent with repeated breaks of the shaft. Examination of hairs in situ showed interlocking of hair shafts. Conclusion: These observations provide an understanding of the physical nature of, and effect of combing on, black African hair. (J Am Acad Dermatol 2000;43:814-20.)

Elastic stiffness of crystalline cellulose in the folded-chain solid state

In previous papers the axial stiffness of crystalline native cellulose has been calculated for two proposed configurations of the cellulose chains within the elementary fibrils: an extended chain configuration, and a configuration in which the chains are folded to form a ribbon which in turn is helically wound into a rather open, spring-like structure. In the present paper two additional folded-chain configurations are treated: a more tightly wound helical configuration in which axial secondary bonds are formed between adjacent turns of the helix; and a configuration in which the chains are folded rather infrequently and remain in a fully-dense close-packed arrangement. It is shown that this last configuration is mechanically equivalent to the extended chain configuration so far as axial stiffness is concerned, and that either helical configuration has a substantially lower axial stiffness than that of the extended chain.