Helicoidal Structure Research Articles

Modulated non-collinear magnetic structure of (Co0.9757Fe0.03)4Nb2O9 as revealed by Mössbauer spectroscopy

In this work, we present detailed 57Fe Mössbauer spectroscopy investigations of (Co0.9757Fe0.03)4Nb2O9 compound to study its possible magnetic structure. We have shown that the previously reported magnetic structures cannot satisfactorily describe our low temperature Mössbauer spectra. Therefore, in combination with theoretical calculations, we have proposed a modulated helicoidal magnetic structure that can be used to simulate the whole series of our low temperature Mössbauer spectra. Our results suggest that the combination of previously reported different magnetic structures are only approximations of the average magnetic structure from our modulated helicoidal model. We anticipate that the proposed modulated non-collinear magnetic structure might shed light on the understanding of the complex magnetoelectric effects observed in this system.

Neutron scattering sum rules, symmetric exchanges, and helicoidal magnetism in MnSb2O6

MnSb$_{2}$O$_{6}$ is based on the noncentrosymmetric $P321$ space group with magnetic Mn$^{2+}$ ($S={5/2}$, $L\approx 0$) spins ordering below $T_{\mathrm{N}}=12$ K in a helicoidal structure. The ground state magnetic structure, expected to be built and originate from 7 Heisenberg exchange constants, has been shown to be coupled to the underlying crystallographic chirality with polar domain switching being reported. We apply neutron spectroscopy to extract these symmetric exchange constants. Given the high complexity of the magnetic exchange network, crystallographic structure and complications fitting linear spin-wave models, we take advantage of multiplexed neutron instrumentation to use the first moment sum rule of neutron scattering to estimate the 7 exchange constants. We then use these parameters to calculate the low-energy spin-waves in the N\'eel state to reproduce the neutron response without strong antisymmetric coupling. Using Green's response functions, the stability of long-wavelength excitations in the context of proposed magnetic structures is then discussed. The results show the presence of strong exchange constants for the chiral exchange pathways and illustrate an underlying coupling between crystallographic and magnetic ``chirality" through predominantely symmetric exchange.

Recent Advances in Electro-Optic Response of Polymer-Stabilized Cholesteric Liquid Crystals

Cholesteric liquid crystals (CLC) are molecules that can self-assemble into helicoidal superstructures exhibiting circularly polarized reflection. The facile self-assembly and resulting optical properties makes CLCs a promising technology for an array of industrial applications, including reflective displays, tunable mirror-less lasers, optical storage, tunable color filters, and smart windows. The helicoidal structure of CLC can be stabilized via in situ photopolymerization of liquid crystal monomers in a CLC mixture, resulting in polymer-stabilized CLCs (PSCLCs). PSCLCs exhibit a dynamic optical response that can be induced by external stimuli, including electric fields, heat, and light. In this review, we discuss the electro-optic response and potential mechanism of PSCLCs reported over the past decade. Multiple electro-optic responses in PSCLCs with negative or positive dielectric anisotropy have been identified, including bandwidth broadening, red and blue tuning, and switching the reflection notch when an electric field is applied. The reconfigurable optical response of PSCLCs with positive dielectric anisotropy is also discussed. That is, red tuning (or broadening) by applying a DC field and switching by applying an AC field were both observed for the first time in a PSCLC sample. Finally, we discuss the potential mechanism for the dynamic response in PSCLCs.

Low-velocity impact resistance behaviors of bionic double-helicoidal composite laminates

To improve the low-velocity impact resistance of composite laminates, a bionic double-helicoidal composite laminate (BDHCL) emulated from the micro-structure of coelacanth scales is proposed. The micro-structure is a double helicoidal Bouligand structure in which two adjacent collagen layers are arranged into an orthogonal bilayer and then rotating at a small helix angle. Three types of helicoidal configurations with different derivatives are designed, including sine rotation angle (SRA), linear rotation angle (LRA), and exponential rotation angle (ERA). The progressive intra-laminar and inter-laminar damage model is adopted to capture the failure behaviors of material. The impact behaviors of BDHCLs with several helix angles and included angles of the bilayer are studied. The results show that the double-helicoidal Bouligand structure has a higher resistance load and shallower penetration depth. The impact resistance of ERA laminate has the maximum improvement among the above three types of helix angle. A certain included angle of bilayer for the BDHCL will exhibit excellent impact resistance.

On the Cation-π capabilities of infinitene (∞). Evaluation of bonding and circular dichroism properties for Infinitene-Ag(I)n (n = 1–4) complexes from relativistic DFT calculations

The helicoidal π-conjugated structure from helicenes is able to feature cation-π bonding towards Ag(I) atoms, resulting in the variation of their inherent high optical rotation values. Recently, the characterization of infinitene ([12]infinitene) as two fused [6]helicene structures with a central crossover section suggests the extension of such coordination characteristics to large and more complex structural landscapes. Herein, we evaluated the possible cation-π features of infinitene towards one to four silver cations from density functional calculations and their role in the optical rotation of the infinity-shaped backbone. The nature of the cation-π interaction for infinitene, shows a favorable formation of single and multiple Ag(I) aggregation, with a slight distortion of the structural backbone. Hence, the formation of cation- π complexes in infinitene is plausible, suggesting the expansion of its chemistry towards the formation of metal complexes in fused helicene species.

Failure mechanism and heat treatment effect of 3D-printed bio-inspired helicoidal CF/PEEK composites

The bio-inspired helicoidal structure has displayed a unique combination of lightweight, high strength, and high impact resistance. This study presents an experimental and numerical investigation of the failure mechanism and heat treatment effect of 3D-printed helicoidal composites by adopting a helicoidal laminate stacking configuration found in the dactyl club in Homarus americanus. The helicoidal specimens were additively manufactured with short carbon fiber reinforced Polyether ether ketone (PEEK) composites and their mechanical performance was characterized through quasi-static three-point bending. The results showed that the flexural strength, flexural modulus, and absorbed energy of the helicoidal composites with the twisted crack pattern were 6.9%, 23.9%, and 5.9% higher than those of the quasi-isotropic composites with the flat crack pattern. Stress analysis was then carried out to investigate the underlying failure mechanism of helicoidal materials. The stress component of σ33 and σ13 in helicoidal specimens showed a milder distribution than that of quasi-isotropic specimens, which plays a major role in the resulting fracture-resistant behavior. Further results showed that the helicoidal specimens under heat treatment of 250 °C over 6 h achieved 24.9% and 20.9% enhancement in flexural strength and modulus. The cracking surface area was increased with extensive translaminar twisted cracks observed with a scanning electron microscope. Experiment results show that heat treatment can exert a significant influence on the crack path of the CF/PEEK helicoidal specimens. Meanwhile, the increased degree of crystallization and crosslinking at the interface could inhibit the initiation of delamination failure.

Biomimetic laminated basalt fiber-reinforced composite with sinusoidally architected helicoidal structure integrating superior mechanical properties and microwave-transmissibility

High-efficient microwave-transmissibility, mechanical durability, and lightweight are in significant demand for wireless communication components. Unfortunately, conventional materials can hardly meet the requirements for emerging radio-frequency (RF) devices so far. Herein, inspired by the sinusoidally architected helicoidal structure of mantis shrimp's dactyl club, a biomimetic laminated basalt fiber-reinforced composite (BL-BFRC) is designed and manufactured through a method combining linear helicoidal layup and hot press forming. Remarkably, the flexural strength and flexural modulus of BL-BFRC are 259.71 MPa and 22.61 GPa, which increase by 28.4% and 36.9% over those of conventional BFRC (202.27 MPa and 16.52 GPa). In addition, BL-BFRC exhibits a dielectric constant of 2.2858 and a loss tangent of 1.55 × 10−3, which are much lower than those of conventional BFRC (3.6459 and 9.73 × 10−3). Accordingly, the microwave-transmissibility increases from 93.53% (conventional BFRC) to 98.34% (BL-BFRC). In fact, the superior mechanical durability of BL-BFRC is mainly attributed to the crack deflection of helicoidal structure and stress dissipation of sinusoidal morphology. And the efficient microwave-transmissibility benefits from dielectric transition effect on non-smooth sinusoidal surface. This work uniquely combines the features of high-efficient microwave-transmissibility and mechanical durability into RF devices and would enable a broad range of wireless communication applications.

The Influence of a High-Voltage Discharge in a Helicoidal Twisted-Pair Structure on Enzyme Adsorption

The effect of a high-voltage discharge in a helicoidal structure on the adsorption properties of an enzyme on mica has been studied with the example of horseradish peroxidase (HRP). The discharge was generated at the expense of a sparkover in a 3 mm gap between two electrodes, to which a 10 kV, 50 Hz AC voltage was applied. The electrodes were connected to a twisted pair, which was wound onto a cone, forming the helicoidal structure. The incubation of the enzyme solution near the top of the helicoidal structure has been found to cause an increase in the degree of aggregation of HRP adsorbed on mica in comparison with the control HRP sample. The results obtained should be taken into account in studies of enzymes using biosensors with helicoidal structures as heating elements, as well as in refining models describing effects of low-frequency alternating current, flowing through helicoidal structures, on proteins and biological objects.

Laser powder bed fusion of mechanically efficient helicoidal structure inspired by mantis shrimp

Abdominal segments are common in crustaceans and are used as biological armor to provide mechanical protection from the surrounding environment. However, little is known about the abdominal segments. Mantis shrimps, a group of crustaceans, evolved abdominal segments that are mechanically robust. Here, we conduct a systematic study of the three-dimensional geometry, material composition, nanoindentation mechanical properties, and microstructural features of mantis shrimp abdominal segments. Inspired by the helicoidally distributed fiber hollow tubes of abdominal segment, a mechanically efficient helicoidal structure is designed and fabricated by laser powder bed fusion (LPBF). The LPBF processed helicoidal structure achieves high forming accuracy (peak deviation = 7.5 μm) and relative density (99.78%). Furthermore, using a combination of compression experiments and finite element simulations, we show that the helicoidal structure exhibits superior specific energy absorption (2.4 times) and plateau stress (2.7 times) to the conventional honeycomb structure and tube structure. The structural optimization results show that the rotation angle and the number of layers have a significant effect on the mechanical properties and the helicoidal structure with a rotation angle of 45° and 3 layers has the best comprehensive energy absorption performance. We demonstrate that the mechanism for this improvement in mechanical properties is the unique twisting stress conduction path of the helicoidal structure under compression. Our results reveal that bio-inspired additive manufacturing paves the way to develop high-performance structures which are in great demand in modern industrial applications.

Neutron diffraction in MnSb2O6 : Magnetic and structural domains in a helicoidal polar magnet with coupled chiralities

${\mathrm{MnSb}}_{2}{\mathrm{O}}_{6}$ is based on the structural chiral $P321$ space group No. 150 where the magnetic ${\mathrm{Mn}}^{2+}$ moments $(S=5/2,$ $L\ensuremath{\approx}0)$ order antiferromagnetically at ${T}_{\mathrm{N}}=12\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Unlike the related iron based langasite $({\mathrm{Ba}}_{3}{\mathrm{NbFe}}_{3}{\mathrm{Si}}_{2}{\mathrm{O}}_{14})$ where the low-temperature magnetism is based on a proper helix characterized by a time-even pseudoscalar ``magnetic'' chirality, the ${\mathrm{Mn}}^{2+}$ ions in ${\mathrm{MnSb}}_{2}{\mathrm{O}}_{6}$ order with a cycloidal structure at low temperatures, described instead by a time-even vector ``magnetic'' polarity. A tilted cycloidal structure has been found [M. Kinoshita et al., Phys. Rev. Lett. 117, 047201 (2016)] to facilitate ferroelectric switching under an applied magnetic field. In this work, we apply polarized and unpolarized neutron diffraction analyzing the magnetic and nuclear structures in ${\mathrm{MnSb}}_{2}{\mathrm{O}}_{6}$ with the aim of understanding this magnetoelectric coupling. We find no evidence for a helicoidal magnetic structure with one of the spin envelope axes tilted away from the cycloidal $c$ axis. However, on the application of a magnetic field $\ensuremath{\parallel}\phantom{\rule{4pt}{0ex}}\mathbit{c}$ the spin rotation plane can be tilted, giving rise to a cycloid---helix admixture that evolves towards a distorted helix (zero cycloidal component) for fields great than $\ensuremath{\approx}2$ T. We propose a mechanism for the previously reported ferroelectric switching based on coupled structural and magnetic chiralities requiring only an imbalance of structural chiral domains.

Multi-scale concurrent analysis for bio-inspired helicoidal CFRP laminates and experimental investigation

The bio-inspired helicoidal structure has been proved to exhibit excellent load-bearing capacity and damage resistance performance. However, there are few relevant researches to analyze its failure process from micro-scale. In this study, three types of carbon fiber reinforced plastics (CFRP) laminate structures were prepared—cross-ply, quasi-isotropic, and helicoidal. The mechanical performance of the laminates was tested using an indentation test. The experiment results verified the excellent mechanical performance of the helicoidal structure. In order to analyze its intrinsic enhancement mechanism, a multi-scale damage model for self-consistent clustering analysis method was proposed, and the damage evolution process of the laminates was simulated at both macro and micro scales. The results indicated that owing to the small rotation angle between adjacent layers in the helicoidal structure, the stress distribution along the thickness direction was more uniform. Moreover, the laminate was more likely to be divided into several sub-laminates by primary delamination. The fibers of each sub-laminate would be cracked in sequence from top to bottom, accompanied by the subprime delamination, resulting in the failure of the laminate. Besides, a twisted crack path could be generated, making the helicoidal structure exhibit excellent load-bearing capacity and damage resistance.

Controlling instabilities of electrified liquid jets via orthogonal perturbations

We present an experimental study on controlling electrified jet instabilities via imposing orthogonal perturbations. A steady helicoidal whipping structure is achieved for the first time in air ambience, which is of significant importance in producing uniform fibers using electrospinning. The flexibility of orthogonal perturbations in output also allows more types of perturbation patterns following Lissajous curves, which demonstrate great potential in applications such as film deposition.

Damage tolerance mechanism of bioinspired hybridization helical composite in lobster homarus americanus

A typical twisted laminar structure was an essential feature for the exoskeleton of the lobster Homarus americanus to resist impact damage and penetration. In fact, two kinds of helical structures were observed in the exoskeleton with different pitch angles and ply thicknesses in the exocuticle and endocuticle. It is worth investigating the evolutionary mechanism of the hybrid helicoidal structure rather than a single helix. In this paper, two pitch angles and ply thickness were introduced to reproduce the hybridization helicoidal structures of crustacean ectoskeletons. Numerical investigations were carried out, including low-velocity impact and compression after impact (CAI). The results showed that the bionic architectures need to balance the protection function provided by the endocuticle and load-carrying performance provided by the exocuticle. In addition, the qualitative relationship between design parameters and performance was analysed. The decrease in angle and the increase in layer thickness will lead to an increase in energy dissipation during the impact process and a decrease in residual strength. The research results provide a reference for the design of hybrid bionic structures with multimechanism cooperation.

Correlation of electronic and vibrational properties with the chiro-optical activity of polyfluorene copolymers

The rationale of this paper is to shed some light on the origin of the optical response of two similar chiral fluorene copolymers in correlation with their vibrational modes, to understand how a chiral center placed in a ramification affects the optical properties of the main chain. Various spectroscopic ellipsometric techniques, in the scope of the Stokes theory were used to characterize the optical-vibrational behavior of the polyfluorenes: ellipsometry in emission (EE), transmission (TE), and Raman (ERS). The results showed that the optical activity and the emission of the circularly polarized light depends substantially on the interaction of the chiral carbon in the ramification and the main chain through specific optically active vibrational modes, for each sample. One interesting achievement was to find the absolute dextrorotatory configuration of the studied molecules, that could induce a helicoidal structure to the entire material.

Surface-field-induced heliconical instability in the cholesteric phase of a mixture of a flexible dimer (CB7CB) and a rodlike molecule (8CB).

In 1968, de Gennes and Meyer independently predicted that a cholesteric phase can form a stable oblique helicoidal (or heliconical) structure provided that K_{3}<K_{2} where K_{3} (K_{2}) is the bend (twist) constant. This structure usually develops under electric field when the material is of positive dielectric anisotropy and was observed for the first time in 2014 by Xiang etal. in a cholesteric phase made of a liquid crystal dimer material (CB7CB) in which K_{3} is anomalously small. Following a recent theoretical prediction by Poy and Žumer, I show that confining a similar cholesteric phase between two glass plates treated for unidirectional anchoring can lead to a similar heliconical instability. In that case, the confinement induces a surface field that acts as an effective electric field E with E≡1/d where d is the sample thickness. The experiment was conducted in a mixture of CB7CB +50 wt% 8CB doped with a small amount of the chiral molecule R811. In addition, I show that this mixture presents an unexpected compensation point near the transition to the N_{TB} phase.

Visualization of Polymer Dynamics in Cellulose Nanocrystal Matrices Using Fluorescence Lifetime Measurements.

Polymer nanocomposites containing self-assembled cellulose nanocrystals (CNCs) are ideal for advanced applications requiring both strength and toughness as the helicoidal structure of the CNCs deflects crack propagation and the polymer matrix dissipates impact energy. However, any adsorbed water layer surrounding the CNCs may compromise the interfacial adhesion between the polymer matrix and the CNCs, thus impacting stress transfer at that interface. Therefore, it is critical to study the role of water at the interface in connecting the polymer dynamics and the resulting mechanical performance of the nanocomposite. Here, we explore the effect of polymer confinement and water content on polymer dynamics in CNC nanocomposites by covalently attaching a fluorogenic water-sensitive dye to poly(diethylene glycol methyl ether methacrylate) (PMEO2MA), to provide insights into the observed mechanical performance. Utilizing fluorescence lifetime imaging microscopy (FLIM), the lifetime of dye fluorescence decay was measured to probe the polymer chain dynamics of PMEO2MA in CNC nanocomposite films. The PMEO2MA chains experienced distinct regions of differing dynamics within Bouligand structures. A correlation was observed between the average fluorescence lifetime and the mechanical performance of CNC films, indicating that polymer chains with high mobility improved the strain and toughness. These studies demonstrated FLIM as a method to investigate polymer dynamics at the nanosecond timescale that can readily be applied to other composite systems.

Cell wall composition determines handedness reversal in helicoidal cellulose architectures of Pollia condensata fruits

Chiral asymmetry is important in a wide variety of disciplines and occurs across length scales. While several natural chiral biomolecules exist only with single handedness, they can produce complex hierarchical structures with opposite chiralities. Understanding how the handedness is transferred from molecular to the macroscopic scales is far from trivial. An intriguing example is the transfer of the handedness of helicoidal organizations of cellulose microfibrils in plant cell walls. These cellulose helicoids produce structural colors if their dimension is comparable to the wavelength of visible light. All previously reported examples of a helicoidal structure in plants are left-handed except, remarkably, in the Pollia condensata fruit; both left- and right-handed helicoidal cell walls are found in neighboring cells of the same tissue. By simultaneously studying optical and mechanical responses of cells with different handednesses, we propose that the chirality of helicoids results from differences in cell wall composition. In detail, here we showed statistical substantiation of three different observations: 1) light reflected from right-handed cells is red shifted compared to light reflected from left-handed cells, 2) right-handed cells occur more rarely than left-handed ones, and 3) right-handed cells are located mainly in regions corresponding to interlocular divisions. Finally, 4) right-handed cells have an average lower elastic modulus compared to left-handed cells of the same color. Our findings, combined with mechanical simulation, suggest that the different chiralities of helicoids in the cell wall may result from different chemical composition, which strengthens previous hypotheses that hemicellulose might mediate the rotations of cellulose microfibrils.

Cholesterol-based nonsymmetric dimers comprising phenyl 4-(benzoyloxy)benzoate core: the occurrence of frustrated phases

ABSTRACT The synthesis and mesomorphism of sixteen new nonsymmetric liquid crystal dimers formed by covalently attaching cholesterol with a three-ring mesogenic core, namely, phenyl 4-(benzoyloxy)benzoate, via an ω-oxyalkanoyl spacer are described. The lengths of the terminal chain attached to the three-ring core, as well as those of the ω-oxyalkanoyl spacer, have been varied; accordingly, they have been grouped into four sets. They display an enantiotropic chiral nematic (N*) behaviour with the isotropization temperature crossing over 200 oC. Four dimers having an odd-parity spacer were found to additionally show blue phase-I/II (BP-I/II), and the smectic A (SmA) phase. Twelve dimers with an even-membered spacer also stabilise either chiral smectic C (SmC*) or twist grain boundary (TGB) phase having SmC* slabs (TGBC* phase). The dimers with longer spacer and terminal tail favour the formation of the TGBC* phase. Thus, the lengths of both the terminal tail and the central spacer are seen to direct the type of mesophase and liquid crystal phase sequences to be formed. X-ray diffraction measurements have been carried out on representative samples to corroborate the structure of the mesophases. The N* phase has been probed with the aid of CD technique to figure out the handedness of the helicoidal structure.

Photonic Cholesteric Liquid-Crystal Elastomers with Reprogrammable Helical Pitch and Handedness.

The unique combination of the rubber-like property and the photonic helicoidal structure of cholesteric liquid-crystal elastomers (CLCEs) results in one-handed circular polarized light reflection, the wavelength of which is dictated by the Bragg relationship. Herein, a highly stretchable mechanochromic photonic CLCE film was fabricated by cross-linking mesogenic oligomers having thiol terminal groups, which further reacted to form disulfide (-S-S-) linkages. The mechanically stretched photonic CLCE film reflected both right- and left-handed circular polarized lights with a blue-shifted color. The helicoidal pitch and handedness controlled by the applied strain were programmed through a dynamic exchange reaction between the -S-S- linkages, thus realizing the patterning at selective regions. The pattern almost vanished under unpolarized daylight but was visible under circularly polarized light when the patterned photonic CLCE film had been heated above its isotropic temperature. The hidden patterns of the heat-treated CLCE film reappeared under unpolarized daylight when stretched, demonstrating a data encryption ability. These patterned photonic elastomers can be uniquely used in sensors, actuators, soft robotics, flexible displays, data encryption, and anticounterfeiting applications with a mechanochromic camouflage response.

Analysis and simulation of fracture behavior in naturally occurring Bouligand structures

Through natural selection processes, refined biological materials could be created that adapt to various environments and exhibit specific functions. Such materials include typical Bouligand structures that can be widely observed in marine creatures that have hard shells. Consisting of a helicoidal arrangement of aligned fibrils, layered single-twisted Bouligand-type structures (SBS) display exceptional fracture and damage resistance. A much more primitive and rarer type of this formation, the double-twisted Bouligand-type structures (DBS), has been discovered in ancient fish scales, and this architecture could provide added rigidity and significantly contribute to toughness when facing fracture risk. In this work, we describe a computational modeling approach to investigating fracture behaviors and toughening mechanisms in Bouligand structures. To achieve qualitative insights into the fracture behaviors of DBS and SBS, we applied these two configurations, which were identified from corresponding biological materials, to analyze load-displacement responses during single edge notched (SEN) tensile testing; the toughening mechanism is also discussed further. The results clearly show that the arrangement of helix fibrils and interlaminar properties play a major role in the resulting fracture behaviors of Bouligand architectures. This is of interest for the future design of engineering materials and structures that require composites with enhanced toughness, and could deepen our understanding of the structure-property relationship of Bouligand-type structures in bionic design. Statement of significanceIn this work, a novel numerical modeling approach based on the extended finite element method (XFEM) has been established to evaluate the fracture behavior of a naturally-occurring Bouligand-type helicoidal structure subjected to the single edge notched (SEN) tensile loading. The roles of the biological features (i.e., layered arrangement of collagen fibrils and interbundle fibrils) on the fracture resistance and toughening mechanism of the Bouligand-type structures have been uncovered and analyzed quantitatively. This is of interest for future design of engineering materials and structures that require composites with enhanced toughness, and can deepen the understanding of the structure-property relationship of the Bouligand-type structure in bionic design.