Helix Pitch Research Articles

Research on design and milling performance of unequal gradient helix and variable pitch end mill

It is difficult to obtain excellent surface quality and dimensional accuracy of the low-rigidity frame and beam parts made by titanium alloy, due to the obvious vibration and large elastic deformation during side milling. The variable helix and pitch cutting tool is usually used to reduce vibration, which does not consider the unequable stiffness in the milling area. Therefore, the concept of Unequal Gradient Helix and Variable Pitch (UGHVP) is proposed in this research. The helix angles of the UGHVP vary with the stiffness of the milling area position, which will lead to milling forces and vibration decreasing as the stiffness weakens. As a result, the surface quality and dimensional accuracy will be increased, of which the advantages are analyzed. The mathematical models of the UGHVP circumferential cutting edges and groove sections are derived, which provide theoretical support for tool grinding. The models are visualized by numerical simulation. Finally, the comparison experiment with the conventional variable helix and pitch end mill was completed by side-milling the designed feature samples. The results showed that the milling forces perpendicular to the milling surface by the UGHVP were smaller and less fluctuating, which could obtain less elastic deformation theoretically. The milling vibration was smoother, especially when cutting in and out. Besides, the larger the axial depth of cut, the more obvious the superior performance of the UGHVP tool. Better surface quality and dimensional accuracy were obtained, which proved the excellent performance of the UGHVP.

Silent piling for offshore jacket foundations in sand: DEM and centrifuge modelling

The race to decarbonise the economy has led to an exponential growth of offshore wind farm developments across the globe. While monopiles are the dominating installed foundations, jacket structures are expected to be more and more common, as wind farms extend to deeper waters [1] and innovative technologies are required to alleviate some important challenges. Firstly, straight shafted piles are not particularly efficient to sustain large tensile demand induced by the push-pull axial loading of the foundations. Secondly, stricter regulations on underwater noise make mitigation methods for pile driving more expensive and silent piling techniques could be used as an alternative [2]. Figure 1(a) describes a new screw pile foundation that meets those two challenges [3]. The foundation is installed by rotary jacking, which avoids any impact related noise. The pile is composed of a large upper shaft, that is designed to resist the lateral load applied by the jacket structure, and a smaller lower shaft which reduces the torque demand during installation. A helix is attached close to the pile tip, to provide an enhanced axial resistance and facilitates the installation of the pile [4]. The main challenge during the pile installation is the very low reaction force that may be available offshore at the pile head, which consists only of the pile self-weight and the installation tool. The aim of this work is to demonstrate that screw piles can be installed for offshore applications by rotary jacking at low reaction force, via (geotechnical centrifuge, [2]) and numerical (DEM, [5]) modelling.
 Figure 1(b) compares the vertical installation force measured in the centrifuge with a DEM simulation, as a function of the advancement ratio (AR), defined as the vertical displacement of the pile during one rotation normalised by the helix pitch. Figure 1(b) shows that at constant installation rate (fixed AR), the lower ARs lead to a tensile force measured at the pile head, i.e. the pile pulls on the installation actuator. The DEM simulation of is in good agreement with the centrifuge result at the same AR and shows that the vertical force trend reverts to a compressive value once the upper part of the shaft enters the ground.
 Figure 1(c) shows the vertical force associated with each pile component during the DEM simulation (AR = 0.5). The greatest penetration resistance (lowest negative value) is the transition piece, reaching -4MN, when the pile tip reaches a depth of 25m. The shaft penetration resistance is small and probably reduced by the rotary movement that changes the inclination of the shear stress along the shaft, as explained in previous publications [6]. The base and helix contributions are largely positive (in tension), as described in Figure 1(c). Consequently, the tension (“thrust”) created by the helix compensates the penetration resistance of the rest of the pile and enables its penetration with a very low reaction force applied at the pile head.
 The effect of the AR on the helix behaviour was previously explained as follows [7, 8]. Helix overflighting (AR< 1) forces a displacement of the sand particles upwards. This forced displacement is opposed by the existing soil, which acts as a non-linear spring, which is progressively compressed. The soil above the helix is then compressed, as depicted in Figure 1(d), which represents the vertical stress in the sand bed and around the pile. Another effect of the pile overflighting is the considerable reduction of the vertical stress under the helix, which facilitates the pile penetration. On the contrary, a large compressive stress under the transition piece is at the origin of its large penetration resistance and its geometry may need to be further optimised.
 In the field, where the pile head condition is a constant reaction force, the AR will simply vary such that the thurst provided by the helix will compensate the pile penetration resistance. Results shown in Figure 1(b) prove that there exists a set of AR between 0.10 and 1.00 such that the necessary force is equal to zero at any depth, thus ensuring the fasibility of its installation at low reaction force, providing a torque of 23MNm can be applied.
 The physical and numerical modelling of a new type of pile have shown that it is a viable option to support jacket structures, as it can be installed without driving and with a very limited reaction force. The DEM simulations have revealed the underlying mechanisms explaining the centrifuge results, while centrifuge results were used to validate the DEM simulations. This project showed that the combination of the two techniques is very powerful to speed up technology development.

Effects of installation advancement ratio on cyclic uplift response of a single-helix screw pile: experimental and numerical investigation in sand

A screw pile consists of one or several helices connected on a central straight shaft or core. These kinds of piles are widely used onshore and typically installed by applying torque at the pile head with additional vertical compressive force (also referred to as crowd force) if required. Current standards and industrial guidelines recommend that installation follows the pitch-matched approach i.e. advancement ratio AR = 1.0 [1], which means that the pile vertical penetration for each rotation equals the helix pitch, because it is suggested that this leads to reduced installation disturbance.
 where is the vertical penetration per rotation and is the helix geometric pitch.
 Recently this kind of pile has been proposed for upscaling from typical onshore sizes as an alternative silent foundation/anchoring solution for future offshore renewable energy application. This may include use as foundations for jacket structures or anchors for wind turbine in deeper water. However, the increase in pile sizes may require prohibitive vertical force if pitch-matched installation is adopted [2] in sand. One of the solutions to this is over-flighting (AR < 1.0) where the pile is over rotated during installation. When the pile is over-flighted, sand below the helix can move upward through the helix opening resulting in a higher stress and potential densified zone above the helix (Figure 1(a)). The increased stress above the helix can push the pile downward and consequently reduce or even eliminate the vertical installation force. In addition, the increased stress and potential densification in the soil above the helix can result in better monotonic uplift performance [3, 4, 5, 6, 7].
 As foundations for offshore renewable energy applications, the screw piles also need to be able to perform under cyclic loading from wind, wave and current for example. Investigations of uplift cyclic performance of pitch-matched screw pile installation have been reported, but the over-flighting effects have not received significant attention. To give confidence in the maintenance of the beneficial effects of over-flighting on cyclic performance, centrifuge tests and discrete element modelling (DEM) were used in this study. The centrifuge tests provide more reliable physical evidence than 1g tests but are less costly and more accessible than field test. Based on the results of centrifuge test, the DEM models are validated and used to allow separate evaluation of the mechanism of the helix and the pile shaft and assessment of soil state evolution during cycling, which are difficult to capture in field or centrifuge tests.
 The centrifuge tests show that the screw piles installed at lower AR accumulate displacement at lower rates with cycling. which is also captured by the DEM (Figure 1(b)). In addition, DEM suggests that shaft behavior controls cyclic performance because the shaft friction resistance is stiffer than the helix bearing resistance at small displacements. The radial stress on the pile shaft can be enhanced by over-flighting installation. Although it degrades with cycling, the enhanced radial stress due to over-flighting remains higher than that of the pitch-matched pile and therefore the over-flighted piles have improved cyclic performance. On the basis of improved cyclic uplift performance and low vertical installation force, over-flighting installation may be utilized for offshore screw piles performing under cyclic uplift loading where installation forces are a concern or loading is predominantly one-way tensile loading.

Thermal storage performance of latent heat thermal energy storage device with helical fin under realistic working conditions

Latent heat thermal energy storage has garnered increasing interest and development as a significant technique for recovering waste heat. In this research, the latent heat thermal energy storage device with helical fin is proposed and its thermal storage performance is also investigated by numerical simulation. First, assorted helix pitches (400 mm, 200 mm, 100 mm and 50 mm) and fin numbers are taken into account to investigate the thermal storage performance with various fin parameters. Then, four types of nanoparticles (SiC, TiO2, CuO and Al2O3) are dispersed into the phase change material to address the inherent defect of poor thermal conductivity of phase change material. Afterwards, several realistic working circumstances, including four inclination angles (θ = 0°, 30°, 60° and 90°) and eight oscillating inlet temperatures among which the frequencies are 1 Hz, 2.5 Hz, 5 Hz and 10 Hz, and amplitudes are 0.25, 0.5, 1 and 2, are selected to explore the impact on melting process. The results indicate that shortening helix pitch and increasing fin number can accelerate the melting rate, while the selection of these two parameters needs to consider the thermal storage performance and the processing difficulty as a whole. Furthermore, the selection of nano phase change materials needs to balance the relationship between thermal storage rate and capacity. Compared to the complete melting time when the device is placed vertically, the complete melting time is reduced by 14.3% when the device is placed horizontally. Utilizing the oscillating inlet temperature reduces the complete melting time by at least 21.2% compared to using the steady inlet temperature. When the frequency value is increased from 1 to 10, the complete melting time is only reduced by 3.4%. Complete melting time of the amplitude of 2 is 12.9% shorter than that of the amplitude of 0.25.

Dirac points, new photonic band gaps, and effect of magnetically induced transparency in dichroic cholesteric liquid crystals with wavelength-dependent magneto-optical activity parameter.

We investigated the properties of cholesteric liquid crystals (CLCs) being in an external static magnetic field directed along the helix axis. We considered a dichroic CLC, that is, CLC with parameters ReΔ=Reɛ_{1}-Reɛ_{2}/2=0 and ImΔ=Imɛ_{1}-Imɛ_{2}/2≠0, where ɛ_{1,2} are the principal values of the local dielectric permittivity tensor. We have shown that in the case of the wavelength dependence of the magneto-optic activity parameter, new features appear in the optics of dichroic CLCs, in particular, in this case new Dirac points appear. Dirac points are points where there is an intersection of any two wave vector curves (they degenerate) and a linear law of the wave vector dependence on the frequency near these points. And moreover, at some Dirac points photonic band gaps (PBGs) appear; at others, lines of magnetically induced transparency (MIT), that is, a full transmission band appears, in an absorbing medium. In this case a polarization-sensitive transmission band appears too. At certain values of the helix pitch of the CLC and of the magnitude of the external magnetic field, three PBGs of different nature appear: a transmittance band, two narrow lines of MIT, and at others a broadband MIT, etc. This system is nonreciprocal, and the nonreciprocity changes over a wide range. It is observed both for reflection and transmittance and for absorption. The soft-matter nature of CLCs and their response to external influences lead to easily tunable multifunctional devices that can find a variety of applications. They can be applied as tunable narrow-band or broadband filters and mirrors, a highly tunable broad/narrow-band coherent perfect absorber, transmitter, ideal optical diode, and in other devices.

Computational fluid dynamics study of gas-liquid mass transfer in concentric tube airlift reactor with helical baffle

This work aims to understand the effect of stationary and rotational helical baffle on gas-liquid mass transfer in concentric tube airlift reactor. A computational fluid dynamics (CFD) method is conducted, and the predicted of overall volumetric mass transfer coefficient (KLa) in concentric tube airlift reactor (0.29 m diameter and 1.5 m initial liquid height) without baffle is validated reasonable agreement with the empirical equations reported in the literatures. The results were simulated basing on this validated model. It indicated that the helical baffle with the same width of riser (40 mm) could increase the KLa by 132∼202% at the superficial gas velocity of 0.00212 m·s−1 when compared with that the airlift reactor without baffle. At the width of helical baffle less than width of riser, the KLa decreased with the decreasing of the width (W) and the rotational speed (n) of helical baffle. When the width of stationary helical baffle is 30 mm, the effect of helical baffle on enhancing mass transfer is negligible. In addition, the KLa decreased with the increasing of the helix pitch (H) of helical baffle and the distance of the lower edge of helical baffle from the bottom of reactor (L).

Three-dimensional finite element analysis for uplifting independent helix plate anchor in uniform clay

Three-dimensional finite element analysis offers a more realistic representation of the geometry of the helix plate anchor. For the helical anchor with a single helix plate or multiple helix plates whose spacing distance is large between each plate, each helix plate behaves independently during uplifting. Although numerous studies have estimated the uplift capacity factor of independent circular plate anchors in uniform clay, rare research comprehensively considers the effects of helix pitch, anchor shaft, and soil rigidity on estimating the uplift capacity factor for independent helix plate anchors. In this paper, three-dimensional finite element analyses were conducted to investigate the impacts of helix pitch, anchor shaft, and soil rigidity on estimating the uplift capacity factor of independent helix plate anchors with shafts in uniform clay. Analysis results demonstrated that the increasing helix pitch and anchor shaft diameter could significantly decrease the uplift capacity factor. The change in the soil rigidity also influenced uplift capacity factor estimation. Therefore, determining the uplift capacity factor should comprehensively account for the effects of embedment depth, helix pitch, anchor shaft, soil rigidity, and overburden pressure. In this study, a procedure was suggested for creating design guidelines to estimate uplift capacity factors for independent helix plate anchors.

A cell wall-localized β-1,3-glucanase promotes fiber cell elongation and secondary cell wall deposition.

β-1,3-glucanase functions in plant physiological and developmental processes. However, how β-1,3-glucanase participates in cell wall development remains largely unknown. Here, we answered this question by examining the role of GhGLU18, a β-1,3-glucanase, in cotton (Gossypium hirsutum) fibers, in which the content of β-1,3-glucan changes dynamically from 10% of the cell wall mass at the onset of secondary wall deposition to <1% at maturation. GhGLU18 was specifically expressed in cotton fiber with higher expression in late fiber elongation and secondary cell wall (SCW) synthesis stages. GhGLU18 largely localized to the cell wall and was able to hydrolyze β-1,3-glucan in vitro. Overexpression of GhGLU18 promoted polysaccharide accumulation, cell wall reconstruction, and cellulose synthesis, which led to increased fiber length and strength with thicker cell walls and shorter pitch of the fiber helix. However, GhGLU18-suppressed cotton resulted in opposite phenotypes. Additionally, GhGLU18 was directly activated by GhFSN1 (fiber SCW-related NAC1), a NAC transcription factor reported previously as the master regulator in SCW formation during fiber development. Our results demonstrate that cell wall-localized GhGLU18 promotes fiber elongation and SCW thickening by degrading callose and enhancing polysaccharide metabolism and cell wall synthesis.

Ferrielectric Liquid Crystal with a Subwavelength Helix Pitch As an Electro-Optic Medium for Phase Spatial Light Modulators

Ferrielectric Liquid Crystal with a Subwavelength Helix Pitch As an Electro-Optic Medium for Phase Spatial Light Modulators

CPT-based design method for helical piles in sand

Helical piles are used extensively for low and medium rise developments and are a popular foundation solution in cases where resistance to significant uplift loads is required. The ability to re-use helical piles and recent interest in their use as foundations in offshore wind and solar energy applications has renewed interest in improving existing design methods. This paper presents the results from field investigations in medium dense and dense sand that examine effects on the axial tension and compression capacity of varying the helix pitch, shaft diameter, advancement ratio and shaft tip geometry. The results from the 20 pile tests conducted (many of which included instrumentation) indicate low sensitivity to a range of the investigated parameters and a strong correlation of the capacity to the cone penetration test resistance. A design method based on these findings is proposed for typical helical pile geometries and is shown to provide predictions that are generally within 10 to 15% of the axial capacities reported for 30 other well documented pile tests reported in the literature. The method, referred to as UWASP-22, incorporates a simple means of estimating the load-displacement response of a helical pile as well as a formulation enabling prediction of the installation torque which is a common quality control measure for helical piles.

Dual-Function Smart Windows Using Polymer Stabilized Cholesteric Liquid Crystal Driven with Interdigitated Electrodes.

In this study, we present an electrically switchable window that can dynamically transmit both visible light and infrared (IR) light. The window is based on polymer stabilized cholesteric liquid crystals (PSCLCs), which are placed between a top plate electrode substrate and a bottom interdigitated electrode substrate. By applying a vertical alternating current electric field between the top and bottom substrates, the transmittance of the entire visible light can be adjusted. The cholesteric liquid crystals (CLC) texture will switch to a scattering focal conic state. The corresponding transmittance decreases from 90% to less than 15% in the whole visible region. The reflection bandwidth in the IR region can be tuned by applying an in-plane interdigital direct current (DC) electric field. The non-uniform distribution of the in-plane electric field will lead to helix pitch distortion of the CLC, resulting in a broadband reflection. The IR reflection bandwidth can be dynamically adjusted from 158 to 478 nm. The electric field strength can be varied to regulate both the transmittance in the visible range and the IR reflection bandwidth. After removing the electric field, both features can be restored to their initial states. This appealing feature of the window enables on-demand indoor light and heat management, making it a promising addition to the current smart windows available. This technology has considerable potential for practical applications in green buildings and automobiles.

Ribonucleic acid genome mutations induced by the Casimir effect

In this paper, we investigate the Casimir effect within a virus RNA, particularizing the study to the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Then, we discuss the possibility of occurring damage or mutation in its genome due to the presence of quantum vacuum fluctuations inside and around the RNA ribbon. For this, we consider the geometry and the nontrivial topology of the viral RNA as having a simple helical structure. We initially compute the non-thermal Casimir energy associated to that geometry, considering boundary conditions that constrain the zero point oscillations of a massless scalar field to the cylindrical cavity containing a helix pitch of RNA ribbon. Then we extend the obtained result to the electromagnetic field and, following, we calculate the probability of occurring damage or mutation in RNA by using the normalized inverse exponential distribution, which suppresses very low energies, and consider cutoff (threshold) energies corresponding to UV-A and UV-C rays, surely responsible by mutations. Then, by taking into account UV-A, we arrive at a mutation rate per base per infection cycle, which in the case of the SARS-CoV-2 is non-negligible. We find a maximum value of this mutation rate for an RNA ribbon radius, applying it for SARS-CoV-2, in particular. We also calculate a characteristic longitudinal oscillation frequency for the helix pitch value corresponding to the local minimum of the Casimir energy. Finally, we consider thermal fluctuations of classical and quantum nature and show that the corresponding probability of mutation is completely negligible for that virus. Therefore, we conclude that only the nontrivial topology and the geometric attributes of the RNA molecule contribute to the possible mutations caused by quantum vacuum fluctuations in the viral genome.

Conformational and Supramolecular Aspects in Chirality of Flexible Camphor-Containing Schiff Base as an Inducer of Helical Liquid Crystals.

The experimental and theoretical study of influence of the conformational state and association on the chirality of the stereochemically nonrigid biologically active bis-camphorolidenpropylenediamine (CPDA) and its ability to induce the helical mesophase of alkoxycyanobiphenyls liquid-crystalline binary mixture was carried out. On the basis of quantum-chemical simulation of the CPDA structure, four relatively stable conformers were detected. A comparison of the calculated and experimental electronic circular dichroism (ECD) and 1H, 13C, 15N NMR spectra, as well as specific optical rotation and dipole moments, allowed to establish the most probable trans-gauche conformational state (tg) of dicamphorodiimine and CPDA dimer with a predominantly mutually parallel arrangement of molecular dipoles. The induction of helical phases in LC mixtures based on cyanobiphenyls and bis-camphorolidenpropylenediamine was studied by polarization microscopy. The clearance temperatures and the helix pitch of the mesophases were measured. The helical twisting power (HTP) was calculated. The decrease in HTP with increasing dopant concentration was shown to be connected with the CPDA association process in the LC phase. The effect of camphor-containing chiral dopants of various structures on nematic LCs was compared. The values of the permittivity and birefringence components of the CPDA solutions in CB-2 were measured experimentally. A strong effect of this dopant on the anisotropic physical properties of the induced chiral nematic was established. A significant decrease in the dielectric anisotropy was associated with the 3D compensation of the LC dipoles during the formation of the helix.

Acoplanarity, Aromaticity, Chirality, and Helical Twisting Power of Chlorin e6 13(N)-Methylamide-15,17-dimethyl Ester Complexes: Effect of a Metal

The experimental and theoretical study of the influence of metal complexing on geometry, aromaticity, chirality, and the ability to twist the nematic phase by complexes based on modified natural chlorin e6 was carried out. The geometry optimization of the chlorin e6 13(N)-methylamide-15,17-dimethyl ester (MADMECl) and its Zn, Cu, and Ni complexes by DFT (CAM-B3LYP/6–31 G(d,p) functional) method was performed. Based on these calculations, the acoplanarity degree of the macrocyclic ligand and the distortion energy of its dianion were estimated, which allowed the arrangement of the MADMECl complexes in the series Ni &gt; Cu &gt; Zn. Aromaticity was evaluated using the NICS criterion (nuclear independent chemical shift). An increase in the degree of aromaticity of the macrocycle upon complex formation was established. At the same time, the aromaticity of the inner conjugation contour corresponds to the same series as the acoplanarity, while the outer π-delocalization is characterized by the reverse sequence. An experimental evaluation of the electron circular dichroism of the Soret and the Q-bands, as well as the g-factor of dissymmetry, was carried out. The growth of these parameters with an increase in the degree of acoplanarity and aromaticity of the internal conjugation contour was determined. The induction of helical phases in mixtures of nematic liquid crystals (LCs) based on cyanobiphenyls and MADMECl macrocyclic metal complexes was studied by polarization microscopy, and the clearance temperatures and helix pitch of the mesophases were measured. A strong effect of the metal on the phase transition temperature and helical twisting power was established.

Fluorinated Biphenylpyrimidine as a Possible Matrix for Ferroelectric Liquid Crystal Mixtures

The results of the synthesis and study of a new fluorinated achiral smectic C liquid crystal, namely, 2-(3-fluoro-4'-pentylbiphenyl-4-yl)-5-hexylpyrimidine, are presented. The possibility to use this compound as a matrix for ferroelectric liquid crystal (FLC) mixtures is evaluated. Two mixtures have been prepared: one (FLC-691-F) is based on new fluorinated compound and another (FLC-691) on its non-fluorinated analogue. The diester of optically active 2-octanol and terphenyldicarboxylic acid was used as a chiral dopant for both mixtures. Investigation of these FLC mixtures parameters: phase transition temperatures, helix pitch, spontaneous polarization and tilt angle show significant influence of fluorine atom. When comparing the phase sequence of FLC-691 and FLC-691-F mixtures, one can note that in the fluorine containing mixture an additional smectic A phase arises. At the same temperatures, the helix pitch of FLC-691-F is 2 – 2.5 times less than the helix pitch of FLC-691. The tilt angle of FLC-691-F is much smaller than that of FLC-691 but its value approaches the optimal value of 22.5 degrees, which ensures maximum light transmittance of electro-optical cells. At the same time the presence of fluorine atom in the matrix is a reason for the decrease of the mixture spontaneous polarization and a significant decrease (almost 4 times) in the driving voltage of electro-optical light shutters. As a result, it was shown that the fluorinated matrix provides new possibilities for controlling the mixture parameters.

Complex Mode Dispersion Characteristics of Dielectric Loaded Radially Thick Helix

In this article, a generalized analytical and computational numerical theory, for both the guided and leaky modes, has been developed to investigate the dispersive behavior of dielectric-loaded radially thick helix (DLRTH). The complex root finding Muller iterative algorithm is used to compute the normalized phase and attenuation constants. A unique way has been adopted for the classification of HE and EH hybrid modes present in the proposed structure, and based on that, for the first time, we report that this structure shows physical leaky and trapped surface wave propagation characteristics. Furthermore, in the guided mode, it is seen that both slow wave (SW) and backward wave (BW) exist due to the inherently skewed boundary of the helical geometry. The parametric study of helix radius and pitch angle as well as dielectric constant gives optimized parameter values, which help in designing the proposed structure in the desired mode and at the desired frequencies. Moreover, it is reported that pitch angle aids in controlling and figuring out the leaky mode bandwidth. The analytical cum numerical MATLAB results are successfully verified using CST Microwave Studio computational analysis and are found to be in good agreement. The improved characteristic equations for guided mode find practical applications in the design and performance evaluation of the SW structure (SWS) for the phase delay applications. The leaky mode will find application for beam scanning helical antennas.

Common Trends of Chiral Induced Spin Selectivity and Optical Dichroism with Varying Helix Pitch: A First‐Principles Study

AbstractElectrons ransported through a chiral molecule become spin−polarized; this phenomenon is known as chiral induced spin selectivity (CISS). It has implications for spintronics, for electrochemical and enantioselective reactions, and for electron transfer in biological systems. The CISS‐induced spin polarization in simulations and in experiment differs by orders of magnitude, and the detailed underlying mechanism is still an open question. Structure−property relationships can help elucidate this question. For this purpose, the effect of helix pitch is studied for a model helix of 20 carbon atoms for two quantities which have been found to correlate in some experiments: spin‐polarization in transmitted electrons and electronic circular dichroism (ECD). We find that even though the chirality of these model helices goes down with increased pitch, ECD and CISS go up, along with ultraviolet−visible (UV/Vis) spectra and magnetic and electric transition dipole moments. Orbital contributions to the most intense UV/Vis transition do not show a consistent qualitative picture. Tentatively, we can assign the increase in these properties to an increase of the electric polarizability with spatial extension of these helices by changing pitch.

Responsive Chiral Photonic Cellulose Nanocrystal Materials

AbstractResponsive photonic crystals have attracted significant attention in fundamental scientific research and technological applications. Upon applying external stimuli, such as mechanical, electrical, magnetic, and optical triggers, the optical properties of photonic crystals can be actively tuned. Among a large number of photonic crystals, cellulose nanocrystals (CNCs) are considered one of the most promising materials due to their renewability, simplicity of preparation, and unprecedented chirality. A comprehensive overview of the fundamental design principles, methodologies, responsive mechanisms, and practical applications of responsive chiral photonic CNC materials is presented here. The changes in helix pitch, refractive index, orientation, and resulting structural and optical properties of chiral photonic CNCs in response to various stimuli are discussed. Thereafter, novel applications of responsive chiral photonic CNCs, such as colorimetric sensors, photonic actuators, rewritable photonic papers, and anti‐counterfeiting smart tags are described. We conclude by discussing future challenges and opportunities for developing high‐performance responsive chiral photonic CNC materials.

Nature-inspired construction of iridescent CNC/Nano-lignin films for UV resistance and ultra-fast humidity response

Exploration of functional materials based on sustainable and renewable biomolecules has been of much interest. Herein, nature-inspired photonic films were proposed by incorporation of bio-based lignin nanoparticles (LNPs) into chiral nematic cellulose crystals (CNCs). Evaporation induced self-assembly (EISA) formed oriented and layered structure of the nanocomposites iridescent films with enlarged helix pitches by intercalation of higher amounts of LNPs. Decreased crystallite sizes and expanding layer gaps indicated the homogeneous distribution and hydrophobic interactions between CNCs and LNPs. Distinguished UV absorption capabilities with over 90 % shielding capabilities in UVB region and increased hydrophobicity with the contact angle of 75° were achieved for the composite films due to the presence of hydrophobic lignin. The proposed optical films also showed outstanding cytocompatibility owing to all-natural components introduced into the materials, which may display great potentials in many fields such as stimuli sensing, anti-counterfeiting and wearable devices.

Purcell factors and Förster-resonance energy transfer in proximity to helical structures

Both spontaneous emission and resonant energy transfer can be enhanced significantly when the emitter is placed in the vicinity of metallic or crystal structures. This enhancement can be described using the electromagnetic Green tensor and is determined by the dominant surface modes of the structure. Here we use the eigenpermittivity formalism to derive the spontaneous emission and FRET rates in the quasistatic regime in a two-constituent medium with an anisotropic inclusion. We then apply our results to a helical structure supporting synchronous vibrations and evaluate the contribution of these modes, which are associated with a strong and delocalized response. We show that this contribution can result in large Purcell factors and long-range FRET, which oscillates with the helix pitch. These findings may have implications in understanding and controlling the interactions of molecules close to helical structures such as the microtubules.