Small Angles Research Articles

Does the contact angle hysteresis control the droplet shapes on cylindrical fibers?

HypothesisEquilibrium droplets on cylindrical fibers are divided into two classes: barreled and clamshell droplets. In the barreled droplet, the liquid body fully envelops the fiber, and the drop forms two boundary contact lines. In the clamshell droplets, some fiber surface under the liquid body remains dry and only one boundary contact line exists. So far, the transition from one shape to the other was predicted by ignoring the contact angle hysteresis. A new series of experiments using drop-on-demand technology were conducted to analyze the shape of droplets. The existing theory cannot explain the obtained data. We hypothesized that the morphological clamshell-barrel transition of droplets significantly depends on the method of drop formation and is largely controlled by the contact angle hysteresis. ExperimentTo test this hypothesis, we investigated two scenarios of drop formation. In the first scenario, the hexadecane drop was growing on the fiber by printing smaller drops on it. In the second scenario, the drop was formed spontaneously from a coating glycerol film due to the Plateau-Rayleigh instability. To obtain a range of contact angles, a set of different silanes were adsorbed on the fiber surfaces. FindingsThe results showed that for small contact angles (<40°) and small contact angle hysteresis, both methods of drop formation led to the same theoretically explainable conditions for clamshell-barrel transition. As the contact angle increases and hysteresis becomes appreciable, the conditions for clamshell-barrel transition become significantly dependent on the method of drop formation. We discovered that no barreled droplets exist for contact angles greater than 60°. The experimental results were documented in a set of diagrams describing the clamshell-barrel transitions. These diagrams can be used in different engineering applications.

Complex Strain Scapes in Reconstructed Transition-Metal Dichalcogenide Moiré Superlattices.

We investigate the intrinsic strain associated with the coupling of twisted MoS2/MoSe2 heterobilayers by combining experiments and molecular dynamics simulations. Our study reveals that small twist angles (between 0 and 2°) give rise to considerable atomic reconstructions, large moiré periodicities, and high levels of local strain (with an average value of ∼1%). Moreover, the formation of moiré superlattices is assisted by specific reconstructions of stacking domains. This process leads to a complex strain distribution characterized by a combined deformation state of uniaxial, biaxial, and shear components. Lattice reconstruction is hindered with larger twist angles (>10°) that produce moiré patterns of small periodicity and negligible strains. Polarization-dependent Raman experiments also evidence the presence of an intricate strain distribution in heterobilayers with near-0° twist angles through the splitting of the E2g1 mode of the top (MoS2) layer due to atomic reconstruction. Detailed analyses of moiré patterns measured by AFM unveil varying degrees of anisotropy in the moiré superlattices due to the heterostrain induced during the stacking of monolayers.

Effect of Process Parameters on the Microstructure and Properties of Cu–Cr–Nb–Ti Alloy Manufactured by Selective Laser Melting

The fabrication of high-performance copper alloys by selective laser melting (SLM) is challenging, and establishing relationships between the process parameters and microstructures is necessary. In this study, Cu-Cr-Nb-Ti alloy is manufactured by SLM, and the microstructures of the alloy are investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), and electron backscatter diffraction (EBSD). The effects of processing parameters such as laser power and scanning speed on the relative density, defects, microstructures, mechanical properties, and electrical conductivity of the Cu-Cr-Nb-Ti alloy are studied. The optimal processing window for fabricating Cu-Cr-Nb-Ti alloy by SLM is determined. Face-centered cubic (FCC) Cu diffraction peaks shifting to small angles are observed, and there are no diffraction peaks related to the second phase. The grains of XY planes have a bimodal distribution with an average grain size of 24-55 μm. Fine second phases with sizes of less than 50 nm are obtained. The microhardness, tensile strength, and elongation of the Cu-Cr-Nb-Ti alloy manufactured using the optimum processing parameters, laser power of 325 W and scanning speed of 800 mm/s, are 139 HV0.2, 416 MPa, and 27.8%, respectively, and the electrical conductivity is 15.6% IACS (International Annealed Copper Standard). This study provides a feasible scheme for preparing copper alloys with excellent performance and complex geometries.

Crystal and morphology development in immiscible nonpolar polymer blend nanocomposite based on low‐density polyethylene and linear low‐density polyethylene in presence of clay and compatibilizer

AbstractIn this work, polyolefin‐blend/clay nanocomposites based on low‐density polyethylene (LDPE), linear low‐density polyethylene (LLDPE), and organically modified clay (OC) were prepared by melt extrusion. Various grades of maleic anhydride (MA) grafted polyethylene (PE‐g‐MA) were used and examined as compatibilizers in these nanocomposites. Differential scanning calorimetry analysis showed that OC and compatibilizer affect the crystallization behavior of LDPE/LLDPE with different mechanisms. Thermodynamic calculations of wetting coefficient based on interfacial energy between OC, LD, and LL, Morphological characterization based on field emission scanning electron microscopy, X‐ray diffraction, small angles X‐ray scattering, and dynamic rheology measurements revealed that the compatibilizer and OC were localized at the interface of LDPE and LLDPE phases with a preferred tendency toward one phase. Results demonstrated that at a specific amount of OC, there is an optimum compatibilizer concentration to achieve nanodispersed OC and beyond that the compatibilizer causes a structural change in the polymer crystalline morphology. It was also found that the tensile property enhancement of LDPE/LLDPE/OC nanocomposites is closely related to the crystalline structure development made by incorporation of both OC and compatibilizer.

Effect of support structures and surface angles on near-surface porosity in laser powder bed fusion

Additive manufacturing (AM) provides tremendous design freedom, but also introduces many sources of defects. Use of AM in safety-critical parts requires an excellent understanding of these defects and their potential impacts. The most common defects studied in laser powder bed fusion (LPBF) are pores. However, most work examines average porosity in the sample bulk while failures commonly originate near the surface where stresses are often highest. This study measures the porosity in 316 SS samples manufactured by LPBF as a function of distance from the down facing surface using various scan parameters, support spacing values, and build angles. These results show that the surface porosity is often much higher than bulk porosity. While parts with small build angles (5°) reach low porosity (<1 %) very quickly (<50–100 μm), at larger build angles porosity commonly remain >1 % at depths >400 μm. The effects of build angle are exacerbated by close support spacing values. These results show the importance of considering near surface porosity rather than just bulk values. These results provide valuable guidance in selecting build conditions and finishing requirements for PBF parts.

Evaluating the impact of the center control touch screen of new energy vehicles on user visual imagery and preferences

In recent years, with the changes in the international energy situation, new energy vehicles (NEVs) have become an essential choice for people to travel. The center control touch screen (CCTS) of NEVs is an important part of the user's interaction with the vehicle, which is quite different from a conventional fuel vehicle. Therefore, it is necessary to conduct research on the CCTS of NVEs. This study integrates multiple research methods to investigate the visual imagery and user preferences for CCTSs in NEVs. The results of the study show that the horizontal version with small rounded angles without a border is the optimal design solution for the visual imagery dimension, whereas the vertical version with large rounded angles and a border is the worst design solution in terms of the visual imagery dimension. The design without a border can increase users' preference and purchase intention to a certain extent. The results of the above study can effectively reflect the potential user demand for NEVs’ CCTSs, which also has positive implications for enhancing the overall user experience of NEVs.

The divergent angle of cavitational devices influences melanoidin degradation. A new look through experimental and computational analysis

In the present work, the influence of the divergent section angle of hydrodynamic cavitation (HC) devices on melanoidin degradation was investigated. The HC reactor was operated alone and in combination with an oxidant at four concentrations of H2O2: 0.9, 2.8, 4.7, 6.6 g L−1. Five Venturi devices (with divergent angles of 4°, 8°, 11°, 14°, and 90°) were made and analyzed at five operating pressures (150, 300, 440, 600, 760 kPa) for a period of 30 min. ANOVA and Fisher's LSD method were used to define the best angle and the best operating pressure for melanoidin degradation. The highest efficiency of melanoidin degradation (14.32 %) was obtained with an angle of 11° and a pressure of 600 kPa. Computational Fluid Dynamics (CFD) simulations were performed to study the influence of the divergent angle on the flow parameters. They showed that at smaller angles (4° and 8°) there was an excess growth of cavities, increasing the chance of coalescence of cavities and reducing the intensity of collapse. On the other hand, at a very high angle (90°), the cavities collapsed before growing, releasing less energy at the time of implosion. The degradation efficiency increased to 60.84 % when 2.8 g L−1 of H2O2 was applied to the HC reactor, at pH 2.5. Higher concentrations of H2O2 reduced the degradation efficiency, indicating a “scavenging” effect in combined systems (HC + H2O2) due to the generation of unreactive radicals. The present study demonstrated that the combination of HC + H2O2 is a promising alternative for melanoidin degradation, with the divergent angle of the device being an important parameter of this treatment process.

Investigations of nonlinear polarization transfer between obliquely intersecting beams

Electromagnetic waves propagating through plasma can interact nonlinearly through a variety of different mechanisms. The excitation of a plasma beat wave (ions or electrons) can create a refractive index modulation that changes the dispersion of the interacting beams. Alternatively, high-intensity beams can enter the regime where relativistic nonlinearities influence the propagation dynamics. In recent studies [Opt. Express 29, 1162 (2021)OPEXFF1094-408710.1364/OE.413064], it was proposed that two beams propagating along the same axis can exchange their polarization state due to nonlinear interaction. Here we present a numerical analysis of two laser beams intersecting in a nonlinear medium at varying angles. Polarization transfer is observed as predicted by analytical theory for a range of angles. For small angles, it is found that filamentation of the interacting beams becomes important. Analytical estimates of the filamentation threshold are presented, and good agreement is found with the simulation data.

Повышение пространственного разрешения цифровых голограмм с помощью одномерного субпиксельного сканирования

The article considers the issue of increasing the spatial resolution during the registration of digital holograms. In practice, discretization is carried out by measuring signal samples using sensors with a certain finite aperture. The resolution is determined by the size of the aperture area over which the averaging takes place. The resolution enhancement method is based on the sampling equation for signals obtained by the subpixel shift method using generalized functions. A subpixel shift is carried out using a spatial shift by a value less than the element of resolution. Apertures of various shapes are used, for example, elliptical, diamond-shaped, hexagonal, but rectangular apertures are most commonly used. The discretization equation involves the use of a two-dimensional aperture function. Below is a way to increase the resolution when using a one-dimensional function. This can be done based on the structure of the holographic signal. In this case, you can use a subpixel shift in only one direction. Digital holography is distinguished by the fact that photodetector matrices which have a spatial resolution much lower than photographic media used in traditional holography methods are used to register the signal,. Therefore, digital holography uses optical schemes with small angles between interfering beams. However, certain restrictions are imposed on the shape of the objects under study. If the objects have a shape that is significantly different from a flat one, it is necessary to use traditional schemes. The article presents mathematical modeling of the resolution enhancement method based on the recovery of signals from a real hologram obtained in the usual way. The increase in resolution is achieved using a one-dimensional subpixel shift. The use of a one-dimensional subpixel shift makes it possible to significantly simplify the optical scheme of the holographic setup.

Excitons in mesoscopically reconstructed moiré heterostructures

Moiré effects in vertical stacks of two-dimensional crystals give rise to new quantum materials with rich transport and optical phenomena that originate from modulations of atomic registries within moiré supercells. Due to finite elasticity, however, the superlattices can transform from moiré-type to periodically reconstructed patterns. Here we expand the notion of such nanoscale lattice reconstruction to the mesoscopic scale of laterally extended samples and demonstrate rich consequences in optical studies of excitons in MoSe2–WSe2 heterostructures with parallel and antiparallel alignments. Our results provide a unified perspective on moiré excitons in near-commensurate semiconductor heterostructures with small twist angles by identifying domains with exciton properties of distinct effective dimensionality, and establish mesoscopic reconstruction as a compelling feature of real samples and devices with inherent finite size effects and disorder. Generalized to stacks of other two-dimensional materials, this notion of mesoscale domain formation with emergent topological defects and percolation networks will instructively expand the understanding of fundamental electronic, optical and magnetic properties of van der Waals heterostructures.

The Influence of Tool Geometry Parameters on Thermo-Mechanical Loads and Residual Stresses Induced by Orthogonal Cutting of AA6061-T6: A Numerical Investigation

The residual stresses state that a mechanical part obtained after machining is a crucial factor that impacts its in-service performance. This stress state is influenced by the thermomechanical loads exerted on the parts during the machining process, which are, in turn, determined by the tool parameters, process, and machining conditions. The aim of the present research was to anticipate how the cutting tool’s edge radius, rake angle, and clearance angle would affect the forces, temperature, and residual stresses induced while orthogonally cutting aluminum AA6061-T6. To achieve this, two-dimensional DEFORM™ software was utilized to develop a finite element model. The residual stresses trend results obtained demonstrated that rake angles of 0° and 17.5–20° values with a small edge radius (5 to 10 µm) and clearance angles of 7 and 17.5° values gave higher compressive stresses. The obtained simulated results were in good agreement with the experiments. The cutting forces, the temperature, and the maximum and minimum machining-induced residual stresses were found to be influenced more by the tool edge radius and the tool rake angle. The influence of the clearance angles on the above-mentioned machining responses was the least. Residual stresses can have a significant impact on the in-service performance of machined parts. The obtained results will help engineers select or design tools that promote a desired surface integrity during machining. This task is not obvious in practice because of difficulties in measuring residual stresses and also because the machining parameters and the tool geometry parameters have different and opposite impacts on thermo-mechanical loads, productivity, and on machining induced residual stresses.

Molecular Dynamics Simulation Study on the Influence of the Abrasive Flow Process on the Cutting of Iron-Carbon Alloys (α-Fe)

The plastic deformation behavior and microstructural changes in workpieces during ultra-precision machining have piqued the interest of many researchers. In this study, a molecular dynamics simulation of nano-cutting iron-carbon alloy (α-Fe) is established to investigate the effects of the fluid medium and cutting angle on workpiece temperature, friction coefficient, workpiece surface morphology, and dislocation evolution by constructing a molecular model of C12H26 as a fluid medium in the liquid phase using an innovative combined atomic approach. It is demonstrated that the presence of the fluid phase reduces the machining temperature and the friction coefficient. The cutting angle has a significant impact on the formation of the workpiece's surface profile and the manner in which the workpiece's atoms are displaced. When the cutting angle is 0°, 5°, or 10°, the workpiece's surface morphology flows to both sides in a 45° direction, and the height of atomic accumulation on the workpiece surface gradually decreases while the area of displacement changes increases. The depth of cut increases as the cutting angle increases, causing greater material damage, and the presence of a fluid medium reduces this behavior. A dislocation reaction network is formed by the presence of more single and double-branched structures within the workpiece during the cutting process. The presence of a fluid medium during large-angle cutting reduces the number of dislocations and the total dislocation length. The total length of dislocations inside the workpiece is shorter for small angles of cutting, but the effect of the fluid medium is not very pronounced. Therefore, small cutting angles and the presence of fluid media reduce the formation of defective structures within the workpiece and ensure the machining quality.

Multislice forward modeling of coherent surface scattering imaging on surface and interfacial structures.

To study nanostructures on substrates, surface-sensitive reflection-geometry scattering techniques such as grazing incident small angle X-ray scattering are commonly used to yield an averaged statistical structural information of the surface sample. Grazing incidence geometry can probe the absolute three-dimensional structural morphology of the sample if a highly coherent beam is used. Coherent surface scattering imaging (CSSI) is a powerful yet non-invasive technique similar to coherent X-ray diffractive imaging (CDI) but performed at small angles and grazing-incidence reflection geometry. A challenge with CSSI is that conventional CDI reconstruction techniques cannot be directly applied to CSSI because the Fourier-transform-based forward models cannot reproduce the dynamical scattering phenomenon near the critical angle of total external reflection of the substrate-supported samples. To overcome this challenge, we have developed a multislice forward model which can successfully simulate the dynamical or multi-beam scattering generated from surface structures and the underlying substrate. The forward model is also demonstrated to be able to reconstruct an elongated 3D pattern from a single shot scattering image in the CSSI geometry through fast-performing CUDA-assisted PyTorch optimization with automatic differentiation.

Moiré Enhanced Potentials in Twisted Transition Metal Dichalcogenide Trilayers Homostructures.

The exploration of moiré superlatticesholds promising potential to uncovernovel quantum phenomena emerging from the interplay of atomic structure and electronic correlation . However, the impact of the moiré potential modulation on the number of twisted layers has yet to be experimentally explored. Here, this work synthesizes a twisted WSe2 homotrilayer using a dry-transfer method and investigates the enhancement of the moiré potential with increasing number of twisted layers. The results of the study reveal the presence of multiple exciton resonances with positive or negative circularly polarized emission in the WSe2 homostructure with small twist angles, which are attributed to the excitonic ground and excited states confined to the moiré potential. The distinct g-factor observed in the magneto-optical spectroscopy is also shown to be a result of the confinement of the exciton in the moiré potential. The moiré potential depths of the twisted bilayer and trilayer homostructures are found to be 111 and 212 meV, respectively, an increase of 91% from the bilayer structure. These findings demonstrate that the depth of the moiré potential can be manipulated by adjusting the number of stacked layers, providing a promising avenue for exploration into highly correlated quantum phenomena.

Path Configuration Complexity Affects Spatial Memory Span on the eCorsi Task but Does Not Influence Performance of a Concurrent Auditory Discrimination Task.

Visuospatial working memory is often assessed using the Corsi block-tapping task where set size is used to estimate capacity. It is well established that characteristics of the Corsi task path configuration such as length, crossings, and angles influence recall accuracy suggesting that more complex path configurations increase the load on working memory. However, the interaction between set size and path configuration is not well understood. Here we used a secondary auditory task to probe if set size and path configuration impose a similar type of load on the system. Nineteen participants (age = 25.3 ± 3.9 years) performed a computerized version of the Corsi test either alone (single) or simultaneously with an auditory tone discrimination task (dual). The eCorsi task involved a set of simple (no crosses, shorter lengths, larger angles) or complex (>2 crosses, longer lengths, smaller angles) paths at set sizes of five to eight blocks. Results showed significantly lower recall accuracy for the complex compared to the simple paths (63.32% vs. 86.38%, p < 0.001) at all set sizes, regardless of task condition (single, dual). Auditory performance (accuracy and response time) was significantly lower in the dual compared to single task (85.34% vs. 99.67%, p < 0.001), but performance was not affected by the complexity of the eCorsi path configuration. These findings suggest that set size and path complexity impose a different type of load on the working memory system and may rely on different resources.

Helical Locomotion in Yield Stress Fluids.

We report three stages for locomotion of a helical swimmer in yield stress fluids. In the first stage, the swimmer must overcome the material's yield strain to generate rotational motion. However, exceeding the first threshold is not sufficient for locomotion. Only when the viscous forces are sufficiently strong to plastically deform the material to a finite distance away from the swimmer will net locomotion occur. Once locomotion is underway in the third stage, the yield stress retards swimming at small pitch angles. Conversely, at large pitch angles, yield stress dominates the flow by enhancing swimming speed. Flow visualizations reveal a highly localized flow near the swimmer in yield stress fluids.

Heterostrain and temperature-tuned twist between graphene/h-BN bilayers

Two-dimensional materials stacked atomically at small twist angles enable the modification of electronic states, motivating twistronics. Here, we demonstrate that heterostrain can rotate the graphene flake on monolayer h-BN within a few degrees (− 4° to 4°), and the twist angle stabilizes at specific values with applied constant strains, while the temperature effect is negligible in 100–900 K. The band gaps of bilayers can be modulated from ~ 0 to 37 meV at proper heterostrain and twist angles. Further analysis shows that the heterostrain modulates the interlayer energy landscape by regulating Moiré pattern evolution. The energy variation is correlated with the dynamic instability of different stacking modes of bilayers, and arises from the fluctuation of interlayer repulsive interaction associated with p-orbit electrons. Our results provide a mechanical strategy to manipulate twist angles of graphene/h-BN bilayers, and may facilitate the design of rotatable electronic nanodevices.

Coexistence of reconstructed and unreconstructed structures in the structural transition regime of twisted bilayer graphene

In twisted bilayer graphene (TBG), a twist-angle-dependent competition between interlayer stacking energy and intralayer elastic energy results in flat rigid layers at large twist angles and lattice reconstruction at small twist angles. Despite enormous scientific interest and effort in the TBG, however, an experimental study of evolution from the rigid lattice to the reconstructed lattice as a function of twist angle is still missing. Here we present a scanning tunneling microscopy and spectroscopy study to reveal the twist-angle-dependent lattice reconstruction in the TBG. Our experiment demonstrates that there is a transition regime between the rigid regime and the relaxed regime, and the reconstructed and unreconstructed structures can coexist in the transition regime. The coexistence of the two distinct structures in this regime may arise from subtle balance between the interlayer stacking energy and intralayer elastic energy in the TBG with intermediate moir\'e sizes. Our results provide an explanation for inconsistent phase diagrams and tunneling spectra reported in TBG around the magic angle.

A comparative analysis of helicopter recovery maneuvers on a SFS by means of PIV and balance measurements

The flow field around a frigate is complex due to flow detachments, high velocity gradients, and flow unsteadiness. These flow patterns can endanger helicopter operations around frigates and increase pilot workload above the flight deck. This paper contains a comparative analysis of three different recovery maneuvers: an approach from the stern in the centerline plane (S); a diagonal maneuver (D); and an L-shaped maneuver. The comparison is made using wind tunnel tests with a scaled frigate and a motorized helicopter. For the three maneuvers, velocity contours around the helicopter with Particle Image Velocimetry are obtained. An internal balance is also used to obtain forces and moments on the helicopter during the flight path of the maneuvers. Those measurements show that the wake of the ship mostly affects longitudinal and thrust forces. In addition, pitch torque is highly reduced when the helicopter is behind the frigate superstructure, and the roll moment is also important when the wind angle increases. In the end, an estimation of pilot workload is presented to conclude that L-shaped maneuver is the best for 0° and small WOD angles and D or S recoveries for moderately high negative WOD angles.

Strong Interlayer Coupling in Twisted Transition Metal Dichalcogenide Moiré Superlattices.

Moiré superlattices in twisted van der Waals materials offer a powerful platform for exploring light-matter interactions. The periodic moiré potentials in moiré superlattices can induce strongly correlated quantum phenomena that depend on the moiré potential associated with interlayer coupling at the interface. However, moiré superlattices are primarily prepared by mechanical exfoliation and manual stacking, where the transfer methods easily cause interfacial contamination, and the preparation of high-quality bilayer 2D materials with small twist angles by growth methods remains a significant challenge. In this work, WSe2 /WSe2 homobilayers with different twist angles by chemical vapor deposition (CVD),using a heteroatom-assisted growth technique, are synthesized. Using low-frequency Raman scattering, the uniformity of the moiré superlattices is mapped to demonstrate the strong interfacial coupling of the CVD-fabricated twist-angle homobilayers. The moiré potential depths of the CVD-grown and artificially stacked homostructures with twist angles of 1.5° are 115 and 45 meV (an increase of 155%), indicating that the depth of moiré potential can be modulated by the interfacial coupling. These results open a new avenue to study the modulation of moiré potential by strong interlayer coupling and provide a foundation for the development of twistronics.