Layer Undulation Research Articles

Highly separated transitional shock-wave/boundary-layer interactions: A spatial modal study

At cruise altitudes, the Reynolds number may become sufficiently low to allow a laminar boundary layer to persist on the suction side of a transonic fan blade up to the shock-wave/boundary-layer interaction (SBLI). In such a transitional SBLI with sufficiently large shock-induced separation, a shock oscillation mechanism occurs, with the source at the upstream growth (until a critical length) and natural suppression (through shear layer instabilities) of the separation bubble. The oscillation cycle is characterized by a temporarily vanishing upstream laminar part of the separation bubble. The suppression of this laminar part is accompanied by downstream advection of turbulence and subsequent entrainment into the bulk separation bubble, affecting the reflected shock movement. In order to study the spatial behavior of the mechanism, particle image velocimetry of a highly separated transitional oblique SBLI at Mach 2.3 is conducted in the high speed aerodynamics laboratory of Delft University of Technology. Statistical quantities, including root mean square velocity fluctuations, phase averages, and spatial modes from proper orthogonal decomposition, are investigated. The entrainment strength varies depending on the phase of the oscillation, and the turbulent shear layer is not fully developed. The main growth and shrinking mode of the separation bubble were extracted, which affects the slip line region size and shock position. Secondary modes that affect the shear layer undulation and upstream effects were also extracted. The study provides quantitative analyses of an important shock oscillation type, with the focus on capturing the separation bubble size variation and upstream effects.

Screw dislocation dynamics in confinement-induced layering of Yukawa liquids after quenching

Screw dislocation filaments (SDFs) with undefined phases and winded around by helical isophase fronts are fundamental topological defects, which can be used for characterizing the disorder of layered solids and unstable traveling waves. Here, we numerically demonstrate the observation of fluctuating SDFs in the transient relaxation of confinement-induced layering of Yukawa liquids after quenching. We classify the generic dynamical behaviors of screw dislocations (SDs) and unravel their topological origins, which are elusive fundamental issues for general layered systems. It is found that the spatiotemporal growth and decay of layer undulation instability causing sequential rupture and reconnection of a stack of layers are the keys for the formation of SD loops or strings of connected SDFs with alternating helicities, and their spatiotemporal fluctuations. The breaking and reconnection of two approaching SDFs with opposite or the same helicities can form two new separated SDFs. This causes an SD loop to be shed from or merged with another SD loop. The total number of SDs decreases and levels off with increasing time from quenching. Published by the American Physical Society 2024

Leaning induced layer undulated tilted smectic phase of asymmetric bent-core liquid crystals.

We report the synthesis and experimental studies on the liquid crystalline phase of a homologous series of compounds comprised of strongly asymmetric bent-core banana shaped molecules. Our x-ray diffraction studies clearly indicate that the compounds exhibit a frustrated tilted smectic phase with layer undulated structure. The low value of the dielectric constant as well as switching current measurements indicate the absence of polarization in this layer undulated phase. In spite of the absence of polarization, a planar aligned sample can be transformed irreversibly to a higher birefringent texture on the application of a high electric field. The zero field texture can only be retrieved by heating the sample to the isotropic phase and cooling it subsequently to the mesophase. We propose a double tilted smectic structure having layer undulation to account for the experimental observations with the layer undulation arising due to the leaning of the molecules in the layer.

Balancing WAAM Production Costs and Wall Surface Quality through Parameter Selection: A Case Study of an Al-Mg5 Alloy Multilayer-Non-Oscillated Single Pass Wall

The purpose of the study was to propose a strategy to assess the potential reduction of the production cost during wire+arc additive manufacturing (WAAM) based on the combination of wire feed speed (related to deposition rate) and travel speed (related to deposition time). A series of experiments, using a multilayer-non-oscillated single pass wall made of an Al-Mg alloy, was conducted. The quality of the wall was assessed through the lateral surface waviness and top layer undulation. The concepts of Surface Waviness and Buy-to-Apply indices were introduced. Initially, the range of travel speed (TS) that provided layers with acceptable quality was determined for a given wire feed speed (WFS), corresponding to a constant current. Then, the effect of the increase of production capacity of the process (though current raising, yet maintaining the ratio WFS/TS constant) on the wall quality for a given condition within the TS range was assessed. The results showed that the useful range of TS prevents too rough a waving surface below the lower limit and top surface undulation over the higher limit. However, inside the range, there is little quality variation for the case under study. Finally, simulations of deposition time were developed to demonstrate the weight of the TS on the final deposition time and wall quality as a function of a target wall width. This respective weight showed the existence of a complex and unpredictable, yet determined, power of a combination of TS, target wall geometry, and dead time between subsequent layers. It was verified to be possible to find optimized TS as a function of different target geometries.

Polarization-Modulated Bent-Core Liquid Crystal Thin Films without Layer Undulation.

Spatial confinement is known to affect molecular organizations of soft matter. We present an important manifestation of this statement for thin films of bent-core smectic liquid crystals. Prior freeze-fracture transmission electron microscopy (FFTEM) studies carried out on nitro-substituted bent-core mesogens (n-OPIMB-NO_{2}) revealed an undulated smectic layer structure with an undulation periodicity of ∼8 nm. We report cryogenic TEM measurements on ∼100 nm thick 8-OPIMB-NO_{2} films. In contrast to FFTEM results, our studies show only density modulation with periodicity b=16.2 nm, and no smectic layer undulation. We show that the discrepancy between the FFTEM and cryogenic transmission electron microscopy (cryo-TEM) results can be attributed to the different sample thicknesses used in the experiments. FFTEM monitors cracked surfaces of a relatively thick (5-10 μm) frozen sample, whereas cryo-TEM visualizes the volume of a thin (0.1 μm) film that was quenched from its partially fluid phase. These results have importance in possible photovoltaics and organic electronics applications where submicron thin films are used.

Stabilized electrically induced Helfrich deformation and enhanced color tuning in cholesteric liquid crystals.

When subjected to an AC electric field perpendicular to its layers, the cholesteric planar state may undergo a periodic layer undulation, known as the Helfrich deformation, which generates a color change of the reflected light. The Helfrich deformation of regular cholesteric liquid crystals is, however, unstable and easily taken over by the focal conic state, and thus the color tuning range is narrow. We demonstrated that the Helfrich deformation can be stabilized in a large electric field region by doping a bent dimer with an anomalously small bend elastic constant and dispersing a small amount of polymer network. By varying the dimer concentration, we were able to systematically change the bend elastic constant and thus verify the theoretical prediction of the undulation threshold electric field and periodicity. We also achieved reflectance tuning with electric field less than 2 V μm-1 and color tuning covering the entire visible light region with electric field as low as 4 V μm-1.

(1)H NMR Relaxometric Study of Molecular Dynamics in a "de Vries" Liquid Crystal.

Liquid crystals that exhibit de Vries smectic A phases are promising materials for new generations of ferroelectric liquid crystal displays and other electro-optical devices. We investigated the molecular dynamic properties of a rod-like de Vries liquid crystal compound, namely the 9HL, a (S)-hexyl lactate derivative, in the whole mesophasic range. This is the first molecular dynamics' investigation on a de Vries phase, and the interest of this system is related to the understanding of the structural and supramolecular organization of de Vries SmA phases, which has been a subject of a broad scientific debate in the last years. Proton NMR spin-lattice relaxation times, T1, were measured in the range 100 MHz to 5 kHz, thus covering a very wide range of motional regimes. (1)H NMR dispersion curves and temperature-dependent relaxation rates were analyzed with a global minimum target fitting approach and the main molecular motions, namely reorientational diffusion, translational self-diffusion, layer undulation, and tilting director fluctuations were fully characterized. The molecular dynamics' behavior observed across the SmA-SmC* phase transitions of 9HL was consistent with the proposed cluster diffuse cone model for the de Vries SmA phase.

Quantum wire‐on‐well (WoW) cell with long carrier lifetime for efficient carrier transport

AbstractA quantum wire‐on‐well (WoW) structure, taking advantage of the layer undulation of an InGaAs/GaAs/GaAsP superlattice grown on a vicinal substrate, was demonstrated to enhance the carrier collection from the confinement levels and extend the carrier lifetime (220 ns) by approximately four times more than a planar reference superlattice. Strained InGaAs/GaAs/GaAsP superlattices were grown on GaAs substrates under exactly the same conditions except for the substrate misorientation (0 and 6 ° off). The growth on a 6 ° off substrate induced significant layer undulation as a result of step bunching and non‐uniform precursor incorporation between steps and terraces, whereas the growth on a substrate without miscut resulted in planar layers. The undulation was the most significant for InGaAs layers, forming periodically aligned InGaAs nanowires on planar wells, a WoW structure. As for the photocurrent corresponding to the sub‐bandgap range of GaAs, the light absorption by the WoW was extended to longer wavelengths and weakened as compared with the planar superlattice. Almost the same photocurrent was obtained for both the WoW and the planar superlattice. Open‐circuit voltage for the WoW was not affected by the longer‐wavelength absorption edge, and the same value was obtained for the two structures. Furthermore, the superior carrier collection in the WoW, especially under forward biases, improved fill factor compared with the planer superlattice. Copyright © 2016 John Wiley & Sons, Ltd.

Thickness-modulated InGaAs/GaAsP superlattice solar cells on vicinal substrates

InGaAs/GaAsP superlattice (SL) is a promising narrow-gap material for III–V multi-junction solar cells on Ge. In metal-organic vapor phase epitaxy (MOVPE) of SL on vicinal substrates, the component layers tend to be undulated due to step bunching occurring at high temperature. In this paper, the effects of growth temperature and thickness modulation of the SL-region on the photovoltaic performance were investigated. Lowering the growth temperature successfully enabled epitaxy of an extremely uniform SL, from which a clear step-like absorption spectrum including sharp exciton peaks was obtained due to layer-by-layer deposition of the individual layers. Larger layer undulation at higher temperature led to poorer in-plane coverage of the InGaAs region, resulting in the reduction of both light absorption and short circuit current. The open circuit voltage, on the other hand, was higher for the cells grown at higher temperature owing to suppressed dark current as a result of reduced crystal defects. Moreover, the lateral thickness variation of the GaAsP barriers in the undulated SL allowed efficient tunnel transport through the thinner part of the barrier, and improved the carrier collection and the fill factor. By optimizing the growth temperature for SL on vicinal substrates, an N-on-P cell including 100-period SL with a bandgap of 1.21 eV achieved 1.11 times higher efficiency than a GaAs reference cell with 36% current enhancement as middle cell performance.

Analyse de la macula par OCT spectral domain dans la chirurgie du décollement de rétine rhegmatogène

Despite surgical reattachment of retinal layers, postoperative functional outcomes after rhegmatogenous retinal detachment (RRD) may be limited. This can be explained by microstructural changes in the macula inherent to the pathology itself as well as the surgery. To evaluate the various changes in the macula by OCT pre- and postoperatively, and correlate them with functional and clinical outcomes in patients with RRD. To establish pre- and postoperative prognostic factors. This was a prospective study of 50 eyes of 50 patients operated for RDD. Each patient underwent a complete clinical examination and macular OCT using the Heidelberg Spectralis; preoperatively and then successively at 7 days, 1 month, 3 months and 6 months after RRD surgery. Preoperative tomographic results revealed an average height of submacular fluid of 742 ± 345 μm. Ninety-six percent of patients exhibited thickening of photoreceptor outer segments (PROS), 62% an outer layer undulation, 60% cystic cavities in the outer and/or inner nuclear layers (ONL, INL), 36% disruptions of the external limiting membrane (ELM) and 64% disruption of the IS/OS junction. Postoperatively, 24% of patients had persistent submacular fluid. The average thickness of the central fovea, the ONL, the IS/OS junction and the PROS were 172 ± 51.3 μm, 88.4 ± 30.9 μm, 11.36 ± 5.4 μm and 19.54 ± 13.1 μm respectively. Postoperative disruptions of the ELM, the IS/OS junction and Verhoeff's membrane (VM) were present in 24%, 60% and 82% of patients respectively. The preoperative tomographic risk factors for poor visual outcome were: submacular fluid height > 800 μm (P<0.001), disruptions of the MLE and/or IS/OS junction (P<0.001), as well as cystic cavities in the ENL and/or INL (P=0.002). Postoperative risk factors were: thinning of the fovea (≤ 250 μm), central fovea (≤ 160 μm), ONL (≤ 90 μm), IS/OS junction (≤ 10 μm) and PROS (≤ 18 μm) layers (P<0.001), as well as a discontinuous or absent appearance of the ELM (P<0.001), IS/OS junction (P<0.001) and VM (P=0.006). Spectral domain OCT allows detection of specific microscopic, quasi-histologic macular changes in rhegmatogenous retinal detachment. These anomalies could be predictive of final postoperative visual outcome.

Low-temperature MOVPE using TEGa for suppressed layer undulation in InxGa1−xAs/GaAs1−yPy superlattice on vicinal substrates

InxGa1−xAs/GaAs1−yPy strain-balanced superlattice (SL) is a promising narrow-gap material that can be lattice-matched to Ge or GaAs. When growing uniform SLs on vicinal substrates for Ge-based application, formation of step-bunching and the resultant layer undulation is a critical problem. The present paper reports low-temperature metal–organic vapor phase epitaxy (MOVPE) with triethylgallium (TEGa) to improve the stability and the uniformity of SL growth. Using trimetylgallium (TMGa), steps were severely bunched especially during InGaAs growth at high temperature, and the average reflectance of SLs significantly decreased due to lattice relaxation above 580°C. Lowering the growth temperature to 550–530°C was effective in order to grow 100-period SLs with a mirror-like surface, but an apparent lateral layer undulation was still confirmed by ex-situ X-ray-diffraction and STEM observation. Growth of SLs below 530°C with TMGa was found to result in serious crystal degradation due to unstable growth in the kinetically-limited temperature range. In contrast, use of TEGa enabled 100-period SLs to be successfully grown even at lower temperature owing to its efficient pyrolysis process. Highly uniform SLs with negligible layer undulation were grown at 510°C with TEGa, showing a photoluminescence spectrum which was as sharp as the one for SLs grown on an exact-oriented substrate. The absorption spectra for the SLs grown with TEGa showed similarly sharp edge with a strong exciton peak.

Spiral layer undulation defects in B7 liquid crystals

In bent-core smectic liquid crystal phases, a strong local preference for layering coupled to the bent molecular shape gives rise to two spontaneous symmetry-breaking instabilities, polar orientational ordering of the molecules and molecular tilt, which combine to produce chiral phases. The B7 is one such phase characterized by layer undulations associated with the formation of periodic, polarization-modulation stripes that can be visualized using FFTEM. By analyzing topological defects in the B7 layer undulation texture, we can determine the molecular organization within the layers which indicates that the layers are homochiral. The layer undulations coupled with the layer chirality forms clockwise or anticlockwise spiral patterns as they nucleate and grow.

Structural Recovery of the Detached Macula after Retinal Detachment Repair as Assessed by Optical Coherence Tomography

PurposeTo investigate correlations between preoperative and postoperative foveal microstructures in patients with macula-off rhegmatogenous retinal detachment (RRD).MethodsWe reviewed the records of 31 eyes from 31 patients with macula-off RRD who had undergone successful re-attachment surgery. We analyzed data obtained from complete ophthalmologic examinations and optical coherence tomography (OCT) before and 9 to 12 months after surgery. All postoperative OCT measurements were taken with spectral-domain OCT, but a subset of preoperative OCT measurements were taken with time-domain OCT.ResultsThe mean duration of macular detachment was 15.5 ± 15.2 days, and mean preoperative best-corrected visual acuity (BCVA, logarithm of the minimum angle of resolution) was 1.03 ± 0.68. Preoperative visual acuity was correlated with retinal detachment height (p < 0.001) and the existence of intraretinal separation (IRS) along with outer layer undulation (OLU) (p = 0.022), but not with macula-off duration. The final BCVA was significantly correlated with integrity of the junction between the photoreceptor inner and outer segments (IS/OS) combined with the continuity of external limiting membrane (ELM) (p = 0.025). The presence of IRS and OLU on a detached macula were highly correlated with the final postoperative integrity of the IS/OS junction and the ELM (p = 0.017).ConclusionsEyes preoperatively exhibiting IRS and OLU showed a higher incidence of disruption to the photoreceptor IS/OS junction and the ELM at final follow-up. Such a close correlation between preoperative and postoperative structural changes may explain why ultimate visual recovery in such eyes is poor.

Analytic Description of Layer Undulations in Smectic A Liquid Crystals

We investigate the layer undulations that appear in smectic A liquid crystals when a magnetic field is applied in the direction parallel to the smectic layers. In an earlier work (Garcia-Cervera and Joo in J Comput Theor Nanosci 7:795–801, 2010) the authors characterized the critical field using the Landau–de Gennes model for smectic A liquid crystals. In this paper, we obtain an asymptotic expression of the unstable modes using Γ-convergence theory, and a sharp estimate of the critical field. Under the assumption that the layers are fixed at the boundaries, the maximum layer undulation occurs in the middle of the cell and the displacement amplitude decreases near the boundaries. Our estimate of the critical field is consistent with the Helfrich–Hurault theory. When natural boundary conditions are considered, the displacement amplitude does not diminish near the boundary, in sharp contrast with the Dirichlet case, and the critical field is reduced compared to the one calculated in the classical theory. This is consistent with the experiments carried out by Ishikawa and Lavrentovich (Phys Rev E 63:030501(R), 2001). Furthermore, we prove the existence and stability of the solution to the nonlinear system of the Landau–de Gennes model using bifurcation theory. Numerical simulations are used to illustrate the predictions of the analysis.

Growth and characterisation on giant magnetoresistance property of metallic multilayers

Metallic multilayers with specific materials configurations have been shown to exhibit giant magnetoresistance effect at room temperature. In this study, microstructural analysis was carried out on magnetron sputtered Co/Cu multilayers using various diffraction and imaging techniques. Dominant characteristic features associated with the multilayers, such as lateral and vertical columnar grain orientations as well as layer undulation and regularity were identified. By deliberately introducing microstructural changes using a buffer layer and via heat treatment, detailed microstructural analysis carried out on the multilayers has provided an insight into the dependence of giant magnetoresistance on the observed microstructures. The present study shows that high giant magnetoresistance effect of the multilayers was associated with highly correlated interfacial profiles, sharp columnar grain boundaries and high degree of lateral coherency in columnar grain growth. While the introduction of iron buffer layer favoured the growth of these structural features that were responsible for large giant magnetoresistance effect, annealing led to deterioration of giant magnetoresistance property of the multilayers.

Atomic Force Microscope Observation of Nano-Ordered Undulation Structure Formed by Bent-Shaped Molecules

The layer undulation, one of the frustration modes, in the X1 phase formed from the 4-bromo-1,3-benzene-bis[4-(4-alkoxyphenyliminomethyl)benzoate] was analyzed by Atomic force microscope (AFM) method. The AFM image shows a clear evidence of undulated layer structure in the homeotropically aligned sample. The layer undulation is observed as a mild sinusoidal wave with the periodic distance of 16–18 nm and the amplitude of 2–3 nm. Further, the undulation occurs in a random direction along the smectic layer, showing that the undulation direction is not related to the polar axis in the preceding B2 phase.

Non-linear rheology of lamellar liquid crystals

We measure the non-linear relation between the shear stress and shear rate in the lyotropic lamellar phase of C12E5/water system. The measured shear thinning exponent changes with the surfactant concentration. A simple rheology theory of a lamellar or smectic phase is proposed with a prediction gamma approximately sigma3/2, where gamma is the shear rate and sigma is the shear stress. We consider that the shear flow passed through the defect structure causes the main dissipation. As the defect line density varies with the shear rate, the shear thinning arises. The defect density is estimated by the dynamic balance between the production and annihilation processes. The defect production is caused by the shear-induced layer undulation instability. The annihilation occurs through the shear-induced defect collision process. Further flow visualization experiment shows that the defect texture correlates strongly with the shear thinning exponent.

Role of Molecular Shape on Bent-Core Liquid-Crystal Structures

We show how frustrations in molecular packing generated by molecular substitutions and electrostatic potentials necessitate a local C1 symmetry and lead to spontaneous polarization splay and layer bending in bent-core liquid crystals. By estimation of the entropic changes due to curvature, we calculated the elastic constants that drive the spontaneous layer bend and polarization splay and obtained length scales of these deformations. Finally, we propose a structural phase diagram which describes how the experimentally observed helical superstructures, layer undulation, polarization splay, and spontaneous fiber formation depend on the direction and magnitude of the spontaneous polarization.

Layer undulations in a smectic C liquid crystal with weak anchoring

The free energy for a sample of smectic C liquid crystal bounded between two semi-infinite plates with weak surface anchoring on the boundaries is considered. A magnetic field is applied across the sample and the ensuing layer undulations are studied. The free energy is minimized to give relations involving the elastic energies, magnetic field strength and wave numbers which govern the layer undulations. Forms for the critical field strength and critical wave numbers are derived and compared to analogous results in a system with strong surface anchoring. An expression for the layer undulation amplitude is derived for weak anchoring which can be compared with the classical Helfrich–Hurault theory for strong anchoring and recent experimental results for cholesteric liquid crystals.

Transmission Electron Microscope Observation by Surface Replica Method for Layer Undulation in Solid X1 Phase of Banana Molecule

The smectic X1 phase formed from the banana 4-bromo-1,3-phenylenebis[4-(4-n-tetradecanoxyphenyliminomethyl)benzoate] (4Br-P-14-O-PIMB) molecule, possesses a distinct two-dimensional structure with a long-range density modulation of ∼17.5 nm along the layer. The X1 phase is formed upon cooling from the well-known B2 phase, and then its characteristic modulated structure is retained at room temperature. A transmission electron microscopic (TEM) observation was performed for shadowed replica film which imprints the surface structure of the homeotropically aligned X1 phase. The modulation was detected as clear stripes with periodicities ranging from 15 to 18 nm, which agree with the modulation length determined by X-ray measurement. The results confirm that the periodicity along the layer is not due to the simple density modulation occurring within a smooth layer, but can be explained by the layer undulation that creates the crests and troughs.