Pancake Structures Research Articles

Beware of Inertial Instability Masquerading as Gravity Waves in Stratospheric Temperature Perturbations

AbstractRapp, Dörnbrack, and Preusse (2018, https://doi.org/10.1029/2018GL079142) uses radio occultation observations and meteorological data from the European Centre for Medium Range Weather Forecasting to quantify the effect of inertial instability on stratospheric temperature variability. A very interesting by‐product of their study is the take‐away message that inertial instability “pancake structures” may be misidentified as being caused by gravity waves. We place this result in the context of inertial instability research. We concur with Rapp et al.'s caution to researchers to first examine the large‐scale meteorological environment to rule out inertial instability when analyzing temperature fluctuations that appear to be gravity wave signals in vertical profile data.

On the globularization of the shapes associated with alpha-precipitate of two phase titanium alloys: Insights from phase-field simulations

In two-phase titanium alloys, α-precipitate exhibits plate morphology which spheroidizes during static globularization. The stability of the 3-dimensional plates to withstand perturbation is well-established. However, based on Rayleigh instabilities, the current understanding rendered by ‘the modified perturbation theory’ holds that the spheroidization of the plates is governed by the stable growth of the ridges. To address this counterintuitive view, a multiphase-field model, which recovers the sharp interface solutions, is employed to investigate the morphological transformation accompanying globularization of shapes that resemble α-precipitate. The present investigation unravels an alternate mechanism, wherein the plates globularize by the successive formation and decay, by lateral expansion, of multiple terminal perturbations. This unique mode of spheroidization, distinctly, characterizes the temporal evolution of the driving force which contradicts the hitherto assumed conventional monotonic-decrease in the curvature-difference. Particularly, it is identified that the ellipsoidal plates exhibit a step-wise change in the curvature-difference while non-monotonic series of peaks are observed during the evolution of the pancake structures. To aid the optimization of the thermal cycle involved in the globularization treatment, a simplified relation between the aspect ratio and spheroidization time, which encompasses the influence of the transformation mechanism, is derived.

List of Abstracts

. Production efficiency as well as the reduction of energy consumption and waste are the main drivers for the improvement of large-scale production in chemical industry. In addition, the demand for new processes – these days especially related to the chemical energy conversion as basis for the Energiewende – are additional triggers for innovation. Catalysis, well recognized by the public e.g. in form of the 3-way automobile catalyst, plays an important role for about 90% of all processes in chemical industry. The development of new and improved catalysts is a challenge for chemists – as is the search for new structural materials for mechanical engineers. Due to the often-necessary noble and thus expensive metals (e.g. palladium) in many catalysts, structural and chemical function have been developed separately in the two communities. Recent progress in catalytically active materials allows replacing expensive noble metals in selected catalytic processes by significant cheaper elements [1]. This offers the new perspective to combine structural and chemical functionality by innovative materials. Within this area, intermetallic compounds with their peculiar combination of crystal and electronic structure [2], represent an interesting class of materials to address this vision as will be shown in the contribution.

Estimating thermohaline variability of the equatorial Pacific Ocean from satellite altimetry

The vertical thermohaline structure in the western equatorial Pacific is examined with a Gravest Empirical Mode (GEM) diagnosis of in-situ mooring measurements. The poor GEM performance in estimating deep thermohaline variability from satellite altimetry confirms a lack of vertical coherence in the equatorial ocean. Mooring observation reveals layered equatorial water with phase difference up to 6 months between thermocline and sub-thermocline variations. The disjointed layers reflect weak geostrophy and resemble pancake structures in non-rotating stratified turbulence. A coherency theorem is then proved, stating that traditional stationary GEM represents in-phase coherent structure and can not describe vertically out-of-phase variability. The fact that stationary GEM holds both spatial and temporal coherence makes it a unique tool to diagnose vertical coherent structure in geophysical flows. The study also develops a non-stationary GEM projection that captures more than 40% of the thermohaline variance in the equatorial deep water.

Characterization of Ceramic Oxide Layer Produced on Commercial Al Alloy by Plasma Electrolytic Oxidation in Various KOH Concentrations

Plasma electrolytic oxidation (PEO) is a promising coating process to produce ceramic oxide on valve metals such as Al, Mg and Ti. The PEO coating is carried out with a dilute alkaline electrolyte solution using a similar technique to conventional anodizing. The coating process involves multiple process parameters which can influence the surface properties of the resultant coating, including power mode, electrolyte solution, substrate, and process time. In this study, ceramic oxide coatings were prepared on commercial Al alloy in electrolytes with different KOH concentrations (0.5 ~ 4 g/L) by plasma electrolytic oxidation. Microstructural and electrochemical characterization were conducted to investigate the effects of electrolyte concentration on the microstructure and electrochemical characteristics of PEO coating. It was revealed that KOH concentration exert a great influence not only on voltage-time responses during PEO process but also on surface morphology of the coating. In the voltage-time response, the dielectric breakdown voltage tended to decrease with increasing KOH concentration, possibly due to difference in solution conductivity. The surface morphology was pancake-like with lower KOH concentration, while a mixed form of reticulate and pancake structures was observed for higher KOH concentration. The KOH concentration was found to have little effect on the electrochemical characteristics of coating, although PEO treatment improved the corrosion resistance of the substrate material significantly.

Key factors determining the development of two morphologies of plasma electrolytic coatings on an Al–Cu–Li alloy in aluminate electrolytes

Abstract The porosity within plasma electrolytic oxidation (PEO) coatings strongly affects their properties, such as the wear resistance. Two typical types of coatings, i.e. bi-layered coatings with large internal pores, which show low wear resistance, and single-layered coatings with excellent wear performance, have been found following PEO of aluminium alloys under pulsed current regimes with an aluminate electrolyte. In this paper, the mechanisms of formation of the different coatings are investigated and discussed based on a thorough investigation of PEO of an Al–Cu–Li alloy using systematic variations of 1000 Hz pulsed current waveforms and electrolyte concentration. Both parameters have important roles in determining the structure of the resultant coatings. The coatings formed in a dilute electrolyte, containing 5 g l− 1 NaAlO2, were bi-layered and contained both large pores and pancake structures irrespective of the application of a negative pulse. In contrast, the application of negative current pulse favored the formation of single-layered coatings that contained fewer pancake structures in an electrolyte 32 g l− 1 NaAlO2. A more concentrated electrolyte, containing 56 g l− 1 NaAlO2, resulted in relatively compact, single-layered coatings independently of a negative current pulse. Similar observations for the coating morphologies were made if the alloy was replaced by high purity aluminium and also for the alloy when the frequency was reduced to 100 Hz. Two different models for the growth of single- and bi-layered coatings are proposed.

Development of high vorticity structures in incompressible 3D Euler equations

We perform the systematic numerical study of high vorticity structures that develop in the 3D incompressible Euler equations from generic large-scale initial conditions. We observe that a multitude of high vorticity structures appear in the form of thin vorticity sheets (pancakes). Our analysis reveals the self-similarity of the pancakes evolution, which is governed by two different exponents e−t/Tℓ and et/Tω describing compression in the transverse direction and the vorticity growth, respectively, with the universal ratio Tℓ/Tω ≈ 2/3. We relate development of these structures to the gradual formation of the Kolmogorov energy spectrum Ek ∝ k−5/3, which we observe in a fully inviscid system. With the spectral analysis, we demonstrate that the energy transfer to small scales is performed through the pancake structures, which accumulate in the Kolmogorov interval of scales and evolve according to the scaling law ωmax ∝ ℓ−2/3 for the local vorticity maximums ωmax and the transverse pancake scales ℓ.

Regular and chaotic dynamics of non-spherical bodies. Zeldovich’s pancakes and emission of very long gravitational waves

In this paper we review a recently developed approximate method for investigation of dynamics of compressible ellipsoidal figures. Collapse and subsequent behaviour are described by a system of ordinary differential equations for time evolution of semi-axes of a uniformly rotating, three-axis, uniform-density ellipsoid. First, we apply this approach to investigate dynamic stability of non-spherical bodies. We solve the equations that describe, in a simplified way, the Newtonian dynamics of a self-gravitating non-rotating spheroidal body. We find that, after loss of stability, a contraction to a singularity occurs only in a pure spherical collapse, and deviations from spherical symmetry prevent the contraction to the singularity through a stabilizing action of nonlinear non-spherical oscillations. The development of instability leads to the formation of a regularly or chaotically oscillating body, in which dynamical motion prevents the formation of the singularity. We find regions of chaotic and regular pulsations by constructing a Poincaré diagram. A real collapse occurs after damping of the oscillations because of energy losses, shock wave formation or viscosity. We use our approach to investigate approximately the first stages of collapse during the large scale structure formation. The theory of this process started from ideas of Ya. B. Zeldovich, concerning the formation of strongly non-spherical structures during nonlinear stages of the development of gravitational instability, known as ‘Zeldovich’s pancakes’. In this paper the collapse of non-collisional dark matter and the formation of pancake structures are investigated approximately. Violent relaxation, mass and angular momentum losses are taken into account phenomenologically. We estimate an emission of very long gravitational waves during the collapse, and discuss the possibility of gravitational lensing and polarization of the cosmic microwave background by these waves.

A comparison of plasma electrolytic oxidation of Ti-6Al-4V and Zircaloy-2 alloys in a silicate-hexametaphosphate electrolyte

Plasma electrolytic oxidation of Ti-6Al-4V and Zircaloy-2 alloys in a silicate-hexametaphosphate electrolyte has been studied in detail by various methods, including scanning electron microscopy, transmission electron microscopy and electrochemical techniques. Different coating growth kinetics, formation efficiencies and microstructures have been revealed for the two alloys. From these results, it is suggested that the insulating nature of the coating on Zircaloy-2 promotes the formation of pancake structures, which are formed by strong discharges. In contrast, the semiconducting properties of the coating on the Ti-6Al-4V alloy promote oxygen evolution, reduce the efficiency of coating growth and favour surface or shallow discharges that result in “coral reef” coating features. Anomalous gas evolution, which is more abundant with Zircaloy-2, was observed on both alloys after the appearance of stronger discharges. However, the current density used for the oxygen generation on Zircaloy-2 was still less than that of the Ti-6Al-4V alloy.

Optimized grafting density of end-functionalized polymers to polar dielectric surfaces for solution-processed organic field-effect transistors.

Polystyrene (PS) grafted to silicon oxide (SiO2, referred to as gPS-SiO2) bilayers generated via a polymer grafting method were used as organic-oxide hybrid gate dielectrics to fabricate solution-processed triethylsilylethynyl anthradithiophene (TES-ADT) organic field-effect transistors (OFETs). The dielectric surface properties were significantly altered by the areal grafting densities of different molecular weight (Mw) PS chains with end-functionalized dimethylchlorosilane attached to the SiO2 surfaces. Lesser grafting densities of longer PS chains increased the surface roughness of the treated SiO2 surfaces from 0.2 to 1.5 nm, as well as the water contact angles from 94° to 88°. Below a critical Mw of end-functionalized PS, the gPS chains on the SiO2 surfaces appeared to form a brush-like conformation with an areal density value greater than 0.1 chains nm(-2), but other high-Mw gPS chains formed pancake structures in which the polymeric layers were easily incorporated with solution-processed TES-ADT as a solute. These findings indicate that low-density gPS layers interfered with the self-assembly of TES-ADT in cast films, causing great decreases in crystal grain size and π-conjugated orientation. The presence of compact gPS chains on the SiO2 surface could yield high electrical performance of TES-ADT OFETs with a field-effect mobility of 2.1 cm2 V(-1) s(-1), threshold voltage of -2.0 V, and on/off current ratio of greater than 10(7) when compared to those developed using less-concentrated gPS-SiO2 surfaces.

The evolution of a stratified turbulent cloud

AbstractLocalized regions of turbulence, or turbulent clouds, in a stratified fluid are the subject of this study, which focuses on the edge dynamics occurring between the turbulence and the surrounding quiescent region. Through laboratory experiments and numerical simulations of stratified turbulent clouds, we confirm that the edge dynamics can be subdivided into materially driven intrusions and horizontally travelling internal wave-packets. Three-dimensional visualizations show that the internal gravity wave-packets are in fact large-scale pancake structures that grow out of the turbulent cloud into the adjacent quiescent region. The wave-packets were tracked in time, and it is found that their speed obeys the group speed relation for linear internal gravity waves. The energetics of the propagating waves, which include waveforms that are inclined with respect to the horizontal, are also considered and it is found that, after a period of two eddy turnover times, the internal gravity waves carry up to 16 % of the cloud kinetic energy into the initially quiescent region. Turbulent events in nature are often in the form of decaying turbulent clouds, and it is therefore suggested that internal gravity waves radiated from an initial cloud could play a significant role in the reorganization of energy and momentum in the atmosphere and oceans.

Wear-resistant coatings formed on Zircaloy-2 by plasma electrolytic oxidation in sodium aluminate electrolytes

Plasma electrolytic oxidation of Zircaloy-2 has been investigated in dilute and concentrated aluminate electrolytes, under a pulsed-bipolar current regime, in order to develop coatings of high wear resistance. Coating growth kinetics, cell potential-time responses and discharging behaviours depend significantly on the electrolyte concentration. The coatings formed in dilute aluminate electrolyte reveal a three-layered structure, with pancake structures at the coating surfaces. “Soft sparks” occur during PEO in dilute aluminate electrolyte, causing a relatively fast growth of the inner layer and resulting in a large amount of alumina-enriched material beneath the pancake structures, and hence an increased wear resistance of the coating. In contrast, more homogenous coatings, free of pancakes, result with the concentrated electrolyte. The main phase in the coatings is t-ZrO2, with γ-Al2O3 also present in coatings formed in the latter electrolyte. The coatings formed in the concentrated electrolyte display a high wear resistance, even for thin coatings formed for short times, which is attributed to the relatively high alumina content of the coatings.

Micro-mountain and nano-forest pancake structure of Nb2O5 with surface nanowires for dye-sensitized solar cells

Niobium oxide produced by an anodization process for application in a high-efficiency dye-sensitized solar cell (DSSC) has been reported. In order to increase the energy conversion efficiency of the cell, we demonstrated niobium oxide micro-mountain and nano-forest pancake structures that contained nanowires on the surface. The increase in efficiency is attributed to the significantly enhanced surface area, which allows for more dye loading and light harvesting, as well as the reduced charge recombination, which provides direct conduction pathways along the crystalline Nb2O5 micro-mountain structures with nanowires. We observed that the photovoltaic performance of the Nb2O5 DSSC increased from 1.03% to 3.35% when the oxide thickness was increased from 9.3 μm to 27.7 μm. Since the anodization process is very cheap and takes place at low temperatures, it has substantial potential for the further development of DSSCs with metal-oxide nano-materials. Anodized oxide advantage is that essentially any form of a niobium surface can be treated in a quick and low-cost approach with Nb2O5 coating.

Structural distribution in mixed ternary noble gas and Lennard-Jones clusters

The geometrical structures and energies of mixed 38- and 55-atom Ar–Kr–Xe clusters are investigated. The interaction is described by ternary Lennard-Jones (TLJ) potential. The stable structures are optimized by adaptive immune optimization algorithm. Results show that in Ar12KrmXen (m+n=26) clusters, there exist amorphous and polytetrahedral configurations, and in Ar13KrmXen (m+n=42) clusters, structures are categorized into four classes, i.e., 8 Mackay icosahedra, 19 amorphous structures, 4 sixfold pancake structures, and 10 ring-like structures. Furthermore, strain energies in Ar–Kr–Xe clusters are studied to propose the possible ways of reducing strain. To investigate the potential energy surface of Ar–Kr–Xe clusters in TLJ model potential, the bond numbers of the Ar12Kr13Xe13 cluster and A12B13C13 TLJ model potential are compared, and the case of the former one is studied in the bond number phase diagram of the latter one.

Study of 40-atom Pt–Au clusters using a combined empirical potential-density functional approach

This work is a theoretical study of 40-atom Pt–Au clusters which are of interest owing to the electronic shell closure of 40-atom noble metal clusters and the current focus on bimetallic Pt–Au clusters as catalysts. The methodology is a complementary combination of a genetic algorithm search for an empirical potential and density functional theory (DFT) reoptimization. Structures based on truncated-octahedral, icosahedral, decahedral and fivefold pancake geometries are found to be energetically favoured for different composition regions at the empirical-potential level and this is partially confirmed at the DFT level. The large HOMO–LUMO gaps found for the icosahedral and fivefold pancake structures indicate electronic shell closure effects, while the truncated-octahedral and decahedral structures have small gaps. The DFT calculations confirm that, for Pt 20 Au 20 truncated-octahedral structures, the Pt core Au shell configuration which has two Au atoms capping the (100) facets is most energetically favoured, and the layered (phase segregated) configuration also has lower energy compared with the Au core Pt shell and mixed configurations.

Possible explanation of theΛ-shape anomaly in polymer surface diffusion

We suggest an explanation for the anomalous behavior of the polymer surface-diffusion coefficient D reported by Zhao and Granick [J. Am. Chem. Soc. 126, 6242 (2004)]: D first increased with increasing polymer surface concentration Gamma, then suddenly dropped at a threshold value Gamma*. Our molecular dynamics simulations show the same behavior. We find that polymers form single-layered pancake structures for Gamma<Gamma*, while double layers form for Gamma>Gamma*. The double-layer structures allow the polymers to better adapt to the substrate corrugation. This increases barriers for lateral diffusion and is thus consistent with small values of D at larger Gamma.

Approximate dynamics of dark matter ellipsoids

The collapse of non-collisional dark matter and the formation of pancake structures in the Universe are investigated approximately. Collapse is described by a system of ordinary differential equations, in the model of a uniformly rotating, three-axis, uniform density ellipsoid. Violent relaxation, mass and angular momentum losses are taken into account phenomenologically. The formation of the equilibrium configuration, secular instability and the transition from a spheroid to a three-axis ellipsoid are investigated numerically and analytically in this dynamical model.

Study of the Kinetics of the Pancake-to-Brush Transition of Poly(N-isopropylacrylamide) Chains

The chemical grafting of thiol-terminated poly(N-isopropylacrylamide) (HS-PNIPAM) chains to a gold surface from a solution was investigated with a quartz crystal microbalance (QCM) in real time. The frequency and energy dissipation responses revealed that short HS-PNIPAM chains exhibit three-regime kinetics of the grafting. In regimes I and II, the HS-PNIPAM chains form pancake and mushroom structures, respectively. In regime III, the chains form brushes. From regime II to regime III, the mushroom-to-brush transition occurs. For the longer HS-PNIPAM chains, because of the strong segment-surface interaction, the segments cannot desorb from the surface, and the chains do not undergo a pancake-to-brush transition.

Kinetic simulation of the coherent radio emission from pulsars

On hand of 3D PIC simulations we show that in a strongly magnetized plasma a relativistic electron beam can be forced to emit highly coherent radio emission by self-induced nonlinear density fluctuations. Such slowly moving nonlinear structures oscillate with the local plasma frequency at which the relativistic electrons are scattered. Beam electrons dissipate a significant amount of their kinetic energy by inverse Compton radiation at a frequency of about γ2ωpe. Since the beam is sliced into pancake structures which experience the same electric field the inverse Compton scattering is coherent. Such a process is a very promising candidate for the coherent radio emission of pulsars.

Kelvin waves in stratospheric temperature observed by the Microwave Limb Sounder

The three‐dimensional (3‐D) structure of stratospheric Kelvin waves is revealed using data from the Microwave Limb Sounder for the period July 1992 to April 1993. Four Kelvin wave modes are identified, two for zonal wave number 1 (k = 1) and two for k = 2, using extended empirical orthogonal functions. The 3‐D structures of the modes agree with linear theory and satisfy the dispersion relation for Kelvin waves, and with periods ranging from 4.5 to 10 days the modes mostly fall near the boundary between “fast” and “slow” Kelvin waves [Canziani et al., 1998]. The analysis also identifies a feature that may correspond with the “pancake structures” associated with inertial instability.