Weak Corrugation Research Articles

Hey, hold on, we are good guys! Colony defense behavior and notes on nesting of Parachartergus fulgidipennis (de Saussure, 1854) (Vespidae: Polistinae: Epiponini)

Parachartergus wasps are known by having a very aggressive behavior, especially the large black species with white wings tip, which have painful stings. They exhibit a unique and interesting characteristic, the capacity to venom spraying during the colony defense or self-defense. However, for the Amazonian species Parachartergus fulgidipennis (de Saussure, 1854), little is known about its behavior and nesting. In this way, we tracked and described the nesting site, the numbers of combs, and performed an ad libitum method to record behavioral activities. The nest was fixed vertically on a man-made concrete column, 1.2 m above the ground, measuring 25.0 x 6.0 cm and contained four combs with approximately 74 cells, with 31 eggs, seven larvae, 30 pupae, and almost 42 adults. With a flat thin grayish envelope with weak corrugations, the nest is very similar to the color of the substrate which turn in to really cryptic. The colony defense behavior presented by this species is unaggressive and very docile, suggesting that is not similar as observed for other Parachartergus species.

Collisionless particle dynamics in trap sections with helical corrugation

Collisionless particle dynamics in a helical magnetic mirror is investigated by methods of the Hamilton dynamics. In the limit of small Larmor radius, the Hamiltonian is fully integrable and particle trajectories can be divided into three types, differing in the direction of the drift velocity. The structure of the trajectories of resonant particles and the width of the resonance are studied. The influence of weak axi-symmetrical corrugation on the particle dynamics and efficiency of plasma flow suppression through a helical mirror are discussed.

Ripple formation on Si surfaces during plasma etching in Cl2

Nanoscale surface roughening and ripple formation in response to ion incidence angle has been investigated during inductively coupled plasma etching of Si in Cl2, using sheath control plates to achieve the off-normal ion incidence on blank substrate surfaces. The sheath control plate consisted of an array of inclined trenches, being set into place on the rf-biased electrode, where their widths and depths were chosen in such a way that the sheath edge was pushed out of the trenches. The distortion of potential distributions and the consequent deflection of ion trajectories above and in the trenches were then analyzed based on electrostatic particle-in-cell simulations of the plasma sheath, to evaluate the angular distributions of ion fluxes incident on substrates pasted on sidewalls and/or at the bottom of the trenches. Experiments showed well-defined periodic sawtooth-like ripples with their wave vector oriented parallel to the direction of ion incidence at intermediate off-normal angles, while relatively weak corrugations or ripplelike structures with the wave vector perpendicular to it at high off-normal angles. Possible mechanisms for the formation of surface ripples during plasma etching are discussed with the help of Monte Carlo simulations of plasma-surface interactions and feature profile evolution. The results indicate the possibility of providing an alternative to ion beam sputtering for self-organized formation of ordered surface nanostructures.

Experimental study of the effects of free stream turbulence on characteristics and flame structure of bluff-body stabilized conical lean premixed flames

This article describes an experimental investigation of the effect of different levels of free stream turbulence intensity on the structure of an unconfined, lean propane–air flame stabilized on an axisymmetric bluff body. Simultaneous imaging of hydroxyl (OH) and formaldehyde (CH2O) by planar laser induced fluorescence and particle image velocimetry (PIV) was used to study the interaction between the flame and the flow field. CH2O fluorescence and the pixel-by-pixel multiplication of OH and CH2O fluorescence signals were utilized to mark preheat and heat release regions respectively. At low turbulence intensity, flame front featured weak corrugations and mostly symmetric flame structures with continuous heat release regions. Pronounced formation of cusps and unburnt mixture fingers were observed as the turbulence intensity was increased from 4% to 14% without discontinuities in the heat release front. The local flame structure was observed to be strongly modified for the high turbulence intensities with the occurrence of flamelet merging, localized extinctions along the shear layer and flame fragmentations. The general shape of the flame was observed to intermittently switch from a symmetric (varicose) to asymmetric (sinuous) mode. Several flame properties were determined to characterize the effects of turbulence–flame interaction which includes the average preheat and reaction zone thicknesses, strain rates and curvature along the flame front, flame brush thickness, flame surface density, flame area ratio and turbulent flame speed. With increasing turbulence levels, the preheat zone was observed to become thicker while reaction zone thickness reduced. The curvature and strain rates pdfs broadened with increasing levels of turbulence intensity. The flame brush thickness showed an initial increase with increasing turbulence levels but saturated beyond 24% turbulence intensity. Measurements showed a reduction in flame surface density with increasing levels of turbulence intensity. The turbulent flame area ratio increased with turbulence intensity but at a reduced rate at higher turbulence intensities.

Shear Waves in an Initially Stressed Elastic Plate with Periodic Corrugations

The influence of the combined effect of initial stress and periodic surface corrugation on ultrasonic SH wave propagation in an elastic plate is investigated. Considering weak surface corrugations, a small parameter is defined using the ratio between the corrugation amplitude and the average thickness of the plate and the perturbation method of strained parameters is used up to the second order. An equation for evaluating the wave phase velocity in the periodically corrugated plate is realized. Numerical examples are provided to graphically illustrate the variations of the phase velocity in periodic plates, with various corrugation amplitudes and wavelengths, under compressive and tensile initial stresses over a frequency spectrum. The study reveals the dispersive nature of SH waves in corrugated plates. It is found that the wave phase velocities are slightly higher (lower) in plates under initial tension (compression) than initially stress-free plates. The influence of initial stresses on SH phase speed decreases as the frequency increases. It is shown that periodic surface roughness has a stronger effect on reducing wave phase speeds than a shallow surface waviness.

On the self-patterning of islands by mechanical constraints during electrochemical deposition

We present a theoretical interpretation for a self-assembly pattern of Cu islands deposited on amorphous NiTi surface by electrochemical deposition. The self-assembly pattern is composed of alternating island-rich and island-depleted zones. We propose that the self-assembly pattern is possibly due to the long-range island–island repulsion mediated by the substrate and the weak corrugation of delaminated NiTi substrate caused by the compressive stress associated with Cu island growth. Based on these two mechanisms, we performed numerical simulations to model the island growth and semiquantitatively reproduced the self-assembly pattern. The good agreement between the simulated patterns and those observed in experiments suggests that the combination of geometrical constraint and electrochemical deposition provides a promising way to fabricate self-assembly patterns for micro- and nanofabrications at low cost.

Phase space tomography of cold-atom dynamics in a weakly corrugated potential

We demonstrate tomographic reconstruction of the phase space distribution of atoms oscillating in a harmonic trap with weak potential corrugation caused by nanoscale imperfections in an atom chip. We find that deformations in these distributions are highly sensitive to anharmonic components of the potential. They are explained in terms of angular velocity dispersion of isoenergetic phase space trajectories. We show that the method is applicable for probing classical and quantum dynamics of cold atoms, and we note its importance for future technological applications.

Epitaxial hexagonal boron nitride on Ir(111): A work function template

Hexagonal boron nitride (h-BN) is a prominent member in the growing family of two-dimensional materials with potential applications ranging from being an atomically smooth support for other 2D materials to templating growth of molecular layers. We have studied the structure of monolayer h-BN grown by chemical vapour deposition on Ir(111) by low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS) experiments and state-of-the-art density functional theory (DFT) calculations. The lattice-mismatch between the h-BN and Ir(111) surface results in the formation of a moir\'e superstructure with a periodicity of $\sim$29 \AA\ and a corrugation of $\sim$0.4 \AA. By measuring the field emission resonances above the h-BN layer, we find a modulation of the work function within the moir\'e unit cell of $\sim$0.5 eV. DFT simulations for a 13-on-12 h-BN/Ir(111) unit cell confirm our experimental findings and allow us to relate the change in the work function to the subtle changes in the interaction between boron and nitrogen atoms and the underlying substrate atoms within the moir\'e unit cell. Hexagonal boron nitride on Ir(111) combines weak topographic corrugation with a strong work function modulation over the moir\'e unit cell. This makes h-BN/Ir(111) a potential substrate for electronically modulated thin film and hetero-sandwich structures.

Inertial focusing of small particles in wavy channels: Asymptotic analysis at weak particle inertia

The motion of tiny non-Brownian inertial particles in a two-dimensional channel flow with periodic corrugations is investigated analytically, to determine the trapping rate as well as the exact position of the attractor, and understand the conditions under which particle trapping and long-term suspension occur. This phenomenon has been observed numerically in previous works and happens under the combined effects of confinement and inertia. Starting from the particle motion equations, a Poincaré map is constructed analytically in the limit of weak inertia and weak channel corrugations. It enables to derive the equation of the attractor, if any, and the corresponding trapping rate. The attractor is close to a streamline, the so-called “attracting streamline”, and is shown to persist in the presence of transverse gravity, provided the channel Froude number is large enough. Particles which are trapped by this streamline can therefore travel over long distances, avoiding deposition. Numerical simulations confirm the theoretical results at small particle response times τ and reveal some non-linear effects at larger τ: the asymptotic attractor becomes unstable at some critical value and splits into multiple branches each with its own basin of attraction.

The stray capacitance effect in Kelvin probe force microscopy using FM, AM and heterodyne AM modes

The effect of stray capacitance on potential measurements was investigated using Kelvin probe force microscopy (KPFM) at room temperature under ultra-high vacuum (UHV). The stray capacitance effect was explored in three modes, including frequency modulation (FM), amplitude modulation (AM) and heterodyne amplitude modulation (heterodyne AM). We showed theoretically that the distance-dependence of the modulated electrostatic force in AM-KPFM is significantly weaker than in FM- and heterodyne AM-KPFMs and that the stray capacitance of the cantilever, which seriously influences the potential measurements in AM-KPFM, was almost completely eliminated in FM- and heterodyne AM-KPFMs. We experimentally confirmed that the contact potential difference (CPD) in AM-KPFM, which compensates the electrostatic force between the tip and the surface, was significantly larger than in FM- and heterodyne AM-KPFMs due to the stray capacitance effect. We also compared the atomic scale corrugations in the local contact potential difference (LCPD) among the three modes on the surface of Si(111)-7 × 7 finding that the LCPD corrugation in AM-KPFM was significantly weaker than in FM- and heterodyne AM-KPFMs under low AC bias voltage conditions. The very weak LCPD corrugation in AM-KPFM was attributed to the artefact induced by topographic feedback.

Electronic structure study of ordering and interfacial interaction in graphene/Cu composites

Graphene CVD-grown on Cu has been studied using Raman spectroscopy, X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES). Raman data indicate the presence of weak compressive strain at the interface of graphene/Cu. Compared with highly ordered pyrolytic graphite (HOPG), new electronic states in the conduction band are observed for graphene/Cu, which are mainly ascribed to the defect states and interfacial interaction between the single graphene layer and Cu surface. Moreover, polarization dependent XAS measurements demonstrate that the graphene/Cu exhibits a high degree of alignment and weak corrugation on the surface. Significant intensity modulation in the resonant XES spectral shape upon different excitation energies near the C K-edge indicates that graphene layer preserves an intrinsic momentum as that of HOPG and the interaction between graphene and Cu shows weak influence on the valence band structure of graphene. However, broad inelastic features and subtle peak shifts are observed in the resonant XES spectra of graphene/Cu in comparison of HOPG, which can be mainly attributed to the electron–phonon scattering and charge transfer from the interfacial interaction of graphene and Cu substrate.

Graphene domain boundaries on Pt(111) as nucleation sites for Pt nanocluster formation

The growth of Pt nanoclusters on a graphene layer on Pt(111) was studied with ultra high vacuum scanning tunneling microscopy. Different periodicities in the moiré patterns of the graphene layer are observed corresponding to different orientations with respect to the Pt(111) lattice. Various graphene orientations are possible because of a relatively weak graphene–Pt interaction. Following Pt deposition onto the graphene-covered surface, small Pt nanoclusters are observed to preferentially form along the moiré domain boundaries. The weak interaction of graphene with Pt(111) leads to a weak corrugation in the superlattice compared to other transition metals, such as Ru, but it is found even this weak corrugation is sufficient to serve as a template for the formation of mono-dispersed one-dimensional Pt nanocluster chains. These Pt nanoclusters are relatively stable and only undergo agglomeration at annealing temperatures above 600K.

Relating temperature dependence of atom scattering spectra to surface corrugation

It is suggested that a measurement of the temperature dependence of the most probableintensity of energy-resolved atom–surface scattering spectra can reveal the strength of thesurface corrugation. To support this conjecture, a classical mechanical theory ofatom scattering from a corrugated surface, valid in the weak corrugation limit,is developed. The general result for the scattering probability is expressed interms of spatial integrals over the impact parameter within a surface unit cell.For the case of a one-dimensional corrugation, approximate expressions for thescattering probability are obtained in terms of analytic closed form expressions. Asan indicator of its relation to experimental measurements, calculations using aone-dimensional corrugation model are compared with data for Ar scattering from amolten Ga surface and an approximate value of the corrugation height parameter isextracted.

Influence of Lipid-Bilayer-Associated Molecules on Lipid-Vesicle Adsorption

Supported lipid bilayers (SLBs) containing different types of bilayer-associated molecules (membrane-bound molecules) where one part of the molecule resides inside the lipid bilayer and another part of the molecule sticks out of the bilayer (e.g., membrane proteins) are important biophysical model systems. SLBs are commonly formed via lipid vesicle adsorption on certain surfaces (e.g., SiO(2)). However, vesicles doped with different types of (bio)molecules often do not form an SLB on the surface, and the reasons for this are not clear. Using a newly developed model of a lipid bilayer, simulations were performed to clarify the influence of the bilayer-associated molecules on vesicle adsorption and rupture. It is shown that by increasing the concentration of membrane-bound molecules in the vesicles the tendency for vesicle rupture decreases markedly and for a certain concentration rupture does not happen. The reason for this is that vesicles containing significant concentrations of such molecules tend to deform less on the surface (lower vesicle strain), especially for a significantly corrugated bilayer-surface potential. After vesicle rupture, membrane-bound molecules face either the surface or the solution in the resulting bilayer patch on the surface, depending on whether the molecules point outward or inward in the original vesicle, respectively. Vesicle surface diffusion is also studied for weak and strong surface corrugation, where vesicles are found to be almost immobile in the latter case.

Experimental Three-Dimensional Description of the Liquid Hexadecane/Graphite Interface

By using an atomic force microscope based on a quartz tuning fork sensor, a 3-dimensional description of the interface between liquid hexadecane and a highly oriented pyrolytic graphite surface can be achieved at room temperature. The C16H34 monolayer in contact with the substrate surface exhibits a lamellar structure whereas no observation at the liquid/graphite interface by scanning tunnelling microscopy was reported for this alkane. The second layer shows very weak corrugations corresponding to lamella boundaries. Force/distance curves show at least four oscillations separated by 0.4 nm except for the first period with a 0.38 nm distance that corresponds to the layer closer the substrate. Such a description agrees well with molecular dynamics results obtained on alkane/solid interfaces.

Excitation of surface plasmon polaritons along the sinusoidal boundary of a metamaterial

We present a detailed analysis about the excitation of surface polaritons (SPs) at the sinusoidally corrugated interface between a conventional dielectric medium and a metamaterial (MM) with arbitrary values of dielectric permittivity and magnetic permeability. The strong impact of SPs on the optical response of the interface is demonstrated by numerical examples obtained with a rigorous electromagnetic code. Conditions leading to total absorption of the incident power and to near-field enhancements are explored in new SP regimes provided by the emergence of MMs. In those regimes where waves incident on the surface without corrugation suffer reactive attenuation, we find SP effects very similar to those occurring for metallic gratings, where the introduction of a weak corrugation in an otherwise highly reflecting surface can turn its reflectivity to zero. New SP effects are identified in regimes corresponding to ideally transparent MMs with a negative index of refraction. In these regimes, contrary to the case of metallic gratings, the introduction of a weak corrugation in an otherwise poorly reflecting surface produces high reflectivities in angular regions corresponding to SP excitation. Moreover, maximal absorption of incident waves and maximal intensity of the excited SP do not occur under the same conditions in these regimes. The complex propagation constants of the SPs supported by the corrugated interface are approximately obtained by fitting a phenomenological expression to our numerical results.

Dynamic and Spectroscopic Studies of Single Molecules Physisorbed on Graphite Substrates. 2. Application to the Ammonia Molecule

In the present paper, the theoretical approach developed in paper 1 is applied to an NH(3) molecule adsorbed on a graphite substrate. The potential energy surfaces (PESs) for the interaction between the molecule and the graphite crystal are described in detail. The molecule exhibits two quasi-equivalent angular position minima of energy ("up" and "down") along the perpendicular axis to the surface. The PES calculations also indicate that the NH(3) molecule has a rotational motion that is moderately hindered, with an energy barrier value of about 14 meV and also a quasi-free lateral translational motion above the surface, indicating a weak corrugation of the graphite (0001) surface. The isosteric heat of adsorption is calculated and is in agreement with the experimental one. Finally, the infrared absorption spectra for the vibrational mode frequency regions are obtained.

Reflection characteristics of a PML with a shallow corrugation

The constitutive characteristics of anisotropic materials can be exploited to construct absorbers that provide reflectionless interfaces for waves at arbitrary angles of incidence. In this paper, we investigate how a weak corrugation affects the reflectivity of the anisotropic perfectly matched absorber developed by Sacks et al. (1995) for a flat interface. To do so, we develop a Rayleigh method to calculate the fields diffracted at the periodically corrugated boundary of an anisotropic absorber with constant constitutive tensors, which correspond to a planar (Cartesian) perfectly matched layer. We present numerical results in the nondiffractive regime (where only a specularly reflected wave can propagate) for sinusoidal corrugations with different groove height-to-period ratios. Our results show that the reflectivity of the anisotropic absorber near normal incidence remains very low (less than 0.4% for a 10% modulation), whereas it changes dramatically near grazing incidences.

On the Two-Dimensional Bragg Resonator

We derive systems of differential and integro-differential equations which allow one to correctly analyze eigenwaves and high-Q oscillations in coaxial systems with a weak two-period corrugation of the walls. The dispersion characteristics of the eigenwaves of a coaxial waveguide with such a corrugation are studied with allowance for the diffraction effects for the quasi-critical waves. In the case of the boundary-value problem with zero boundary conditions, which describes a specific type of coaxial resonators considered in [1], we obtain simplified formulas for the complex eigenfrequencies and Q-factors. The obtained expressions are indicative of the possible spectrum crowding of high-Q oscillations, which takes place as the longitudinal and/or transverse sizes of the resonator increase.

The weakly nonlinear interfacial stability of a core–annular flow in a corrugated tube

A core–annular flow, the concurrent axial flow of two immiscible fluids in a circular tube or pore with one fluid in the core and the other in the wetting annular region, is frequently used to model technologically important flows, e.g. in liquid–liquid displacements in secondary oil recovery. Most of the existing literature assumes that the pores in which such flows occur are uniform circular cylinders, and examine the interfacial stability of such systems as a function of fluid and interfacial properties. Since real rock pores possess a more complex geometry, the companion paper examined the linear stability of core–annular flows in axisymmetric, corrugated pores in the limit of asymptotically weak corrugation. It found that short-wave disturbances that were stable in straight tubes could couple to the wall's periodicity to excite unstable long waves. In this paper, we follow the evolution of the axisymmetric, linearly unstable waves for fluids of equal densities in a corrugated tube into the weakly nonlinear regime. Here, we ask whether this continual generation of new disturbances by the coupling to the wall's periodicity can overcome the nonlinear saturation mechanism that relies on the nonlinear (kinematic-condition-derived) wave steepening of the Kuramoto–Sivashinsky (KS) equation. If it cannot, and the unstable waves still saturate, then do these additional excited waves make the KS solutions more likely to be chaotic, or does the dispersion introduced into the growth rate correction by capillarity serve to regularize otherwise chaotic motions?We find that in the usual strong surface tension limit, the saturation mechanism of the KS mechanism remains able to saturate all disturbances. Moreover, an additional capillary-derived nonlinear term seems to favour regular travelling waves over chaos, and corrugation adds a temporal periodicity to the waves associated with their periodical traversing of the wall's crests and troughs. For even larger surface tensions, capillarity dominates over convection and a weakly nonlinear version of Hammond's no-flow equation results; this equation, with or without corrugation, suggests further growth. Finally, for a weaker surface tension, the leading-order base flow interface follows the wall's shape. The corrugation-derived excited waves appear able to push an otherwise regular travelling wave solution to KS to become chaotic, whereas its dispersive properties in this limit seem insufficiently strong to regularize chaotic motions.