Surface Undulations Research Articles

RETRACTED: FEM analysis of the impact of surface undulations on the natural convective flow of viscous fluid in a permeable trapezoidal enclosure

Examination of the transport mechanism in a permeable trapezoidal enclosure with an undulation effect is commenced. A formulation describing naturally convective flow in a permeable domain is conceded by employing Boussinesq and Darcy approximations. Uniform temperature is provided at the circular cylinder and base wall of the enclosure, whereas non-parallel side extremities are kept cold. No heat flux condition is applied at a wavy surface (upper) to maintain the potential difference in temperature for generation of convection. A finite element scheme is opted to resolve the governing system for accounted physical problems. The grid sensitivity test is also executed to assure the credibility of the code and results. A wide range of physical parameters is selected to comprehend their impact on streamlines and isotherm patterns. Results are revealed comparatively for zero undulation (upper solid straight wall) and with undulations (wavy wall). Heat flux and kinetic energy are also enumerated as key quantities against concerning parameters. It is depicted that the average Nusselt number and kinetic energy are more in the absence of undulations than when it is present. Additionally, it is manifested that the placement of a heated cylinder helps transfer heat in the domain and the production of thermal convective potential.

Influence of applied stress on the corrosion resistance of micro-arc oxide coatings on AZ31B magnesium alloy

ABSTRACT A micro-arc oxidised (MAO) protective coating was formed onto AZ31B magnesium alloy samples which were then loaded into ‘C' rings to apply compressive and tensile stresses. The influence of these applied stresses on the protective ability of the coating in simulated body fluid was assessed by surface characterisation and electrochemical testing. It was found that the undulating surface and pore structure of the MAO ceramic coatings was affected by the stress during immersion with a greater number of closed pores under compression and more open pores, with additional cracks, under tension. During the earlier stages of immersion, compressive stresses enhanced the corrosion resistance whilst tensile stresses accelerated corrosion. However, at later stages of immersion both compressive and tensile stress caused enhanced corrosion with tensile stresses consistently worse than compressive stresses.

Effect of Mg/Si Ratio on the Bendability and Anisotropic Bend Behavior of Extruded 6000-Series Al Alloy.

Extruded Al-Mg-Si profiles applied in the automotive industry are required to achieve an appropriate combination of strength and bendability. In order to investigate the effect of Mg/Si ratio on the bendability and anisotropic bending behavior, AA6005 and AA6061C were extruded to 2 mm thick plates. More Goss texture and anisotropic particle clusters exist in AA6005 alloys with a low Mg/Si ratio, which leads to a high tendency of surface roughing and cracking and to strong anisotropy in their bendability. However, more low-angle grain boundaries, cubic texture and comparatively random distribution of particles exists in AA6061C alloys with a high Mg/Si ratio, which blunts the surface roughing and crack process. The surface undulation is the outcome of the strain-intense localization of several layers of grains in the vicinity of the outer elongated surface. The strain localization and surface undulation lead to shear band initiation near the valleys. Several cooperating micro-mechanisms in AA6005, including grain clusters with Goss and Cubic orientation, heterogeneously nucleated particles and grain boundary spatial arrangements, lead to the grain boundary decohesion along a shear direction. AA6005 shows for predominately intergranular fractures in nature, with some areas exhibiting grain boundary decohesion during bending in the TD. However, AA6061C shows a predominately transgranular in nature, with some areas exhibiting intergranular fracture, which is affected by shear band development.

Flow over a hydrofoil subjected to traveling wave-based surface undulation: effect of phase difference between surface waves and wave number

Flow over a hydrofoil subjected to traveling wave-based surface undulation: effect of phase difference between surface waves and wave number

Surface elasticity and area incompressibility regulate fiber beading instability

A continuum body endowed with an energetic surface can exhibit different mechanical behavior than its bulk counterpart. Soft polymeric cylinders under surface effects become unstable and form surface undulations referred to as the elastic Plateau–Rayleigh (PR) instability, exclusively driven by competing surface and bulk properties. However, the impact of surface elasticity and area compressibility, along with bulk compressibility, on the PR instability of soft solids remains unexplored. Here we develop a theoretical, finite deformations framework to capture the onset of the PR instability in highly deformable solids with surface tension, surface elasticity, and surface compressibility, while retaining the compressibility of the bulk as a material parameter. In addition to the well-known elastocapillary number, surface compressibility and a dimensionless parameter related to the surface modulus are found to govern the instability behavior. The results of the theoretical framework are analyzed for an exhaustive list of bulk and surface parameters and loading scenarios, and it is found that increasing surface elasticity and surface incompressibility preclude PR instability. Theoretical results are compared with high-fidelity numerical simulation results from surface-enhanced isogeometric finite element analysis and an excellent agreement is observed across a broad range of material parameters and large deformation levels. Our results demonstrate how surface effects can be used to (i) render stable soft structures and prevent PR instability when it occurs as an unwanted by-product of manufacturing techniques or (ii) tune the instability behavior for possible applications involving polymer fibers.

Energy Flows in Static and Programmable Catalysts

Programmable catalysts that change on the time scale of a catalytic cycle provide a new opportunity to control the flow of energy to reactants and products to promote faster and more selective chemistry. While traditional chemical manufacturing processes consume energy to achieve favorable reaction conditions, programmable catalysts aim to dynamically add or remove energy to catalytic cycles through perturbations of the catalytic surface via strain, charge, or light. These surface energy flows are quantified by the changes in adsorbate binding energy with time, and the overall efficiency relating energy inputs to catalytic performance is defined by the characteristics of the undulating catalytic surface. Understanding and quantifying energy flows in programmable catalysts provides baseline definitions and metrics for comparing dynamic conditions and identifying optimal catalytic performance for more efficient chemical manufacturing.

Normal and anomalous diffusion in a bouncing ball over an irregular surface.

The problem of a bouncing ball on a nonplanar surface is investigated. We discovered that surface undulation adds a horizontal component to the impact force, which acquires a random character. Some aspects of Brownian motion are found in the horizontal distribution of the particle. On the x axis, normal and superdiffusion are observed. For the probability density's functional form, a scaling hypothesis is presented.

Real Space and Time Imaging of Collective Headgroup Dipole Motions in Zwitterionic Lipid Bilayers

Lipid bilayers are supramolecular structures responsible for a range of processes, such as transmembrane transport of ions and solutes, and sorting and replication of genetic materials, to name just a few. Some of these processes are transient and currently, cannot be visualized in real space and time. Here, we developed an approach using 1D, 2D, and 3D Van Hove correlation functions to image collective headgroup dipole motions in zwitterionic phospholipid bilayers. We show that both 2D and 3D spatiotemporal images of headgroup dipoles are consistent with commonly understood dynamic features of fluids. However, analysis of the 1D Van Hove function reveals lateral transient and re-emergent collective dynamics of the headgroup dipoles-occurring at picosecond time scales-that transmit and dissipate heat at longer times, due to relaxation processes. At the same time, the headgroup dipoles also generate membrane surface undulations due a collective tilting of the headgroup dipoles. A continuous intensity band of headgroup dipole spatiotemporal correlations-at nanometer length and nanosecond time scales-indicates that dipoles undergo stretching and squeezing elastic deformations. Importantly, the above mentioned intrinsic headgroup dipole motions can be externally stimulated at GHz-frequency scale, enhancing their flexoelectric and piezoelectric capabilities (i.e., increased conversion efficiency of mechanical energy into electric energy). In conclusion, we discuss how lipid membranes can provide molecular-level insights about biological learning and memory, and as platforms for the development of the next generation of neuromorphic computers.

Irreversible thermodynamics of surfaces and interfaces: Special reference to the strained thin solid films on the substrates: Theory and practice

The realization of nanoscale devices largely depends on our ability to control and manipulate interfacial interactions and, thus, understanding of the mechanisms of surface/interface instabilities. In this work, theoretically as well as technologically important and distinct two thermodynamic systems, which are exposed to (isobaric) and isolated from (isochoric) external body forces and surface tractions, are formulated by using irreversible thermodynamics in combination with the generalized variational method. The starting point for the present formulation closely follows up the Fowler and Guggenheim [Statistical Thermodynamics (University Press, Cambridge, 1952)] interpretation of the Planck inequality [Über Prinzip Vermehrung Entropie: Ann. Phys. Series 2(32), 462 (1887)] for isothermal reversible and irreversible (natural) infinitesimal changes in heterogeneous systems (multi-phase and multi-component). By combining this fundamental principle with the interlink between the dissipation function and global internal entropy production postulates, two distinct sets of governing equations for the surface drift-diffusion flux as well as the rate of evaporation/condensation and/or the growth/recrystallization of amorphous solid thin films are obtained for isochoric and isobaric systems. The role of Eshelby's energy-momentum tensor in the generalized potential for the interface displacement is found to differ (opposite in sign) for isochoric and isobaric systems. To demonstrate the importance of these sign conflicts, two sets of computer experiments are performed on isochoric and isobaric systems. They showed us that the elastic strain energy density contribution to the generalized driving force for surface drift-diffusion alone favoring flat and smooth surfaces in isobaric systems regardless of the sign of the uniaxial stress (healing), rather than causing the surface roughness and even catastrophic crack initiation as the case in internally strained isochoric systems. Computer simulations allowed us to track down the dynamical behavior of test modules by furnishing surface and strain energy variations, combined with the Global Helmholtz free change, which indicates the existence of two regimes: initial smooth surface undulations followed up by the rather chaotic crack formation and propagation stage at the middle of the thin film supported by the stiff substrate. In this study, we mainly focused on the development kinetics of “Stranski–Krastanow” island-type morphology, initiated by the nucleation route rather than the surface roughening scheme. The physicomathematical model, which is based on the irreversible thermodynamics treatment of surfaces and interfaces with singularities [T. O. Ogurtani, J. Chem. Phys. 124, 144706 (2006)], furnishes us to have autocontrol on otherwise free-motion of the triple junction contour line between the substrate and the droplet without presuming any equilibrium dihedral contact (wetting) angles at edges. We have also demonstrated the formation of the Stranski–Krastanow (SK)-type doublet islanding (quantum dots) as a stationary nonequilibrium state in an epitaxially strained thin flat droplet on a rigid substrate by introducing the wetting potential—invoked by the quantum confinement—into the scenario and carefully selecting the system parameters (size and shape) for the isochoric system represented by [Ge/Si (100)]. It has been also shown that on the contrary to common perceptions, the Stranski–Krastanow islands are in genuine stationary nonequilibrium states in the sense of Prigogine if one invokes proper free-moving boundary conditions at triple junctions deduced from the irreversible thermodynamics rather than ad hoc periodic or reflecting constrains at the edges.

Microstructure enhanced biocompatibility in laser additively manufactured CoCrMo biomedical alloy

The present work investigated biocompatibility of the unique nanostructural surface morphology inherently evolved in laser-based additively manufactured CoCrMo after biocorrosion in simulated body fluid at physiological temperature (37 °C). The extremely rapid thermokinetics intrinsically associated with the laser-based additive manufacturing technique resulted in heterogeneous cellular dendritic solidification morphologies with selective elemental segregation along the cell boundaries within CoCrMo samples. Consequently, a selective and spatially varying electrochemical response resulted in generation of a nanoscale surface morphology (crests and troughs) due to differential localized electrochemical etching. Also, depth of the trough regions was a function of the applied potential difference during potentiodynamic polarization which resulted in samples with varying morphological ratio (depth of trough/width of cell wall). CoCrMo with such nanoscale surface undulations were proposed for enhanced biocompatibility in terms of viability, spreading, and integration of MT3C3 pre-osteoblasts cells elucidated via MTT assay, immunofluorescence, and microscopy techniques. Furthermore, the influence of the morphological ratio, characteristic to the additively deposited CoCrMo after electrochemical etching (biocorrosion) on biocompatibility of MT3C3 pre-osteoblasts cells was qualitatively and quantitatively compared to a mirror-polished flat CoCrMo surface.

Room-temperature in situ fabrication of ultrathin undulating layered hydroxide salt membranes for efficient H2/CO2 separation

Two-dimensional (2D) materials have demonstrated their superiorities as molecular sieve membranes for strategic gas separations. The promotion of these 2D membranes is confronted with great challenges such as elaborate fabrication procedures and expensive raw materials. Layered hydroxide salts (LHSs) are a kind of lamellar material involving economical ingredients and are quite easy-to-fabricate. However, they have yet to be developed into gas separation membranes and their separation potential has not been investigated, despite their merits. Here, we report the successful fabrication of ultrathin Zn5(OH)8(NO3)2·2H2O membranes by using a rapid and facile in situ growth strategy at room temperature. By virtue of the few-layered membrane thickness, the obtained membranes exhibited intriguing surface undulations, which endowed the membranes with extremely tortuous interlayer channels and therefore an auspicious molecular sieving ability. Consequently, these membranes showed an optimal H2/CO2 mixture separation selectivity of 792 ± 38 and a remarkable H2 permeance of 1607 ± 99 GPU. Moreover, with a demonstration of a gentle framework dehydration, the molecular sieving capacity of the LHS membrane was further sharpened at 150 °C owing to the narrowed interlayer channels. This work introduced a low-cost 2D membrane and manifested the advantages of fabricating few-layered 2D membranes, opening a new avenue for membrane future scale-up.

Experimental investigation on deballing and surface finishing of selective laser melted 18Ni300 steel using polymer rheological abrasive medium

Additively manufactured metallic parts compete with casted and forged parts in physical and mechanical properties. However, the presence of various surface defects, such as balling and staircase effects, lack of fusion, surface undulations, etc., hinders the direct functional application of the as-built part and demands further post-processing. Abrasive flow finishing (AFF) is an extensively employed post-processing technique to finish additively manufactured parts. This process uses polymer rheological abrasive medium (abrasive medium) to finish internal and external surfaces. Even though significant research has been carried out on the surface finishing of selective laser melted metallic parts, the mechanism of material removal and surface finish improvement is not explored much. In the present work, the mechanism of deballing and surface finish improvement on selective laser melted 18Ni300 steel using the AFF process is studied. A newly developed polysaccharide-based abrasive media with monomodal and bimodal abrasive particle distribution are employed for finishing. At an extrusion pressure of 7.5 MPa, the influence of abrasive particle distribution and the number of finishing cycles on the amount of material removed and percentage change in surface roughness (% ΔRa) is observed. It was found that the abrasive medium with bimodal abrasive particle distribution resulted in more material removal and % ΔRa than monomodal. Surface roughness reduction of up to 93% was obtained using the abrasive medium with bimodal abrasive particle distribution. It was noticed that the loosely adhered powder particles were removed within the initial 15 cycles, and the balling effect was completely eliminated within 75 cycles. After 375 cycles, the as-built side surface was free from all other surface defects, such as micro-cracks, shallow pits, material ridges etc. Based on the obtained results, a suitable material removal mechanism in selective laser melting parts using the AFF process was proposed.

Influence mechanism of laser delay on internal defect and surface quality in stitching region of 316L stainless steel fabricated by dual-laser selective laser melting

Multi-laser stitching additive manufacturing technology is an effective solution to break through the size limitation in single-laser additive manufacturing based on a powder-bed process, where excellent metallurgical bonding and high surface quality of the multi-laser stitching region (overlap) are the critical issues to ensure the final forming quality. The above metallurgical bonding and surface quality are not only affected by common process parameters but also sensitive to laser delay (including on and off delay) parameters because they influenced the starting and end regions of melt tracks. In this study, the influence of laser delay parameters on the forming quality of a single 316L stainless steel melt track was first investigated, thereby obtaining an optimization range of laser delay parameters. Then, the influence mechanism of laser delay on the internal pore defects and surface quality in the dual-laser stitching region was systematically studied. The results show that with the increase of laser on delay, there are four kinds of internal defects in the stitched region, namely keyhole, irregular pore, large-size continuous pore and non-overlap. In addition, due to the high energy density at the starting/end point of the melt tracks, powder denudation and solidified bulge were observed near the stitching line, which leads to the undulation problem with layer-by-layer accumulation on the surface in the stitching region. It is found that the surface undulation with a height of 202 μm and a width of 723 μm still exists at the optimized laser delay parameters (porosity of 0.96 %), which means new process strategies are also needed to be developed besides optimization of laser on/off delay parameters. In addition, a trade-off should be made between the low porosity and high surface quality due to the different influence trends of laser delay on them. This study provides a theoretical basis and technical support for reducing internal defects and improving surface quality in the stitching region fabricated by multi-laser selective laser melting (SLM).

Thermo-Convective Flows of Mono- and Hybrid-Nanofluids Over Horizontal Undulated Surfaces in a Porous Medium

This paper presents a comparative study between the thermo-convective flows of two mono- and one hybrid-nanofluid over three uniformly heated undulated surfaces (described by sinusoidal, sawtooth or triangular waveforms) embedded in a porous medium. The base fluid for each nanofluid is water, and the nanoparticles are copper, alumina or a copper-alumina mixture. Two different types of materials of the porous medium have been considered: aluminum foam and glass balls. This problem is governed by a system of nonlinear, coupled partial differential equations, which is solved using the Keller-Box method. The influences of each porous medium and the pertinent parameters on the nanofluid flows and heat transfer have been explored. It is seen that secondary flow occurs at large amplitudes of the surface undulations for the sinusoidal and triangular waveforms, but no such flow is observed in the case of the sawtooth waveform and the flat surface. To assess the heat transfer properties, the mean Nusselt number has been calculated. It is observed that the mean Nusselt number is higher in the porous medium of glass balls than aluminum foam. The heat transfer on the undulated surfaces is the highest in the case of a hybrid-nanofluid and the least for water-alumina.

Baroclinic critical layer in a viscous stratified boundary layer flow on an undulated tilted surface

The present paper investigates theoretically and experimentally the boundary layer generated by a stably stratified fluid flowing horizontally along a surface tilted in the transverse direction and deformed by sinusoidal undulations with crests perpendicular to the flow direction. In the absence of undulations, a weak transverse velocity proportional to the normal velocity is created such that the flow remains purely horizontal. In the presence of undulations of amplitude $h$ , a stronger transverse flow is generated that exhibits a singular behaviour at the critical altitude where the frequency of the perturbation matches the buoyancy frequency of the fluid. This baroclinic critical layer was previously analysed by Passaggia et al. (J. Fluid Mech., vol. 751, 2014, pp. 663–684) for a boundary layer flow with a small sliding velocity on the surface. Here, the no-slip boundary condition of the experimental flow is applied. For this purpose, we solve the viscous sub-layer to obtain a complete theoretical model for the solution in the critical layer without any adjusting parameter. The theoretical predictions for the transverse velocity are compared with experimental measurements, and a good quantitative agreement is demonstrated. Compared with the sliding case, the no-slip boundary condition on the surface reduces the amplitude of the critical layer solution by a factor $Re^{-1/3}$ , where the Reynolds number Re is defined using the velocity at infinity and the thickness of the boundary layer. As a consequence, the transverse velocity has a maximum in the critical layer of order $h$ , but it still induces a shear rate of order $h\,Re^{1/3}$ .

Finite-element simulation for crystals with surface undulations based on Takagi–Taupin theory

A finite-element method based on a weak formulation of the Takagi–Taupin equations was adopted to study the X-ray diffraction of crystals with surface undulations. A general diffraction geometry was simulated to investigate the diffraction features caused by the surface undulations. The numerical results reveal that the effects of surface undulations on Bragg diffraction are local for those limited in the low-frequency range and physically result from the refractive effect, brought on by the variation of the local asymmetry angle. Thus, a formula based on the local perfect flat crystal approximation was introduced to efficiently evaluate the change in the direction of the diffracted wave caused by surface undulations.

High-temperature oxidation and hot corrosion of Ni-based single crystal superalloy in the incubation stage

Early stages of high-temperature oxidation and hot corrosion of Ni-based single-crystal superalloy were studied at 900 °C by introducing sulfate coating. Detailed elemental and microstructural analysis revealed layered surface oxide, atomic diffusion, faceted Al-rich precipitation, and γ′-phase depletion, together with grain coarsening and reorientation in the near-surface region. Undulated surface, multiple layered outer oxide scale, inner CrS scale (solely localized in γ-matrix), and an additional γ′-depleted region without crystal reorientation were observed in hot corrosion rather than in high-temperature oxidation. The corrosion behavior is discussed in terms of atomic diffusion and oxide growth. The impact of sulfate-coating is elucidated.

Converging-diverging shock-driven instabilities along soft hydrogel surfaces.

Intense surface eruptions are observed along the curved surface of a confined cylindrical film of hydrogel subject to laser-induced converging-diverging shock loading. Detailed numerical simulations are used to identify the dominant mechanisms causing mechanical instability. The mechanisms that produce surface instability are found to be fundamentally different from both acoustic parametric instability and shock-driven Richtmyer-Meshkov instability. The time scale of observed and simulated eruption formation is much larger than that of a single shock reflection, in stark contrast to previously studied shock-driven instabilities. Moreover, surface undulations are only found along external, as opposed to internal, soft solid boundaries. Specifically, classic bubble surface instability mechanisms do not occur in our experiments and here we comment only on the new surface undulations found along the outer boundary of solid hydrogel cylinders. Our findings indicate a new class of impulsively excited surface instability that is driven by cycles of internal shock reflections.

A reasonable inlet boundary for wind simulation based on a trivariate joint distribution model

In mountainous regions, the height of the atmospheric boundary layer (ABL) plays a crucial role in accurately simulating the wind field. In order to uncover the correlation between the complex terrain and the ABL height, the required samples were obtained via the WRF experiment taking a mountainous region in Southwestern China as the research object. The Pair-Copula decomposition method was then employed to create a trivariate joint distribution model of the atmospheric boundary layer elevation, the terrain elevation, and the surface relief degree based on the samples. The trivariate joint distribution model shows that the ABL field fluctuates more in the regions with higher surface undulation and less in the regions with lower surface undulation. Furthermore, the model can be used to evaluate the probability distribution of the ABL height over the complex terrain where a projected bridge will locate, thereby providing a more reasonable inlet boundary to simulate the wind environment surrounding the bridge. The method presented here for correlating the ABL height with terrain is universal and can be extended to wind field simulation in many regions.

Study on Secondary Electron Yield of Dielectric Materials

The secondary electron emission coefficient of dielectric materials will have an important impact on the performance of electronic devices and equipment. In order to understand the secondary electron emission coefficient of common dielectric materials, the collection electrode negative bias method is introduced in this study. First, the measurement method of secondary electron emission coefficient of dielectric materials is analyzed, and 20 common dielectric materials are introduced, the secondary electron emission coefficients of 20 kinds of common dielectric materials were measured and analyzed, which laid the foundation for the subsequent application of dielectric materials. The results show that among common dielectric materials, the secondary electron emission coefficients of alumina, silica and muscovite are relatively high, while those of polyimide, polycarbonate and polyester are relatively low; The surface roughness of the material will also have an important impact on the secondary electron emission coefficient. With the gradual increase of the surface roughness of the material, the undulation of the material surface will have a shielding effect and absorption effect on the secondary electrons, which will eventually lead to the gradual reduction of the secondary electron emission coefficient of the material. The research results can provide data support for the establishment of the database of secondary electron emission characteristics of common dielectric materials.