Interface Profile Research Articles

Spontaneous dynamics of two-dimensional Leidenfrost wheels

Recent experiments have shown that liquid Leidenfrost drops levitated by their vapor above a flat hot surface can exhibit symmetry-breaking spontaneous dynamics (A. Bouillant et al., Nature Physics, 14 1188-1192, 2018). Motivated by these observations, we theoretically investigate the translational and rotational dynamics of Leidenfrost drops on the basis of a simplified two-dimensional model, focusing on near-circular drops small relative to the capillary length. The model couples the equations of motion of the drop, which flows as a rigid wheel, and thin-film equations governing the vapor flow, the profile of the deformable vapor-liquid interface and thus the hydrodynamic forces and torques on the drop. In contrast to previous analytical models of Leidenfrost drops levitating above a flat surface, which predict only symmetric solutions, we find that the symmetric Leidenfrost state is unstable above a critical drop radius: $R_1$ for a free drop and $R_2>R_1$ for an immobilized drop. In these respective cases, symmetry breaking is manifested in supercritical-pitchfork bifurcations into steady states of pure rolling and constant angular velocity. In further qualitative agreement with the experiments, when a symmetry-broken immobilized drop is suddenly released it initially moves at an acceleration $\alpha g$, where $\alpha$ is an angle characterizing the slope of the liquid-vapor profile and $g$ is the gravitational acceleration; moreover, $\alpha$ exhibits a maximum with respect to the drop radius, at a radius increasing with the temperature difference between the surface and the drop.

A Semianalytical Method to Fast Delineate Seawater‐Freshwater Interface in Two‐Dimensional Heterogeneous Coastal Aquifers

AbstractThe freshwater‐seawater interface is one of the most important regions in coastal aquifer systems, delineating the subsurface into zones with distinct fluid density and biogeochemical properties. Heterogeneity in hydraulic conductivity is inherent to geological formations, resulting in distinct interface profiles. Currently available analytical solutions are limited to homogeneous and stratified aquifers, and numerical simulations of variable‐density flow and transport are the only option to characterize the interface in a two‐ or three‐dimensional heterogeneous aquifer. This study presents a novel semianalytical method to fast delineate the steady seawater‐freshwater interface in a two‐dimensional, confined, heterogeneous aquifer with a constant flux inland boundary and indistinct seepage face at the vertical coastal boundary. The heterogeneous domain is conceptualized as a series of thin columns of stratified aquifers with the horizontal flow in each of them. The proposed approach is based on the concept of local transmissivity parameters and shows high accuracy in the interface delineation. Leveraging the ability to delineate the interface rapidly along with the Monte Carlo approach, we numerically investigate the stochastic behavior of the interface profile. The uncertainty in seawater intrusion is heavily influenced by the degree of heterogeneity and weakly influenced by the scale of heterogeneity. To homogenize the aquifer, we found that geometric mean generally underestimates the seawater intrusion and cannot be used as an effective parameter. We also showed that the near‐coast, near‐top region of the aquifer is the most influential region and should be characterized at a higher resolution to reduce uncertainties in estimating seawater intrusion.

Computational prediction of key heat transfer mechanisms and hydrodynamic characteristics of critical heat flux (CHF) in subcooled vertical upflow boiling

The present study investigates extensively the capability of computational fluid dynamics (CFD) to predict two-phase fluid flow and heat transfer characteristics of FC-72 flow boiling in vertical upflow. The computational method performs transient analysis to model the entire flow field and bubble behavior for highly subcooled flow boiling in a rectangular channel heated on two opposite walls at high heat flux conditions of about 81% of critical heat flux (CHF). Transient variations of local vapor fraction, thermal energy transfer and concentration, and wall temperature are estimated through the computational methodology based on the Volume of Fluid (VOF) approach combined with shear-lift modeling to overcome a fundamental weakness in modeling multiphase flows. Detailed information about bubble distribution in the vicinity of the heated surface, thermal conduction inside the heating wall, local heat fluxes passing through the solid-fluid interface, and velocity and temperature profiles, which are not easily observed or measured by experiments, is carefully evaluated. Predictions are compared to the experimental data for three sets of operating conditions. Overall, computationally predicted flow pattern, bubble behavior, and heat transfer parameters (such as wall temperature excursion and thermal energy concentration) clearly represent phenomena of premature CHF which take place slightly earlier than actual operating conditions. But, despite these slight differences, the present computational work does demonstrate the ability to effectively predict the severe degradation in heat transfer performance commonly encountered at heat fluxes nearing CHF.

Dynamic variation of interface shape in a liquid oxygen tank under a sinusoidal sloshing excitation

In this study, a numerical model is built to simulate the coupled process of fluid flow and thermal physical performance in a cryogenic liquid oxygen storage tank. Both the external heat leak and liquid-vapor phase change are taken into consideration. The convection thermal boundary is used to simulate the heat inputs. The sinusoidal sloshing excitation is applied on the tank wall and realized by User-defined functions. Moreover, the mesh motion treatment is adopted and coupled with the volume of fluid model to predict the movement of the liquid-vapor interface. The pressure data during fluid sloshing is selected to validate the proposed numerical model, which shows the present model has great accuracy in predicting fluid sloshing with the relative error being less than 5.0%. The grid-independence study is conducted as well. Calculated by the present numerical model, the phase distribution is investigated. With the dynamic monitors being set, variations of the interface profiles are obviously displayed under different sloshing amplitudes. It shows the initial sloshing amplitude has obvious influences on the dynamic response of the liquid-vapor interface. During the whole sloshing process, the vapor is cooled by the subcooled liquid. While for the effect of the phase change on variation of the free interface, it becomes greatly prominent with the increase of the sloshing amplitude. Moreover, with the consideration of interface phase change, both the sloshing moment and liquid pressure decrease with time, which is largely different from the parameter variations calculated without considering of the interface phase change. With some valuable conclusions obtained, the present study is significant to the in-depth investigation on the non-isothermal sloshing dynamic process in ocean engineering.

Evaporation driven detachment of a liquid bridge from a syringe needle in repose

In this paper, a study of the stability of an evaporating semi-unbounded axisymmetric liquid bridge that forms between a syringe needle tip and a horizontal interface by using both theory and experiments is presented. Here, the evaporation produces slow quasistatic motion such that it allows one to use hydrostatics to analyze interface profiles via solutions to the Young–Laplace equation. The two main parameters, in the hydrostatic limit, are the familiar Bond number and a slenderness parameter that often appears in the literature that studies liquid bridge stability. The axisymmetric Young–Laplace equation yields a semi-analytical solution for capillary pressure at zero Bond number using boundary conditions appropriate for this study. At finite Bond numbers, computation of interface profiles is used to estimate the maximum slenderness. Experiments using water for Bond numbers 0.01 < Bo < 0.1 show good agreement for the maximum slenderness when comparing those results with predictions based on solutions to the Young–Laplace equation.

Effect of Weld Parameters on the Microstructure of Aluminum-Copper Joints Produced by Flash Welding

The effects of operational conditions on dissimilar Al-Cu flash welded joint quality have been studied using a series of industrial welding experiments. Macro- and microstructural analyses of dissimilar aluminum-copper joints produced by flash butt welding are performed in the present work. Various process conditions including standard and non-standard operational conditions were assessed during the welding experiments. Optical and scanning electron microscopy and electron dispersive x-ray spectroscopy are used to investigate and assess the metallurgical joints created through this process. For suboptimal process conditions, voids (5 mm wide, 1 mm deep) were observed at the joint interface. Some of the voids contained remnants of molten metal while some were empty. Al-Cu intermetallic compounds at the joint area were investigated, and their chemical compositions are determined. The profile of the joint interface was observed to be wavy which is attributed to the voids on the copper side and insufficient material flow of the copper during the upset. The effects of welding parameters on the waviness of the interfaces are investigated. The results show that the waviness of the interface increases with increasing the heat intensity (heat/joint area). No correlation was observed between the flashing speed and the waviness of the interfaces.

Steady-state multiple near resonances of periodic interfacial waves with rigid boundary

Steady-state resonant interfacial waves in a two-layer fluid within a frictionless duct are investigated theoretically. A combination of the homotopy analysis method (HAM) and Galerkin’s method is used to search for accurate steady-state resonant solutions with multiple near resonances. In the HAM, a piecewise parameter in the auxiliary linear operators is introduced to remove the small divisors caused by nearly resonant components. Convergent series solutions are then provided to the Galerkin iterations to accelerate the convergence rate. It is found that weakly nonlinear steady-state resonant waves form a continuum in the parameter space. As nonlinearity (wave steepness) increases, energy appears to be progressively shifted to sideband frequency components, effectively broadening the spectrum. The corresponding interfacial wave profile exhibits an almost fixed spatial pattern of repeated relatively high frequency, high-amplitude bursts followed by low-amplitude, longer waves. On examining the influence of density ratio, though changing slightly, the upper layer enlarges the amplitude of components near primary ones, which reduces the amplitude of higher frequency components, enlarges the wave steepness, and reduces the horizontal velocity in the wave field. Our results indicate that steady-state systems with resonant interactions among periodic interfacial wave components could occur naturally in the ocean. All these should enhance our understanding of periodic resonant interfacial waves.

Smoothed particle hydrodynamics simulation of converging Richtmyer–Meshkov instability

The Smoothed Particle Hydrodynamics (SPH) method based on the Harten–Lax–van Leer Riemann solver is improved to study converging Richtmyer–Meshkov instability (RMI). A new density summation algorithm is proposed, which greatly suppresses the pressure oscillation at the material interface. The one-dimensional Sod problem is first simulated for code verification. Then, the SPH program is extended to two dimensions to simulate the converging RMI at a square air/SF6 interface, and the numerical results compare well with the experimental ones [Si et al., “Experimental investigation of cylindrical converging shock waves interacting with a polygonal heavy gas cylinder,” J. Fluid Mech. 784, 225–251 (2015)]. Nonlinear mode coupling and pressure disturbance are found to act evidently, causing a very fast growth spike. Performing a Fourier analysis of the interface profiles, amplitude growths of the first three harmonics are obtained. The first harmonic presents an increasing growth rate at early stages due to geometric convergence. The second harmonic experiences a long period of linear growth due to the counteraction between geometric convergence and nonlinearity, whereas the third harmonic saturates very early for stronger nonlinearity. For all three harmonics, the perturbation growth rate reduces evidently at the late stage due to the Rayleigh–Taylor stabilization caused by interface deceleration. It is found that the instability growth at early stages depends heavily on the incident shock strength, while the late-stage asymptotic growth rate is nearly constant, regardless of shock strength. It is also found that intensifying the incident shock is an effective way to produce extreme thermodynamic state at the geometric center even though it causes a faster instability growth.

Numerical platform for the interface analysis of magnetic fluids by use of the Boundary Element Method

For the stability analysis or dynamic analysis of the interface phenomena of magnetic fluids as the Rosensweig instability, this paper presents mainly a magnetic analysis with reduced loads, since magnetic fields must be re-calculated every time step the interface changes. This method adopts the indirect boundary element method (IBEM), since the behavior of the surface can be explained mainly by the magnetic stress difference obtained from the interface magnetic field. Starting with Green’s theorem applied for harmonic fields, the magnetic potential and the normal magnetic induction are obtained separately through the density of singular sources on boundaries. The problems of sharp-pointed peaks on the interface, or edges and corners where boundaries cross are avoided easily. The magnetic field, the interface stresses and the fluid velocity are calculated on the largely-deformed interface in a two-layered system of fluid and vacuum domain under a homogeneous vertical magnetic field. In consideration of the nonlinear equation of motion of the pattern amplitude (Gollwitzer et al., 2010), the sum of interface stresses and the normal fluid velocity are shown in the phase space of the height of the solitary-wave interface profile and the intensity of the applied magnetic induction. This IBEM is compared with another method called Magnetic Analysis for General Use (MAGU, Mizuta, 2011) on the formulation basis, and their extension to the nonlinear magnetization is discussed.

Effects of interfacial stress in phase field approach for martensitic phase transformation in NiAl shape memory alloys

A phase field approach for phase transition between austenite to martensitic variants and twinning between martensitic variants is presented with the main focus on the effects of interfacial stress on phase transformations. In this theory, each variant-variant phase transformations and twinnings within each martensitic variant can be represented by only one-order parameter. Thus, it allows us to get the analytical solution of the martensite-martensite interface profile, energy, and width. Moreover, this model allows us to include interface stress which is consistent with the sharp interface limit. The finite element method is utilized to solve the coupled phase field and elasticity equations for a cubic to tetragonal phase transformation in NiAl shape memory alloy. The stress fields are obtained and the effects of interfacial stress on the stress field at the interfaces are studied for both austenite–martensite and martensite–martensite interfaces in detail. Additionally, the temperature-induced growth of the martensitic phase inside austenite, martensitic phase transformation, and twining are solved. The evolution of the microstructure and stress fields are obtained and the effects of interfacial stress on the morphological evolution of martensitic nanostructures are examined. It is found that the interfacial stress is an important factor, influencing the stress distribution at interfaces and the phase field solution significantly. This theory can be extended for electric, reconstructive, and magnetic phase transformations.

Bactericidal coating to prevent early and delayed implant-related infections

The occurrence of an implant-associated infection (IAI) with the formation of a persisting bacterial biofilm remains a major risk following orthopedic biomaterial implantation. Yet, progress in the fabrication of tunable and durable implant coatings with sufficient bactericidal activity to prevent IAI has been limited. Here, an electrospun composite coating was optimized for the combinatorial and sustained delivery of antibiotics. Antibiotics-laden poly(ε-caprolactone) (PCL) and poly`1q`(lactic-co glycolic acid) (PLGA) nanofibers were electrospun onto lattice structured titanium (Ti) implants. In order to achieve tunable and independent delivery of vancomycin (Van) and rifampicin (Rif), we investigated the influence of the specific drug-polymer interaction and the nanofiber coating composition on the drug release profile and durability of the polymer-Ti interface. We found that a bi-layered nanofiber structure, produced by electrospinning of an inner layer of [PCL/Van] and an outer layer of [PLGA/Rif], yielded the optimal combinatorial drug release profile. This resulted in markedly enhanced bactericidal activity against planktonic and adherent Staphylococcus aureus for 6 weeks as compared to single drug delivery. Moreover, after 6 weeks, synergistic bacterial killing was observed as a result of sustained Van and Rif release. The application of a nanofiber-filled lattice structure successfully prevented the delamination of the multi-layer coating after press-fit cadaveric bone implantation. This new lattice design, in conjunction with the multi-layer nanofiber structure, can be applied to develop tunable and durable coatings for various metallic implantable devices. This is particularly appealing to tune the release of multiple antimicrobial agents over a period of weeks to prevent early and delayed onset IAI.

Numerical simulation on internal stress evolution based on formation of thermally grown oxide in thermal barrier coatings

A finite element (FE) analysis method was developed to estimate internal stress in thermal barrier coatings (TBCs). The evolution equation of thermally grown oxide (TGO) based on oxygen diffusion in a bond coat (BC) was combined with the inelastic constitutive equation for a top coat (TC). Thermal cycling analysis to simulate start and stop of the gas turbine was performed using the developed analysis code. To investigate the influence of the TC/BC interfacial profile on thermal stress in the TBC, the profile of the analysis model was modeled with a sinusoidal waveform, with varying combinations of wavelength and amplitude. The results revealed that the growth of the TGO layer was a principal factor that increases thermal stress in the TC. The stress was therefore increased as it approached the TC/BC interface. Additionally, an investigation of the effect of the TC/BC interfacial profile on the stress revealed that stress increases with the increase in radius of curvature at the top of the sinusoidal waveform.

Impact of AlGaN/GaN Interface and Passivation on the Robustness of Low-Noise Amplifiers

Poststress dc characteristics of AlGaN/GaN HEMTs can be used to study the effect of high-power stress on the noise figure (NF) and gain of low-noise amplifiers (LNAs) subjected to large input overdrives. This enables a shift from circuit- to transistor-level measurements to investigate the impact of variations in HEMT design parameters on the robustness (including both recovery time and survivability) by mimicking LNA operation. Using this method, a tradeoff between survivability and recovery time is demonstrated for different AlGaN/GaN interface profiles (sharp interface, standard interface, and AlN interlayer). Furthermore, the impact of different surface passivation schemes (Si-rich, Si-poor, and bilayer SiN x ) on robustness is investigated. The bilayer passivation, which features low leakage current and small gain compression under overdrive stress, exhibits relatively weak survivability. The mechanisms influencing the robustness are analyzed based on transistor physics. The short recovery time is mainly due to impeding the injection of hot electrons into surface traps and high reverse current, whereas the survivability is dependent on the local or global peak electrical fields around the gate under high power stress.

Salt-responsive monoolein cubic phase containing polyethyleneimine gel

Monoolein (MO) cubic phases responsive to salt ions were prepared by immobilizing polyethyleneimine (PEI) in its water channel through its electrostatic attraction with octanoic acid (OA) intercalated in MO bilayer and ionically gelling PEI using tripolyphosphoric acid (TPPA). The interfacial tension of PEI/OA solution markedly decreased with increasing concentration and it was typical of the interfacial tension profile of a surfactant solution. TPPA remarkably lowered the transition temperature of MO cubic phase from about 63 °C to below 46 °C, possibly because the hydroxyl groups of TPPA participated in hydrogen bonding with the glycerol residue of MO. The cubic phases showed finger print–like patterns on TEM micrographs and neither OA, nor PEI, nor TPPA affected the structure of the cubic phases. When PBS was used as a release medium, the release degree of a dye (i.e. Rhodamine B) loaded in the cubic phase was more than two times higher than that obtained using distilled water. The salt ions contained in the buffer solution would be able to dissolve PEI gel via ion exchange and promote the release of dye out of the cubic phase.

Dual Monitoring of Surface Reactions in Real Time by Combined Surface-Plasmon Resonance and Field-Effect Transistor Interrogation.

By combining surface plasmon resonance (SPR) and electrolyte gated field-effect transistor (EG-FET) methods in a single analytical device we introduce a novel tool for surface investigations, enabling simultaneous measurements of the surface mass and charge density changes in real time. This is realized using a gold sensor surface that simultaneously serves as a gate electrode of the EG-FET and as the SPR active interface. This novel platform has the potential to provide new insights into (bio)adsorption processes on planar solid surfaces by directly relating complementary measurement principles based on (i) detuning of SPR as a result of the modification of the interfacial refractive index profile by surface adsorption processes and (ii) change of output current as a result of the emanating effective gate voltage modulations. Furthermore, combination of the two complementary sensing concepts allows for the comparison and respective validation of both analytical techniques. A theoretical model is derived describing the mass uptake and evolution of surface charge density during polyelectrolyte multilayer formation. We demonstrate the potential of this combined platform through the observation of layer-by-layer assembly of PDADMAC and PSS. These simultaneous label-free and real-time measurements allow new insights into complex processes at the solid-liquid interface (like non-Fickian ion diffusion), which are beyond the scope of each individual tool.

Nonlinear dynamics of thin liquid films subjected to mixed-frequency electrical field

The nonlinear dynamics of the interface between both perfect and leaky dielectric liquid films, interposed between two parallel electrodes, are investigated under the effect of mixed-frequency electric fields. A coupled system of evolution equations is derived in dimensionless form, by employing the long-wave approximation. The linear stability analysis is implemented in accordance with the characteristics of each specific case, namely, the constant (DC) and the altering (AC) fields. In particular, the response of the system to the multi-mode AC electrical field is analyzed. Assisted by the conclusions of the theoretical investigation, the initial-boundary-value problem associated with the coupled system of evolution equations is solved numerically for several parameter sets. The system behavior is studied by monitoring the evolution process and by examining the steady/quasi-steady pillar formations in the nonlinear regime. The possibility to generate interface profiles of diverse topological forms, to manipulate their features, and to control the time dependent progress and the film rupture by imposing different combinations of frequencies and/or amplitudes of the corresponding mode is confirmed.

Surface Propensity of Aqueous Atmospheric Bromine at the Liquid-Gas Interface.

Multiphase reactions of halide ions in aqueous solutions exposed to the atmosphere initiate the formation of molecular halogen compounds in the gas phase. Their photolysis leads to halogen atoms, which are catalytic sinks for ozone, making these processes relevant for the regional and global tropospheric ozone budget. The affinity of halide ions in aqueous solution for the liquid-gas interface, which may influence their reactivity with gaseous species, has been debated. Our study focuses on the surface properties of the bromide ion and its oxidation products. In situ X-ray photoelectron spectroscopy carried out on a liquid jet combined with classical and first-principles molecular dynamics calculations was used to investigate the interfacial depth profile of bromide, hypobromite, hypobromous acid, and bromate. The simulated core electron binding energies support the experimentally observed values, which follow a correlation with bromine oxidation state for the anion series. Bromide ions are homogeneously distributed in the solution. Hypobromous acid, a key species in the multiphase cycling of bromine, is the only species showing surface propensity, which suggests a more important role of the interface in multiphase bromine chemistry than thought so far.

Chiral pasta: Mixed phases at the chiral phase transition

Interiors of neutron stars are ultra-dense and may contain a core of deconfined quark matter. Such a core connects to the outer layers smoothly or through a sharp microscopic interface or through an intermediate macroscopic layer of inhomogeneous mixed phases, which is globally neutral but locally contains electrically charged domains. Here I employ a nucleon-meson model under neutron star conditions that shows a first-order chiral phase transition at large densities. In the vicinity of this chiral transition I calculate the free energies of various mixed phases - different 'pasta structures' - in the Wigner-Seitz approximation. Crucially, chirally broken nuclear matter and the approximately chirally symmetric phase (loosely interpreted as quark matter) are treated on the same footing. This allows me to compute the interface profiles of bubbles, rods, and slabs fully consistently, taking into account electromagnetic screening effects and without needing the surface tension as an input. I find that the full numerical results tend to disfavor mixed phases compared to a simple step-like approximation used frequently in the literature and that the predominantly favored pasta structure consists of slabs with a surface tension $\Sigma \simeq 6\, {\rm MeV}/{\rm fm}^2$.

COMPUTATION OF ROUGHNESS AT SUBSTRATE COATING INTERFACE ON DEPOSITION OF ALUMINIUM OVER MEDIUM CARBON STEEL BY FRICTION SURFACING

Friction surfacing is happening to be a solid-state lining process, which proposes a mean of producing perfect coating between alike and unlike materials utilizing mechanical interlocking at the coating interface. During friction surfacing, the low rotational speed with high axial force normally produces good quality bonding; however, poor mechanical interlocking and cavitation at coating substrate interface resulted in an adverse effect on bond quality. In this present work, the substrate plate was preheated at three different temperatures of 100°C, 200°C and 300°C to create a coating of aluminium 6063 over EN8 medium carbon steel using friction surfacing. The mechanical interlocking was recognized from the specimen image of longitudinal cross-sectional slices, received from optical microscopy. Image recognition software (ImageJ) which depicts the substrate coating interface produce the interfacial roughness profile. The deepness of interlocking at coating interfacial region was monitored at different substrate preheating temperatures done by filter profile graph. The degree of interlocking has been calculated by interfacial roughness values of the profile graph by the use of Python programming language. Then this was compared with a bond strength to identify the relationship among them. Further push off and bonding strength caused by the dovetail shaped interlocking have become higher compared to narrow shaped interlocks stated by profile graph.

Shape evolutions of moving fluid threads under asymmetrical confinements

This paper presents a combined experimental and numerical investigation designed to improve our understanding of how the shape of moving fluid threads evolves under asymmetrical confinements in both circular and square microchannels. Microfluidic devices with two junctions are designed to control the length of the fluid thread at the first junction and the deformation of the fluid thread at the second junction. Three different flow modes: nonbreakup, loosely confined breakup, and tightly confined breakup, are identified for varying lengths of fluid threads and capillary number, and two boundaries are identified between the three modes. The deformation dynamics of the fluid threads evolving as difference modes are addressed to consider the effects of thread length and capillary number. Numerical simulations are carried out to determine how the curvature evolves for different flow modes in the square microchannel. The evolution of interface profiles is obtained numerically over a wide range of capillary number. Stop-flow simulations are then carried out to identify both the critical shape for the onset of the capillary instability during tightly confined breakup and the corresponding curvature distribution. This critical shape is found to be corresponding to the fluid thread with the critical length at the transitive boundary between the loosely confined and tightly confined situations.