Interface Perturbations Research Articles

On Richtmyer–Meshkov finger collisions in a light fluid layer under reshock conditions

The fingers known as bubbles (spikes) resulting from the penetration of light (heavy) fluids into heavy (light) fluids are significant large-scale features of Richtmyer–Meshkov instability (RMI). Through shock-tube experiments, we study finger collisions in light fluid layers under reshock conditions. Four unperturbed fluid layers with varying thicknesses are created to analyse the motion of waves and interfaces during finger collisions. The wave dynamics, sensitive to initial layer thicknesses, are characterized by a one-dimensional theory. Eight perturbed fluid layers, with four thicknesses and two interface phase combinations, are generated to explore the finger collision mechanism. It is shown that after reshock, the initial in-phase and anti-phase cases undergo spike–bubble rear-end collisions (SBCs) and spike–spike head-on collisions (SSCs), respectively. Compared with SBCs, SSCs significantly suppress spike growth, leading to the attenuation of perturbation growth, especially for larger thicknesses. As the initial thickness decreases, an SSC impedes the downstream interface from reversing its phase, resulting in abnormal RMI, thereby reducing the SSC's effectiveness in attenuating growth. The effects of rarefaction waves enhance both interfaces’ amplitudes and the whole layer's thickness, diminishing the intensity of finger collisions, while the second reshock exerts an opposing influence. Linear and nonlinear models, incorporating the influence of reshocks and rarefaction waves, are developed to predict the interface perturbation growth before and after finger collisions.

Asymptotic analysis of sign-changing transmission problems with rapidly oscillating interface

We study the asymptotic behavior of a sign-changing transmission problem, stated in a symmetric oscillating domain obtained by gluing together a positive and a negative material, separated by an imperfect and rapidly oscillating interface. The interface separating the two heterogeneous materials has a periodic microstructure and is a small perturbation of a flat interface. The solution of the transmission problem is continuous and its flux has a jump on the oscillating interface. Under certain conditions on the properties of the two materials, we derive the limit problem and we prove the convergence result. The T-coercivity method is used to handle the lack of coercivity for both the microscopic and the macroscopic limit problems. For more information see https://ejde.math.txstate.edu/Volumes/2024/60/abstr.html

Experimental investigation of three-dimensional Rayleigh–Taylor instability of a gaseous interface

Validating the theoretical work on Rayleigh–Taylor instability (RTI) through experiments with an exceptionally clean and well-characterized initial condition has been a long-standing challenge. Experiments were conducted to study the three-dimensional RTI of an SF $_6$ –air interface at moderate Atwood numbers. A novel soap film technique was developed to create a discontinuous gaseous interface with controllable initial conditions. Spectrum analysis revealed that the initial perturbation of the soap film interface is half the size of an entire single-mode perturbation. The correlation between the initial interface perturbation and Atwood numbers was determined. Due to the steep and highly curved feature of the initial soap film interface, the early-time evolution of RTI exhibits significant nonlinearity. In the quasi-steady regime, various potential flow models accurately predict the late-time bubble velocities by considering the channel width as the perturbation wavelength. Differently, the late-time spike velocities are described by these potential flow models using the wavelength of the entire single-mode perturbation. These findings indicate that the bubble evolution is influenced primarily by the spatial constraint imposed by walls, while the spike evolution is influenced mainly by the initial curvature of the spike tip. Consequently, a recent potential flow model was adopted to describe the time-varying amplitude growth induced by RTI. Furthermore, the self-similar growth factors for bubbles and spikes were determined from experiments and compared with existing studies, revealing that a large amplitude in the initial soap film interface promotes the spike development.

Constructed Mott–Schottky Heterostructure Catalyst to Trigger Interface Disturbance and Manipulate Redox Kinetics in Li-O2 Battery

HighlightsA carbon free self supported Mott-Schottky heterostructure was constructed as an efficient cathode catalyst for lithium oxygen batteries, achieving homogeneous contact between the two materials for strong interfacial interactions.The heterostructure triggered interfacial perturbations and band structure changes, which accelerated oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) kinetics, resulting in an extremely long cycle life of 800 cycles and an extremely low overpotential of 0.73 V.Combined with advanced characterization techniques and density functional theory calculations, the underlying mechanism behind the boosted ORR/OER activities and the electrocatalytic mechanism were revealed.

Molecular insights into the impact of mineral pore size on methane hydrate formation

Natural gas hydrates are not only substantial energy sources but also have significant applications in the chemical industry and other fields. Although investigating hydrate formation in sediment minerals is crucial for their development and utilization, the underlying hydrate formation mechanism remains unclear. Here, molecular simulations were conducted in systems incorporating hydrophobic and hydrophilic pores of different sizes to investigate methane hydrate formation processes. The findings suggest that, as the hydrophobic slit size increases, there is a larger number of dissolved methane after the system reaches a metastable equilibrium state. The probability of cage formation indicates that hydrate cages readily form on hydrophobic surfaces or in the solution phase near the solution/gas interface. The larger slits are preferred for hydrate nucleation, regardless of whether the surface is hydrophobic, with most initial nuclei located near the liquid/methane interface. However, the interface perturbation can lead to the movement and growth of hydrate nuclei near the solution/methane interface into the bulk solution phase. Additionally, hydrate can nucleate and grow on the hydrophobic surface, facilitated by the adsorbed methane molecules and nonstandard cages. Pores hinder methane storage capacity in the hydrate phase due to the confinement effect and the amorphous nature of the hydrate formed. These molecular-level findings enhance our understanding of hydrate formation in sedimentary environments and porous materials, benefiting the development of natural gas hydrates and the use of porous materials for gas storage and transportation.

Fragmentation of colliding liquid rims

We present direct numerical simulations of the splashing process between two cylindrical liquid rims. This belongs to a class of impact and collision problems with a wide range of applications in science and engineering, and motivated here by splashing of breaking ocean waves. Interfacial perturbations with a truncated white noise frequency profile are introduced to the rims before their collision, whose subsequent morphological development is simulated by solving the two-phase incompressible Navier–Stokes equation with the adaptive mesh refinement technique, within the Basilisk software environment. We first derive analytical solutions predicting the unsteady interfacial and velocity profiles of the expanding sheet forming between the two rims, and develop scaling laws for the evolution of the lamella rim under capillary deceleration. We then analyse the formation and growth of transverse ligaments ejected from the lamella rims, which we find to originate from the initial corrugated geometry of the perturbed rim surface. Novel scaling models are proposed for predicting the decay of the ligament number density due to the ongoing ligament merging phenomenon, and found to agree well with the numerical results presented here. The role of the mechanism in breaking waves is discussed further and necessary next steps in the problem are identified.

Numerical modeling of a confined falling liquid film sheared by a gas flow in a plate heat exchanger of an NH3/H20 absorption chiller

In recent years, the demand for cooling has seen a substantial increase, primarily due to rising summertime temperatures. Conventional mechanical compression air conditioners have played a predominant role in fulfilling this demand, albeit at the cost of significant electricity consumption. In response, absorption chillers have emerged as an eco-friendly alternative, powered by sustainable heat sources like solar energy or industrial waste heat. The present work is conducted within the framework of developing compact absorption chillers incorporating plate heat exchangers. It builds upon prior research by improving an existing numerical model for falling film plate heat exchangers within an NH3/H2O absorption chiller. This former numerical model evaluates the coupled heat and mass transfer between the liquid film and the vapor flow while excluding hydrodynamic interactions at the liquid-vapor interface. The current investigation aims to address compactness and cost-efficiency limitations in absorption chillers. The consequences of operation within confined spaces were numerically evaluated. An innovative analytical model was developed to quantify the influence of the interactions at the liquid-vapor interface. Shear stress and interfacial perturbations were considered compared to the classical Nusselt model, enabling the evaluation of the changes in the average film thickness and the flooding phenomenon. This analytical model was then added to the previously developed heat and mass transfer model. The numerical model was applied to both the desorber and absorber units. The results show that increased confinement has minimal effects on the transfer coefficients. Additionally, an approach is suggested for estimating flooding in confined spaces and the results are compared with various correlations available in the literature.

Refraction of a triple-shock configuration at planar fast–slow gas interfaces

This paper characterizes the refraction of a triple-shock configuration at planar fast–slow gas interfaces. The primary objective is to reveal the wave configurations and elucidate the mechanisms governing circulation deposition and velocity perturbation on the interface caused by triple-shock refraction. The incident triple-shock configuration is generated by diffracting a planar shock around a rigid cylinder, and four interfaces with various $Z_{{r}}$ (i.e. acoustic impedance ratio across the interface) are considered. An analytical model describing the triple-shock refraction is developed, which accurately predicts both the wave configurations as well as circulation deposition and velocity perturbation. Depending on $Z_{{r}}$ , three distinct patterns of transmitted waves can be anticipated: a triple-shock configuration; a four-shock configuration; a four-wave configuration. The underlying mechanism for the formation of these wave configurations is elucidated through shock polar analysis. A novel physical insight into the contribution of triple-shock refraction to the interface perturbation growth is provided. The results indicate that the reflected shock in an incident triple-shock configuration makes significant negative contribution to both circulation deposition and velocity perturbation. This investigation elucidates the underlying mechanism responsible for the relatively insignificant contribution of baroclinic circulation to the Richtmyer–Meshkov-like instability induced by a non-uniform shock, and provides an explanation for the decrease in growth rate of interface perturbation amplitude with increasing Atwood number.

Segmental dynamics of polystyrene near polymer-polymer interfaces.

This study investigated the segmental dynamics of polymers near polymer-polymer interfaces by probing the rotation of polymer-tethered fluorescent molecules using imaging rotational fluorescence correlation microscopy. Multilayered films were utilized to provide spatial selectivity relative to different polymer-polymer interfaces. In the experimental setup, for the overlayer polymer, polystyrene (PS) was employed and a 15 nm-thick probe-containing layer was placed ≈25nm apart from different underlayer polymers with glass transition temperatures (Tg) either lower or higher than that of PS. The underlayer of poly-n-butyl methacrylate had 72K lower Tg than that of PS, whereas polymethyl methacrylate and polysulfone had 22 and 81K higher Tg, respectively, than that of PS. Two key dynamic features of the glass transition, the non-Arrhenius temperature dependence and stretched relaxation, were examined to study the influence of soft and hard confinements on the segmental dynamics of the overlayer polymer near the polymer-polymer interfaces. Although complications exist in the probing location owing to the diffusion of the polymer-tethered probe during the annealing protocol to consolidate the multilayers, the results suggest that either the segmental dynamics of the polymer near the polymer-polymer interface do not change owing to the soft and hard confinements or the interfacial perturbation is very short ranged.

Vibration effect on Rayleigh–Taylor instability of sedimenting suspension

In this paper, the effect of vertical vibration on the Rayleigh–Taylor instability at the lower interface of a sedimenting suspension layer sandwiched between viscous fluids is studied numerically and theoretically. It is shown numerically that the vibration suppresses the amplitudes of interface perturbations and enlarges the widths of granular fingers, postponing the nonlinear growth stage. By modeling the suspension as a fluid with its effective density and viscosity, stability analyses are carried out, and the dominant growth rate and the dominant wave number predicted by the dispersion relation agree with the numerical results, especially for cases with strong vibrations. Because of the small-scale flow induced by the particles, high vibration acceleration amplitude and low vibration frequency, which correspond to a large vibration power input, increase the kinetic energy spectra at the high wave number regime, a phenomenon contrary to the vibration effect for pure fluid cases.

Probing the Dissociation Pathway of a Kinetically Labile Transthyretin Mutant.

Aggregation of transthyretin (TTR) is associated with devastating amyloid diseases. Amyloidosis begins with the dissociation of the native homotetramer (a dimer of dimers) to form a monomeric intermediate that assembles into pathogenic aggregates. This process is accelerated in vitro at low pH, but the process by which TTR dissociates and reassembles at neutral pH remains poorly characterized due to the low population of intermediates. Here, we use 19F-nuclear magnetic resonance (NMR) and a highly sensitive trifluoromethyl probe to determine the relative populations of the species formed by the dissociation of a destabilized variant, A25T. The A25T mutation perturbs both the strong dimer and weak dimer-dimer interfaces. A tetramer ⇌ dimer ⇌ monomer (TDM) equilibrium model is proposed to account for concentration- and temperature-dependent population changes. Thermodynamic and kinetic parameters and activation energetics for dissociation of the native A25T tetramer, as well as a destabilized alternative tetramer (T*) with a mispacked F87 side chain, were extracted by van't Hoff and 19F-NMR line shape analysis, saturation transfer, and transition state theory. Chemical shifts for the dimer and T* species are degenerate for 19F and methyl probes close to the strong dimer interface, implicating interfacial perturbation as a common structural feature of these destabilized species. All-atom molecular dynamics simulations further suggest more frequent F87 ring flipping on the nanosecond time scale in the A25T dimer than in the native A25T tetramer. Our integrated approach offers quantitative insights into the energy landscape of the dissociation pathway of TTR at neutral pH.

The magnetised plasma Richtmyer–Meshkov instability: elastic collisions in an ion–electron multifluid plasma

The influence of an applied magnetic field on the collisional plasma Richtmyer–Meshkov instability (RMI) is investigated through numerical simulation. The instability is studied within the five-moment multifluid plasma model without any simplifying assumptions such as infinite speed of light, negligible electron inertia or quasineutrality. The plasma is composed of ion and electron fluids, and elastic collisions are modelled with the Braginskii transport coefficients. A collisional regime is investigated and the magnetic field is applied in the direction of shock propagation, which is perpendicular to the density interface. The primary instability is influenced by several terms affecting the evolution of circulation, the most significant of which are the baroclinic, magnetic field torque and intraspecies collisional terms. The applied magnetic field results in a reduction of interface perturbation growth, agreeing qualitatively with previous numerical simulations for the case of an ideal multifluid plasma RMI. The only major difference in the present case's instability mitigation by applied magnetic field, relative to the ideal case with applied magnetic field, is that the elastic collisions replace and obstruct the secondary vorticity suppression mechanism through collisional dissipation of vorticity. Additionally the collisions, influenced by the combination of self-generated and the applied magnetic field, introduce anisotropy to the problem. The primary suppression mechanism for the RMI is unchanged relative to the ideal case, i.e. the magnetic field torque resisting baroclinic deposition of vorticity in the ion fluid.

Groundwater dynamics in intertidal zones of beach aquifers during salt-fingering flow

At aquifer-sea interfaces, submarine groundwater discharge (SGD) and related chemical loads are closely related to groundwater environment in the intertidal zone of coastal aquifers. Although unstable salinity distribution in the intertidal zone associated with salt-fingering flow has been known as a natural phenomenon for some coastal aquifers, a quantitative understanding of groundwater dynamics in the intertidal zone during salt-fingering flow is still lacking. This study investigated the movement features of salt fingers and the response laws of freshwater discharge zone (FDZ) in the intertidal zone, considering the fixed and seasonal inland conditions. The results revealed that during salt-fingering flow, the finger was repeatedly generated and detached from the intertidal zone. Horizontally, the finger moved toward the sea, and the finger speed first increased at the development stage, then remained unchanged at the separation stage, and finally decreased at the mergence stage. The movement speed of salt fingers was closely related to ambient freshwater flow in the FDZ. In the vertical direction, the finger first permeated downward with a decreasing speed, and then the upward discharge of fresh groundwater in the FDZ region with an increased flow velocity pushed the finger upwards with an increasing speed. In contrast to the results in a stable intertidal regime, the width of FDZ at the aquifer-sea interface fluctuated over time, and the variation trends possessed a well agreement with the flux of fresh groundwater discharge. This suggested that the width of FDZ at the aquifer-sea interface can be used as an indicator of fresh groundwater discharge, even in an unstable regime, to provide a basis for field monitoring. The intensity of salt-fingering flow varied over time under the condition of seasonal freshwater inflow. The mean duration of salt fingers decreased slightly with the increase of seasonal freshwater periods due to a decrease in the interface perturbation for the intertidal environment. These findings lay the foundation for gaining a better understanding of the potential implications of salt-fingering flow on SGD and related solute transports.

In situ observation of destabilization of a nanostructured Ag/Cu multilayer fabricated via multicomponent accumulative roll bonding

Nanostructured metallic multilayered composites have many excellent properties due to their high-density heterophase interfaces, but they also pose challenges to the thermal stability of their multilayered structures. In this work, destabilization behavior of nanostructured Ag/Cu multilayer with an individual layer thickness of 40 nm prepared by multicomponent accumulative roll bonding is directly observed by transmission electron microscope. The results show that the destabilization of the nanostructured Ag/Cu multilayer originates from the interfacial instability perturbation driven by compositional gradient and capillary force. The subsequent destabilizing process is closely related to the cross-sectional ratio of the lamellar structure. When the thickness difference between adjacent lamellae is large, the lamellae with smaller thickness will follow a destabilizing sequence of perturbation → necking → pinching → termination migration. While the thickness of adjacent lamellae is similar, the perturbed lamellae tend to form stable grooves during roughening.

Dynamic Regulation of the Interfacial pH for Highly Reversible Aqueous Zinc Ion Batteries.

An electrolyte additive, with convenient operation and remarkable functions, has been regarded as an effective strategy for prolonging the cycle life of aqueous zinc ion batteries. However, it is still difficult to dynamically regulate the unstable Zn interface during long-term cycling. Herein, tricine was introduced as an efficient regulator to achieve a pH-stable and byproduct-free interface. The functional zwitterion of tricine not only inhibits interfacial pH perturbation and parasitic reactions by the trapping effect of an anionic group (-COO-) but also simultaneously creates a uniform electric field by the electrostatic shielding effect of a cationic group (-NH2+). Such synergy accordingly eliminates dendrite formation and creates a chemical equilibrium in the electrolyte, endowing the Zn||Zn cell with long-term Zn plating/stripping for 2060 h at 5 mA cm-2 and 720 h at 10 mA cm-2. As a result, the Zn||VS2 full cell under a high cathodic loading mass (8.6 mg cm-2) exhibits exceptional capacity retention of 93% after 1000 cycles.

Experimental and analytical approaches toward controlling viscous fingering at the interface of two immiscible fluids using the optimal constant piecewise injection rate

Viscous fingering (VF) at the interface of two immiscible fluids can have disruptive effects in many technological applications. Therefore finding methods for reducing it with an effective and simple approach plays a significant role. A proposed solution that restrains the expansion of such complex morphologies, is the time-dependent injection rate. This proposed solution is often done either analytically or by numerical simulations which need to be experimentally proven. In this study, we perform multiple experiments of low viscosity fluid injection into high viscosity fluid in order to study the formation of VF and find injection rates or protocols that reduce VF at the onset of the nonlinear regime and in the fully nonlinear regime by applying several constant piecewise injection rates. Moreover, analytically and numerically studies are done, in which we derive the optimal parameters in piecewise injection profiles to limit the VF and the tip-splitting effect. This optimal constant piecewise injection rate includes specified number of stages as well as the injection time and its rate in each stage. The obtained results revealed that depending on being at the beginning of the nonlinear regime or in the fully nonlinear regime, the optimal constant piecewise injection rate can considerably reduce the length of fingers and the interfacial fingering perturbations. Empirical results show similar trend in terms of instability reduction by applying the same optimal number of piecewise injection rate stages. In addition, evaluating the difference between this piecewise results and the results of linear injection rates, which we have done before, reveals that employing linear injection rates is more effective in controlling the instability in lower injection rates, while for higher injection rates, it is required to utilize a piecewise injection rate.

A decoupled mechanism of interface growth in single-mode hydrodynamic instabilities

One of the most significant issues in hydrodynamic interfacial instabilities is the growth rate of the interfacial perturbations, which plays an important role in both scientific research (e.g. supernova explosion) and engineering applications (e.g. inertial confinement fusion). Yet the underlying mechanisms of such flow phenomena remain unclear or controversial. In this paper the decoupled mechanisms of two effects are found to dominate the interface growth of the single-mode Rayleigh–Taylor instability (RTI) and Richtmyer–Meshkov instability (RMI) via Layzer's potential-flow model. One is the inertial effect induced by the interfacial density gradient and the acceleration, and the other is the frontal distortion effect stemming from interface shape evolution. The former determines the dominant features of interface evolution, while the latter influences the local concavity and convexity of growth rate such as the overshoot phenomenon. These two effects can be approximated as a linearly decoupled analytical solution if their nonlinear interaction term is neglected. With the decoupled solution, the theoretical growth rates agree well with high-fidelity numerical simulation results. The present result indicates that the long-time evolution of fluid interface in both RTI and RMI at all density ratios can be accurately predicted if both inertia and frontal distortion effects are taken into account. Furthermore, the strong dependence of instability evolution on initial amplitude is quantified based on the effects of decoupling, which sheds light on the physical origin of the overshoot phenomenon.

On the interfacial deformations and thermal characteristics exhibiting self-similar behavior under the action of a line heat source

Interfacial dynamics resulting from a heating source located near the interface play a crucial role in dictating the heat and momentum transport in the near-interface region. This paper aims toward simultaneous characterization of interfacial deformation and thermal behavior under the action of a line heating source placed below the interface. Experiments have been conducted on aqueous glycerol with a heating wire at different power inputs and depths from the interface. The interfacial deformations are mapped and quantified by employing moon-glade background oriented schlieren, which offers real-time, non-intrusive whole field measurements based on the deflection of light rays from liquid interface. Infrared thermography is used to measure transient interfacial temperature variations. Results show that the interface exhibits a convex-shaped deformation under the influence of the heating wire for all cases of heating power and depth. The maximum interface temperature coincides with the peak interfacial deformation. However, the region of thermal influence is smaller compared to the deformed region. Non-dimensionalization of transient interface deformation and temperature profiles establishes the underlying similarity of the phenomenon as non-dimensional interface perturbation profiles overlap for all cases of height and heating power. These characteristics are also observed for normalized temperature profiles at different wire depths.

Ultrathin Conductive Bithiazole‐Based Covalent Organic Framework Nanosheets for Highly Efficient Electrochemical Biosensing

AbstractCovalent organic frameworks (COFs) with unique structural merits show substantial potential in the construction of biosensors. However, high‐performance COF‐based biosensors have rarely been reported due to special requirements for electrochemical biosensing. Here, the ultrathin nitrogen and sulfur‐rich bithiazole‐based COF nanosheets (COF‐Bta‐NSs) with the thickness of ≈1.95 nm are developed by using an interfacial perturbation growth strategy, and are further integrated with acetylcholinesterase (AChE) through strong supramolecular interactions to construct a high‐performance biosensor for organophosphorus pesticides (OPs) detection. By virtue of the excellent electrical conductivity and abundant edge unsaturated sites of COF‐Bta‐NSs, such unique biosensors can be used for the detection of various OPs, showing a wide detection range, ultralow detection limit, and high stability. Significantly, the portable biosensing device is further set up based on commercialized screen‐printed electrode (SPE), which is sensitive and reliable with the actual samples collected from river water and leafy vegetables, confirming the practical applicability. This research provides a novel insight into the development of advanced COF‐based biosensors with excellent performance for biological and environmental analysis.

The effect of collisions on the multi-fluid plasma Richtmyer–Meshkov instability

The Richtmyer–Meshkov instability (RMI) results from the impulsive acceleration of a density interface where the RMI itself or the acceleration is perturbed. The RMI is ubiquitous in shock environments and may arise due to an interface of fluid species, isotopes, temperature, or more. The plasma RMI can be significantly influenced by electromagnetic effects and can be modeled more accurately by a multi-fluid plasma (MFP) model rather than conventional magnetohydrodynamics, though with increased computational expense. MFP modeling of the plasma RMI has revealed many phenomena but has only been completed within the ideal regime. Modeling the effects of elastic collisions is vital for understanding the behavior of the instability in a dense plasma. The Braginskii transport coefficients provide theoretically based relations modeling thermal equilibration, inter-species drag, viscous momentum- and energy-transfers, and thermal conductivity. Our numerical simulations of the MFP RMI with these relations show that the key changes from the ideal case are (1) reduction of relative motion between the ion and electron fluids (consequently affecting the self-generated electromagnetic fields), (2) introduction of anisotropy in momentum and energy via transport coefficients, and (3) damping of high frequency electromagnetic waves and plasma waves. Under the conditions studied, the net effect is a reduction in the MFP RMI amplitude width and the growth rate to levels approaching the neutral fluid instability, as well as a reduction in large scale perturbations along the ion fluid density interface, a positive for inertial confinement fusion efforts. There are, however, two important caveats: small-scale density interface perturbations remain, and the conditions simulated are a few relevant points in a large parameter space that requires further investigation.