Interface Perturbations Research Articles

Effect of Periodic Oscillation on the Interfacial Instability of Two Superposed Fluid Layers in a Fully Saturated Porous Media

We investigate the effect of horizontal periodic oscillation on the interfacial instability of two immiscible and viscous fluids of different densities in a fully saturated porous media. A linear stability analysis of the viscous and time-dependent basic flow leads to a periodic oscillator describing the evolution of the interfacial perturbation amplitude. The horizontal oscillation leads to the occurrence of two types of instability, the Kelvin–Helmholtz’s instability and the parametric resonance. These instabilities appear at the frontier between water and petroleum and have a practical interest in oil reservoir engineering. The results show that, an increase of the oscillation frequency destabilizes the Kelvin–Helmholtz instability and displaces the parametric instability regions toward the short wavelength perturbation. Also, we examine mainly how the other physical parameters of the system affect the instabilities for various permeability and porosity values of the porous medium as well as for relative heights of the two fluid layers.

Elastically driven Kelvin–Helmholtz-like instability in straight channel flow

Originally, Kelvin-Helmholtz instability (KHI) describes the growth of perturbations at the interface separating counterpropagating streams of Newtonian fluids of different densities with heavier fluid at the bottom. Generalized KHI is also used to describe instability of free shear layers with continuous variations of velocity and density. KHI is one of the most studied shear flow instabilities. It is widespread in nature in laminar as well as turbulent flows and acts on different spatial scales from galactic down to Saturn's bands, oceanographic and meteorological flows, and down to laboratory and industrial scales. Here, we report the observation of elastically driven KH-like instability in straight viscoelastic channel flow, observed in elastic turbulence (ET). The present findings contradict the established opinion that interface perturbations are stable at negligible inertia. The flow reveals weakly unstable coherent structures (CSs) of velocity fluctuations, namely, streaks self-organized into a self-sustained cycling process of CSs, which is synchronized by accompanied elastic waves. During each cycle in ET, counter propagating streaks are destroyed by the elastic KH-like instability. Its dynamics remarkably recall Newtonian KHI, but despite the similarity, the instability mechanism is distinctly different. Velocity difference across the perturbed streak interface destabilizes the flow, and curvature at interface perturbation generates stabilizing hoop stress. The latter is the main stabilizing factor overcoming the destabilization by velocity difference. The suggested destabilizing mechanism is the interaction of elastic waves with wall-normal vorticity leading to interface perturbation amplification. Elastic wave energy is drawn from the main flow and pumped into wall-normal vorticity growth, which destroys the streaks.

Role of convective acceleration in the interfacial instability of liquid-gas coaxial jets

When a high-speed gas flow destabilizes a liquid jet into a spray (atomization), the gas-liquid interface undergoes instabilities that play a key role in the breakup events. We use high-speed backlit imaging to track the maxima of such interfacial perturbations as they get stretched and accelerated by the gas phase. The spatial gradients of the local ensemble-averaged velocity statistics exhibit two regimes with respect to the gas Reynolds numbers that are in good agreement with changes in atomization regimes.

Tissue/Biofluid Specific Molecular Cartography of Leishmania donovani Infected BALB/c Mice: Deciphering Systemic Reprogramming.

Pathophysiology of visceral leishmaniasis (VL) is not fully understood and it has been widely accepted that the parasitic components and host immune response both contribute to the perpetuation of the disease. Host alterations during leishmaniasis is a feebly touched area that needs to be explored more to better understand the VL prognosis and diagnosis, which are vital to reduce mortality and post-infection sequelae. To address this, we performed untargeted metabolomics of Leishmania donovani (Ld) infected, uninfected and treated BALB/c mice’s tissues and biofluids to elucidate the host metabolome changes using gas chromatography–mass spectrometry. Univariate and multivariate data treatments provided numerous significant differential hits in several tissues like the brain, liver, spleen and bone marrow. Differential modulations were also observed in serum, urine and fecal samples of Ld-infected mice, which could be further targeted for biomarker and diagnostic validations. Several metabolic pathways were found to be upregulated/downregulated in infected (TCA, glycolysis, fatty acids, purine and pyrimidine, etcetera) and treated (arginine, fumaric acid, orotic acid, choline succinate, etcetera) samples. Results also illustrated several metabolites with different pattern of modulations in control, infected and treated samples as well as in different tissues/biofluids; for e.g. glutamic acid identified in the serum samples of infected mice. Identified metabolites include a range of amino acids, saccharides, energy-related molecules, etcetera. Furthermore, potential biomarkers have been identified in various tissues—arginine and fumaric acid in brain, choline in liver, 9-(10) EpOME in spleen and bone marrow, N-acetyl putrescine in bone marrow, etcetera. Among biofluids, glutamic acid in serum, hydrazine and deoxyribose in urine and 3-Methyl-2-oxo pentanoic acid in feces are some of the potential biomarkers identified. These metabolites could be further looked into for their role in disease complexity or as a prognostic marker. The presented profiling approach allowed us to attain a metabolic portrait of the individual tissue/biofluid modulations during VL in the host and represent a valuable system readout for further studies. Our outcomes provide an improved understanding of perturbations of the host metabolome interface during VL, including identification of many possible potential diagnostic and therapeutic targets.

Investigation of Rayleigh-Taylor instability in copper plate under explosive loading

The Rayleigh–Taylor instability in metal is of great practical interest to a diverse range of fields, and it is significantly different from the traditional fluid interface instability. In this paper, a set of high explosives driven Rayleigh–Taylor instability experiments in copper plate are presented for intensively understanding the perturbation growth behavior of metal interface instability. The perturbation growth history on copper interface at a specific time is recorded by flash radiography. The experimental results show that the evolution of interface perturbation is sensitive to its initial perturbation characters under a given driving pressure, the larger the initial perturbation amplitude, the faster the perturbation grows, but the perturbation wavelength of the interface remains almost unchanged at the explosive loading. The perturbation on the front interface will influence the interface features of the back free interface over time. Namely, the position corresponding to the perturbation trough on the front interface gradually evolves into a peak and vice versa.

On interplay of surface tension and inertial stabilization mechanisms in the stable and unstable interface dynamics with the interfacial mass flux

Non-equilibrium dynamics of interfaces and mixing are omnipresent in fluids, plasmas and materials, in nature and technology, at astrophysical and at molecular scales. This work investigates dynamics of an interface separating fluids of different densities and having interfacial mass flux, and being influenced by the acceleration and the surface tension. We derive solutions for the interface dynamics conserving mass, momentum and energy, find the critical acceleration values separating stable and unstable regimes, and reveal the macroscopic inertial mechanism as primary mechanism of the interface stabilization. We show that while the surface tension influences only the interface, its presence leads to formation of vortical structures in the bulk. For large accelerations the conservative dynamics is unstable, leading to the growth of interface perturbations and the growth of the interface velocity. This new instability can be unambiguously discerned from other instabilities; for strong accelerations it has the fastest growth-rate and the largest stabilizing surface tension value when compared to Landau-Darrieus and Rayleigh-Taylor instabilities. We further find the values of initial perturbation wavelengths at which the conservative dynamics can be stabilized and at which it has the fastest growth. Our results agree with existing observations, identify extensive theory benchmarks for future experiments and simulations, and outline perspectives for application problems in nature and technology.

Modulation Instability of Surface Waves in the Model with the Uniform Wind Profile

The modulation instability of surface capillary-gravity water waves is analysed in a shear flow model with a tangential discontinuity of velocity. It is assumed that air blows along the surface of the water with a uniform profile in the vertical direction. Such a model, despite its simplicity, plays an important role in hydrodynamics as the reference model for investigating basic physical phenomena of wave–current interactions and acquiring insights into a series of complex phenomena. In certain cases where the wavelength of interfacial perturbations is much bigger than the width of the shear flow profile, the model with the tangential discontinuity in the velocity is adequate for describing physical phenomena at least within limited spatial and temporal frameworks. A detailed analysis of the air-flow conditions under which modulation instability sets in is presented. It is also shown that the interfacial waves are subject to dissipative or radiative instability when negative-energy waves appear at the interface.

On the role of interfacial elasticity in morphological instability of a heteroepitaxial interface

In this paper, we discuss a theoretical approach to morphological instability analysis of the coherent interphase boundaries in strained heterostructures. Taking into account the fact that, under certain conditions, the atomic arrangement of solid–solid interfaces is thermodynamically unstable, the evolution equation describing the kinetics of the relief formation is obtained. The considered process is controlled by interface diffusion activated by the nonuniform stress field which occurs due to mismatch between materials and the initial interface perturbation. To define the stress distribution along the curved interface, we use constitutive equations of bulk and surface/interface elasticity modeling the interphase domain as a negligibly thin layer adhering to the bulk phases. This allows us to take into account the surface energy variation related to the interface relief evolution and analyze, in addition to other parameters, the effect of elastic constants that characterize the mechanical behavior of the interphase layer. With the use of the first-order approximation of the boundary perturbation method, the solution of the linearized evolution equation leads to an estimation of equilibrium surface shape.

Evolution of adsorbed layers of asphaltenes at oil-water interfaces: A novel experimental protocol

Hypothesis: Asphaltenes can form rigid interfacial films surrounding water droplets rendering water separation from crude oil sluggish. Therefore, the quantitative characterization of such complex film formation is of great importance. As the adsorbed layers of asphaltene illustrate crumpling under compression at certain conditions, the evolution process from soft to rigid states of the film can be evaluated considering standard deviations from Young-Laplace shape fitting. ExperimentalIn this study, novel experimental protocols are introduced to investigate the evolution of adsorbed asphaltene layer to a film of aggregates at model oil/water interface by means of dynamic interfacial tension (IFT) and dilational surface rheology measurements. In particular, the surface elasticity and standard deviation from the Young-Laplace shape fitting (YL-SD) are introduced as important indicators for the transformation of a regular asphaltene adsorbed layer to a film of aggregates. Different parameters affecting the film formation and stability, such as aging time, asphaltene concentration, and history of interfacial dynamics, are discussed and linked to emulsion stability.Findings: It is shown for the first time that the standard deviation of drop profile fitting from the Young-Laplace equation can be used as a rigorous parameter to reveal the properties of the interfacial asphaltene film, which cannot be recognized by regular IFT measurements. Via this novel technique, it is revealed that the transformation of an asphaltene adsorbed layer to a rigid film depends not only on the asphaltene concentration but also on the aging time and the interfacial area perturbations. The results of this new method are supported by measurements of the dilational surface elasticity, which is known as an important parameter for the characterization of complex adsorbed layers, and further verified by an emulsion stability analysis.

Nonlinear limiting dynamics of a shrinking interface in a Hele-Shaw cell

Abstract

Control of fingering instability by vibrations

In applications involving the injection of a fluid in a porous medium to displace another fluid, a main objective is the maximization of the displacement efficiency. Displacement fronts moving in porous media are subjected to hydrodynamic instability when a liquid of low viscosity displaces a high-viscosity liquid and consequently finger-like structure forms along the interface. This finger instability is usually undesirable in technical applications and natural filtration processes. We discuss the external periodic forcing as one of the promising ways to control the instability and perform numerical simulation of an initially spherical drop in a porous media under vertical vibrations. The drop is a favorable object to study since in this case one can observe the effect of vibrations on fluid interface domains inclined by different angles with respect to vibration axis. It is shown that under vibrations small-scale perturbations of interface are suppressed and in the case of vibrations of large enough intensity the drop becomes stable. The stability criterion is derived.

Stability analysis of a nanopatterned bimaterial interface

In the article it is shown that the nanopatterned interface of bimaterial is unstable due to the diffusion atom flux along the interface. The main goal of the research is to analyze the conditions of interface stability. The authors developed a model coupling thermodynamics and solid mechanics frameworks. In accordance with the Gurtin—Murdoch theory of surface/interface elasticity, the interphase between two materials is considered as a negligibly thin layer with the elastic properties differing from those of the bulk materials. The growth rate of interface roughness depends on the variation of the chemical potential at the curved interface, which is a function of interface and bulk stresses. The stress distribution along the interface is found from the solution of plane elasticity problem taking into account plane strain conditions. Following this, the linearized evolution equation is derived, which describes the amplitude change of interface perturbation with time.

Is it Necessary for the Use of Fluorinated Compounds to Formulate Reverse Micelles in a Supercritical Fluid? Searching the Best Cosurfactant to Create "Green" AOT Reverse Micelle Media.

Herein, we report the effect of employing two different alcohols, such as n-pentanol and 2,2,3,3,4,4,5,5-octafluoro pentanol (from now on F-pentanol), into 1,4-bis-2-ethylhexylsulfosuccinate (AOT) reverse micelles (RMs), to determine the interfacial activity and establish the best candidate to act as a cosurfactant in supercritical RMs. Dynamic light scattering (DLS), Fourier transform infrared (FT-IR), and fluorescence emission spectroscopy allowed us to determine and understand the behavior of alkanols in RMs. As a result, we found interesting displacements of alkanol molecules within the RMs, suggesting that the electrostatic interaction between SO3- and Na+ weakens because of new interactions of n-pentanol with SO3- through H-bonds, changing the curvature of the micellar interface. According to FT-IR and DLS studies, F-pentanol forms a RM polar core interacting through intermolecular H-bonds, suggesting no perturbations of the AOT RM interface. Hence, n-pentanol was selected as a cosurfactant to form supercritical RMs, which is confirmed by red edge excitation shift studies, using C343 as a molecular probe. Herein, we were able to create RMs under supercritical conditions without the presence of modified surfactants, fluorinated or multitailed compounds, which, to the best of our knowledge, was not shown before.

Experimental and numerical investigation of the Rayleigh-Taylor instability of the Newtonian and dilatant fluid system

The present work is devoted to the Rayleigh–Taylor instability of dilatant and Newtonian fluids. The main aim of this work was to carry out experiment, numerical simulation of mixing of two media, and making a comparison between gained data. A series of experiments was carried out to study the Rayleigh–Taylor instability of a dilatant liquid, which is a mixture of potato starch and water. The initial perturbation of the liquid-air interface is given by the shape of the metal profile separating the two media. The difference in the development of this instability in water-air and dilatant fluid-air systems is shown.

The effect of initial conditions on mixing transition of the Richtmyer–Meshkov instability

Abstract

Modeling the amplitude growth of Richtmyer–Meshkov instability in shock–flame interactions

This paper discusses the shock–flame interactions and the aspects associated with it, including the types of interactions, role of interactions in turbulent flames, high pressure generation during interactions, initial pressure effects on interactions, equivalence ratio effects on turbulent interactions, and the Richtmyer–Meshkov instability (RMI). In particular, the theory of RMI and the models associated with its amplitude growth with time have been discussed. Then, a new analytical model, Al-Thehabey model, is presented based on the impulsive acceleration of Richtmyer, not the gravitational acceleration. This model predicts the amplitude growth of the interface perturbation in terms of Atwood number (A), wave number of the perturbation (k), interface velocity (uc), and time (t). This new model’s prediction of the amplitude growth of the RMI is tested on six different combinations of fluids at different interface velocities. The results of the new model are compared with the results of four other existing analytical models and the new model’s performance fared very well. In addition, the new model’s performance has been compared with the experimental results from a shock wave incident on CO2–air, at Mach number, M = 3.08, interface velocity, u = 699.1 m/s, Atwood number, A = 0.206, and wavelength, λ = 990.0 × 10−6 m. The new model showed much closer results with the experimental ones than all other models used in the evaluation. The advantage of this new model is that it is capable of predicting the amplitude growth for both linear, at the early stages of the instability, and non-linear later regimes of the instability. In addition, it covers a larger time-domain than both the Alon et al. and the Sadot et al. models.

The magnetised Richtmyer–Meshkov instability in two-fluid plasmas

Abstract

Structure-based inhibitors targeting the alpha-helical domain of the Spiroplasma melliferum histone-like HU protein

Here we report bisphenol derivatives of fluorene (BDFs) as a new type of chemical probes targeting a histone-like HU protein, a global regulator of bacterial nucleoids, via its dimerization interface perturbation. BDFs were identified by virtual screening and molecular docking that targeted the core of DNA-binding β-saddle-like domain of the HU protein from Spiroplasma melliferum. However, NMR spectroscopy, complemented with molecular dynamics and site-directed mutagenesis, indicated that the actual site of the inhibitors’ intervention consists of residues from the α-helical domain of one monomer and the side portion of the DNA-binding domain of another monomer. BDFs inhibited DNA-binding properties of HU proteins from mycoplasmas S. melliferum, Mycoplasma gallicepticum and Escherichia coli with half-maximum inhibitory concentrations in the range between 5 and 10 µM. In addition, BDFs demonstrated antimicrobial activity against mycoplasma species, but not against E. coli, which is consistent with the compensatory role of other nucleoid-associated proteins in the higher bacteria. Further evaluation of antimicrobial effects of BDFs against various bacteria and viruses will reveal their pharmacological potential, and the allosteric inhibition mode reported here, which avoids direct competition for the binding site with DNA, should be considered in the development of small molecule inhibitors of nucleoid-associated proteins as well as other types of DNA-binding multimeric proteins.

Pulsing Liquid Alloys for Nanomaterials Synthesis.

Although it remains unexplored, the direct synthesis and expulsion of metals from alloys can offer many opportunities. Here, such a phenomenon is realized electrochemically by applying a polarizing voltage signal to liquid alloys. The signal induces an abrupt interfacial perturbation at the Ga-based liquid alloy surface and results in an unrestrained discharge of minority elements, such as Sn, In, and Zn, from the liquid alloy. We show that this can occur by either changing the surface tension or inducing a reversible redox reaction at the alloys' interface. The expelled metals exhibit nanosized and porous morphologies, and depending on the cell electrochemistry, these metals can be passivated with oxide layers or fully oxidized into distinct nanostructures. The proposed concept of metal expulsion from liquid alloys can be extended to a wide variety of molten metals for producing metallic and metallic compound nanostructures for advanced applications.

Instability and interface coupling of coaxial liquid jets in a driving stream

The behavior of jet breakup and interface coupling in a co-flow focusing (CFF) process is studied theoretically. A physical model of coaxial liquid jets moving in an infinite annular driving stream is established, and the dimensionless dispersion relation for temporally axisymmetric perturbations is solved numerically. The effects of process parameters such as flow velocities, liquid physical properties, and radius ratio between the inner and outer jets on the jet instability are analyzed. The evolutions of interface perturbations are observed in CFF experiments, and the perturbation wavelengths under different liquid flow rates are measured in comparison with theoretical predictions. Moreover, the coupling of interface instabilities in CFF is studied through changing the radius ratio between the inner and outer liquid jets. In particular, two simplified single jet models under the assumption of minimum inner and outer liquid flow rates are proposed to reveal the transition from weak coupling to strong coupling of jet interfaces. This work provides great insight into the physical mechanism of interface instability in CFF advantageous for producing monodisperse microdroplets with fine robustness and high throughput.