Interface Destabilization Research Articles

On the role of unsteadiness in impulsive-flow-driven shear instabilities: precursors of fragmentation

Shear instabilities at the interface of two fluids, such as classical Kelvin–Helmholtz instability (KHI), is the precursor of interface destabilization, leading to fluid fragmentation critical in a wide range of applications. While many insights into such instabilities are derived for steady background forcing flow, unsteady impulse flows are ubiquitous in environmental and physiological processes. Yet, little is understood on how unsteadiness shapes the initial interface amplification necessary for the onset of its topological change enabling subsequent fragmentation. In this combined theoretical, numerical and experimental study, we focus on an air-on-liquid interface exposed to canonical unsteady shear flow profiles. Evolution of the perturbed interface is formulated theoretically as an impulse-driven initial value problem using both linearized potential flow and nonlinear boundary integral methods. We show that the unsteady airflow forcing can amplify the interface's inherent gravity–capillary wave, up to wave-breaking transition, even if the configuration is classically KH stable. For impulses much shorter than the gravity–capillary wave period, it is the cumulative action, akin to total energy, that determines amplification, independent of the details of the impulse profile. However, for longer impulses, the details of the impulse profile become important. In this limit, akin to a resonance, it is the entangled history of the interaction of the forcing, i.e. the impulse, that changes rapidly in amplitude, and the response of the oscillating interface that matters. The insights gained are discussed and experimentally illustrated in the context of interface distortion and destabilization relevant for upper respiratory mucosalivary fluid fragmentation in violent exhalations.

Dimer Interface Destabilization of Photodissociative Dronpa Driven by Asymmetric Monomer Dynamics.

Photoswitchable Dronpa (psDronpa) is a unique member of the fluorescent protein family that can undergo reversible photoinduced switching between fluorescent and dark states and has recently been engineered into a dimer (pdDronpaV) that can dissociate and reassociate as part of its photoswitchable pathway. However, the specific details of the protein structure-function relationship of the dimer interface along with how the dimer proteins interact with each other upon chromophore isomerization are not yet clear. Classical molecular dynamics simulations were performed on psDronpa as monomers and dimers as well as the pdDronpaV dimer and with cis/trans chromophore structures. Analysis of the cis and trans isomers of the chromophore illustrated key differences between their interactions with residues in the protein in both the monomer and dimer forms of psDronpa. Examination of the psDronpa dimer showed nonidentical chromophore interactions between the domains, indicating domain directional favoring. Examination of the trans form of pdDronpaV illuminated the importance of hydrogen bonding between the monomeric domains in maintaining their association, as well as illustrating the motion of dissociation of the domains. This discovery offers important information for possible future mutations of pdDronpaV that might be made to accelerate dissociation.

Dimer Interface Destabilization of Photodissociative Dronpa Driven by Asymmetric Monomer Dynamics

Dimer Interface Destabilization of Photodissociative Dronpa Driven by Asymmetric Monomer Dynamics

Microstructural pattern formation during liquid metal dealloying: Phase-field simulations and theoretical analyses

In recent years, liquid metal dealloying (LMD) has emerged as a promising material processing method to generate micro and nano-scale bicontinuous or porous structures. Most previous studies focused on the experimental characterization of the dealloying process and on the properties of the dealloyed materials, leaving the theoretical study incomplete to fully understand the fundamental mechanisms of LMD. In this paper, we use theoretical models and phase-field simulations to clarify the kinetics and pattern formation during LMD. Our study starts from a theoretical analysis of the 1D dissolution of a binary precursor alloy, which reveals that the 1D dissolution process involves two regimes. In the first regime, due to the low solubility of one of the elements in the melt, it accumulates at the solid-liquid interface, which reduces the dissolution kinetics. In the second regime, the interface kinetics reaches a stationary regime where both elements of the precursor alloy dissolve into the melt. Previous works revealed that in the early dealloying stage, the dealloying front is destabilized by an interfacial spinodal decomposition, which triggers the formation of interconnected ligaments. We extend this line of work by proposing a linear stability analysis able to predict the initial length-scale of the ligaments formed in the initial stage of the dealloying. Combining this analysis with the 1D dissolution model proposed here enables us to better understand the initial conditions (composition of the precursor alloy and the melt) leading to a planar dissolution without interface destabilization. Finally, we report a strong influence of solid-state diffusion on dealloying that was overlooked in previous studies. Although the solid-state diffusivity is four to five orders of magnitude smaller than in the liquid phase, it is found to affect both dissolution kinetics and ligament morphologies.

Inertial dynamics of an interface with interfacial mass flux: Stability and flow fields’ structure, inertial stabilization mechanism, degeneracy of Landau’s solution, effect of energy fluctuations, and chemistry-induced instabilities

This work focuses on the long-standing problem of inertial dynamics of an interface with interfacial mass flux and reports new mechanisms for the interface stabilization and destabilization. The interface is a phase boundary separating fluids of different densities and having interfacial mass flux. To analyze the interface dynamics from a far field, we develop and apply the general matrix method to rigorously solve the boundary value problem involving the governing equations in the fluid bulk and the boundary conditions at the interface and at the outside boundaries of the domain. We find the fundamental solutions for the linearized system of equations and analyze the interplay of interface stability with flow fields’ structure by directly linking rigorous mathematical attributes to physical observables. We find that the interface is stable when the dynamics conserves the fluxes of mass, momentum, and energy; the stabilization is due to an inertial mechanism causing small oscillations of the interface velocity. In the classic Landau’s dynamics, the postulate of perfect constancy of the interface velocity leads to the development of Landau–Darrieus instability. This destabilization is also linked to the imbalance of the perturbed energy at the interface. The classic Landau’s solution is found to have degeneracy; lifting of the degeneracy may lead to singularity and self-similar dynamics. Our results compare well with traditional theories of combustion and propose new experiments to study the dynamics of the interface and the flow fields in combustible systems. We further conduct reactive molecular dynamics simulations to elucidate the complexity of chemical processes, to study the destabilizing effect of energy fluctuations on the interface stability, and to illustrate the chemistry-induced instabilities. In summary, we identify the extreme sensitivity of the interface dynamics to the interfacial boundary conditions, including the formal properties of fundamental solutions and the qualitative and quantitative properties of the flow fields. This provides new opportunities for studies, diagnostics, and control of multiphase flows in a broad range of processes in nature and technology.

The Effect of Water Concentration in Carbonate-Based Electrolytes on the Si Anode/Electrolyte Interface

Lithium-ion batteries (LiBs) have been successfully utilized as power sources in various applications ubiquitous in our daily lives, ranging from portable electronic devices to electric vehicles. To meet the increasing energy demand, however, development of LiBs with a higher energy density is inevitable. Using silicon (Si) as an anode is promising due to its high theoretical capacity (approximately 4200 mAhg-1), appropriate operating voltage (~0.4 V vs. Li/Li+), abundance, and environmentally benign nature.1 Despite the aforementioned advantages, several challenges remain before the commercial utilization of Si anodes, namely the low conductivity of its readily-formed surface oxide (SiO2), a large volume change (up to 400 %) during lithiation/delithiation, and the instability of Si/electrolyte interface (SiEI).2-3 As the large volume change leads to fracture and continuous evolution of SiEI, achieving a stable SiEI is a key to developing successful LiBs with Si anodes. Water, which is known to react with LiPF6 salt in carbonate-based electrolytes to generate corrosive hydrofluoric acid (HF),4 is a source for the electrode/electrolyte interface destabilization. Thus, it is essential to understand how the presence of even trace amounts of water in the system affects the interfacial chemistry of the LiB electrodes. While the performance of LiBs with graphite or lithium titanate negative electrodes have been previously studied with excess water in the electrolyte,5-6 information on the effect of water concentration in the electrolyte on Si anodes, and the SiEI in particular, is limited. In this work, we evaluated the electrochemical performance of Si anodes in a standard electrolyte (1.2 M LiPF6 in EC:EMC (3:7 wt%)) with varying concentration of water (10 - 1,000 ppm). Subsequent analyses on the cycled Si anodes were performed utilizing a variety of surface and bulk characterization techniques. The surface chemistry (e.g., composition and evolution) of SiEI was analyzed using Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, time-of-flight secondary ion mass spectrometry (TOF-SIMS), and X-ray photoelectron spectroscopy (XPS). Surface morphologies of cycled Si anodes were measured with tapping-mode atomic force microscopy (AFM), and the surface resistivities were mapped in contact-mode with scanning spreading resistance microscopy (SSRM). The information obtained is utilized to understand water concentration effects on the properties, reactivity, and evolution of the SiEI and the electrochemical cell performance. References Feng, K.; Li, M.; Liu, W.; Kashkooli, A. G.; Xiao, X.; Cai, M.; Chen, Z., Silicon-Based Anodes for Lithium-Ion Batteries: From Fundamentals to Practical Applications. Small 2018, 14 (8), 1702737.Chae, S.; Ko, M.; Kim, K.; Ahn, K.; Cho, J., Confronting Issues of the Practical Implementation of Si Anode in High-Energy Lithium-Ion Batteries. Joule 2017, 1 (1), 47-60.Philippe, B.; Dedryvere, R.; Allouche, J.; Lindgren, F.; Gorgoi, M.; Rensmo, H.; Gonbeau, D.; Edstrom, K., Nanosilicon Electrodes for Lithium-Ion Batteries: Interfacial Mechanisms Studied by Hard and Soft X-ray Photoelectron Spectroscopy. Chem. Mater. 2012, 24 (6), 1107-1115.Xu, K., Nonaqueous Liquid Electrolytes for Lithium-Based Rechargeable Batteries. Chem. Rev. 2004, 104 (10), 4303-4417.Burns, J. C.; Sinha, N. N.; Jain, G.; Ye, H.; VanElzen, C. M.; Scott, E.; Xiao, A.; Lamanna, W. M.; Dahn, J. R., The impact of intentionally added water to the electrolyte of Li-ion cells. I. Cells with graphite negative electrodes. J. Electrochem. Soc. 2013, 160 (11), A2281-A2287.Burns, J. C.; Sinha, N. N.; Jain, G.; Ye, H.; Van Elzen, C. M.; Scott, E.; Xiao, A.; Lamanna, W. M.; Dahn, J. R., The impact of intentionally added water to the electrolyte of Li-ion cells: II. Cells with lithium titanate negative electrodes. J. Electrochem. Soc. 2014, 161 (3), A247-A255.

Interface dynamics: Mechanisms of stabilization and destabilization and structure of flow fields

Interfacial mixing and transport are nonequilibrium processes coupling kinetic to macroscopic scales. They occur in fluids, plasmas, and materials over celestial events to atoms. Grasping their fundamentals can advance a broad range of disciplines in science, mathematics, and engineering. This paper focuses on the long-standing classic problem of stability of a phase boundary—a fluid interface that has a mass flow across it. We briefly review the recent advances in theoretical and experimental studies, develop the general theoretical framework directly linking the microscopic interfacial transport to the macroscopic flow fields, discover mechanisms of interface stabilization and destabilization that have not been discussed before for both inertial and accelerated dynamics, and chart perspectives for future research.

Nucleation and Growth of Ordered Copolymer Structures at Reactive Interfaces between PA6 and MA-g-HDPE

We have studied the effect of the interfacial chemical reaction between PA6 and MA-g-HDPE in static conditions at a macroscopically flat interface. Interface destabilization and the growth of instabilities, somehow similar to myelin figures observed in surfactants put in the presence of water, are observed. For the first time in this system, it is shown that ordered microphase-separated copolymer domains, whose morphologies depend on the architecture of the copolymer, namely, essentially on the relative length of the blocks on each side of the interface, may nucleate and grow at a static interface between reactive polymers. We discuss the stability of the plane interface in the case of nonsymmetrical formed graft copolymers. The density of copolymers in the interface (coverage) can be estimated accurately from the long period of the formed structures. We confirm the predictions of Berezkin et al. This observation is very important since it confirms that nanometric domains are certainly generated during reactive extrusion, in addition to micrometric domains formed by rheological processes.

Excitation of Fountain and Entrainment Instabilities at the Interface between Two Viscous Fluids Using a Beam of Laser Light

We report on two instabilities, called viscous fountain and viscous entrainment, triggered at the interface between two liquids by the action of bulk flows driven by a laser beam. These streaming flows are due to light scattering losses in turbid liquids, and can be directed either toward or forward the interface. We experimentally and numerically investigate these interface instabilities and show that the height and curvature of the interface deformation at the threshold and the jet radius after interface destabilization mainly depend on the waist of the laser beam. Analogies and differences between these two instabilities are characterized.

Reaction process in polycarbonate/polyamide bilayer film and blend

Bisphenol-A polycarbonate (PC) and amorphous polyamide (aPA) were used as reactive system to study the interfacial interchange reaction between condensation polymers. Aminolysis is the main process during thermal annealing at 160–180 °C. The simultaneously scission of PC chains and formation of PC-aPA copolymer chains during the reaction process, can act as interfacial compatibilization agents between incompatible homopolymers. Reaction kinetics measured by in-situ Fourier transform infrared spectrum (FTIR) at interfaces of well-separated bilayer films and phase separation blends were compared. The reaction follows a first-order diffusion controlled mechanism and three time regimes were observed. First, the functionalized chains located in the vicinity of the interface participate into the reaction and the annealing time dependence of the integral area of difference IR spectra follows a power law relationship as [Area] ∼ t1/4. Second, a depletion layer of reactants in the interfacial region is formed with the progression of reaction, and center-of-mass diffusion of reactants is required for further reaction which results in a transition of the power law relationship into [Area] ∼ t1/2. Finally, the potential barrier arising from the previously formed copolymers at the interface suppressed the reaction. When the conversion reaches a critical value, thermal fluctuation can induce interface destabilization. An acceleration of reaction rate observed by FTIR for sample annealed at 180 °C is synchronous with the interfacial roughness development analyzed by atomic force microscopy.

Effect of transverse magnetic field on the liquid-solid interface stability and morphology of Al-0.85wt%Cu alloy during directional solidification

The influence of transverse magnetic field on the liquid-solid interface stability and morphology has been investigated in directionally solidified Al-0.85wt%Cu alloy. Experimental results show that the transverse magnetic field causes the interface to be instable and the interface shape to be depressed on one side along the radius. The interface instability increases with increasing magnetic field. Increasing the solidification velocity reduced extent of interface destabilization by the magnetic field. The depression of the interface with the magnetic field is more dramatic at low solidification velocities. These phenomena are attributed to the thermoelectromagnetic convection (TEMC) on the interface and cellular scale.

Numerical analysis of constitutional supercooling during directional solidification of alloys

Some limitations of Tiller’s morphological stability criterion are discussed in the present study. This criterion assumes a purely diffusive regime in the melt as well as a planar solid–liquid interface and a constant solidification rate. But experimental works in agreement with previous numerical modeling have shown a significant decrease of the growth rate and a variable interface curvature during the concentrated semiconductor alloys solidification. The mathematical expression of the morphological stability criterion was derived by using Tiller’s equation, predicting the solute distribution in the liquid. The numerical computations performed in this study show a significant disagreement between the numerical results and Tiller’s formula. Numerical modeling conducted in conditions when the supercooling should occur, show that the Tiller’s stability criterion cannot predict the moment of interface destabilization. The interface destabilization is numerically observed when some fluctuations appear in the liquid solutal profiles and cause the appearance of a supercooled zone inside the liquid at small distance from the interface. The present numerical results are not in contradiction with the basic elements of the classical constitutional supercooling theory, providing only that the stability criterion cannot predict the moment of the interface destabilization.

Electrohydrodynamic actuation of co-flowing liquids by means of microelectrode arrays

Electric fields induce forces at the interface between liquids with different electrical properties (conductivity and/or permittivity). We explore how to use these forces for manipulating two coflowing streams of liquids in a microchannel. A microelectrode array is fabricated at the bottom of the channel and one of the two liquids is labelled with a fluorescent dye for observing the phenomenon. The diffuse interface between the two liquids is deflected depending on the ac signal and conductivity (or permittivity) ratio between the liquids. Only a few volts are needed for observing the interface destabilization, in contrast with other electrode configurations where hundreds of volts are applied.

Gas bubbles in shaped sapphire

Shaped sapphire single crystals almost always contain a typical defect: small bubbles also known as “micro-voids”. The goal of this paper is to review the existing literature and give a survey of our recent work on this specific defect. As a conclusion to our review we propose a mechanism for bubble formation and its occurrence and distribution in shaped sapphire. The bubbles contain CO which originates as a result of complex reactions between the molten alumina, Mo crucible and graphite pieces. The CO dissolves in the molten alumina and is probably present as a saturated solution whatever the growth conditions. Then the dissolved gas is rejected at the solid–liquid interface and bubble nucleation occurs where the concentration of CO is highest. Therefore, the location of the bubbles in the crystal is governed by the hydrodynamics in the meniscus, which include Marangoni convection. The bubble is finally incorporated in the crystal when it reaches a critical size which depends only on the growth rate. When a central capillary channel is used, the bubbles are located in a layer close to the surface of the crystal from where they can be easily removed by polishing. Also, all the literature agrees on the fact that interface destabilization at high velocity results in the presence of bubbles throughout the entire crystal and should be avoided.

Molecular dynamics simulation analyses of viral fusion peptides in membranes prone to phase transition: effects on membrane curvature, phase behavior and lipid-water interface destabilization

To gain insight into the atomistic details of membrane fusion induced by fusogenic peptides, molecular dynamic simulations of synthetic peptides, derived from viral fusion proteins, contained in lipid bilayers were performed. A 20 amino acid peptide from the N-terminus of the influenza HA fusion peptide (WT20) assumed the oblique orientation at the interface between water and the membrane made up of dipalmitoylphosphatidylcholine (DPPC)/palmitic acid (PA), as reported previously for different membranes. Simulations of WT20 embedded in bilayer membranes made up of dioleoylphos-phatidylethanolamine (DOPE) and DPPC/PA showed a positive curvature-inducing effect, whereas WT20 showed a negative curvature-inducing effect on a DPPC bilayer. In phase re-constitution analyses starting from a random mixture of DPPC, PA and water molecules, WT20 weakly stabilized an inverted hexagonal phase. In the latter analyses WT20 preferentially assumed a transmembrane orientation as opposed to the interfacial orientation, regardless of the phase to which the system settled (lamellar vs. inverted hexagonal). In another set of analyses using systems containing a water layer between the apposed DPPC/PA (and DOPE) monolayers, the behavior of WT20 during the formation of an intermembrane connection (or stalk) was examined. Comparison among the mutants supports a view that the oblique orientation of WT20 facilitates the perturbation of the lipid-water interface and the stalk formation. Taken together, these results imply that the influenza HA fusion peptide can have substantial effects on the membrane curvature and can assume a wide range of orientation/position in membranes depending on the local environment of the lipid/water system. Its movability and oblique orientation appear to be associated with its ability to perturb membrane/water interfaces.

Confined suspension jet and long-range hydrodynamic interactions: A destabilization scenario

The collective dynamics of a quasi-two-dimensional suspension jet, of non-Brownian particles, confined in a thin cell and driven by gravitational force is studied both numerically and theoretically. We present a theoretical scheme aimed to describe such a system in the Stokes regime. We focus on the dynamics of the interface between the suspension and the pure fluid. Numerical simulations solving Newton’s equations for all particles show that the jet free surface becomes unstable: the fastest growing modes at small sizes coarsen up to the largest structures reaching the jet lateral scale. In the bulk, structural waves develop and travel at slightly slower speed than the jet average fall. An analytical model, based on hydrodynamic-like equations for the suspension, is derived and predicts the development of the interfacial instability. It captures in essence the collective effects driving the interface destabilization, i.e., the long-range hydrodynamic interactions coupled with the abrupt interface, and no relation to surface tension is found.

Mechanism of Crude-Oil/Water Interface Destabilization by Silicone Demulsifiers

SummaryOil exploitation is always accompanied by the nondesired formation of emulsions caused by the presence of naturally occurring surface-active molecules such as asphaltenes and resins. Because their presence stabilizes the oil/water interface, it is necessary to break the emulsions by adding other surface-active molecules. Formulations based on polysiloxane molecules were selected as effective nontoxic products to enhance the oil/water phase separation. Establishing the relation between the efficiency of formulations and the interfacial properties of silicone molecules is the objective of this study. Formulation selection is based on bottle tests using turbidimetric measurements and on dynamic tests using a "dispersion rig" setup that allows the formation of emulsions under pressure and temperature conditions and injection of additives into the formed emulsion online. Rapid kinetics of separation and high levels of separated water were observed. Dynamic interfacial measurements were performed using complementary techniques. The drop-volume technique allows the measurement of the evolution of crude-oil/water interfacial tension with time. The Langmuir trough technique is used to obtain 2D compression isotherms and to deduce the elastic properties of interfaces. The coalescence of water droplets leading to the destabilization of emulsions and consequently to the oil/water separation efficiency can be related to the rheological properties of water/crude-oil interface. Therefore, such comprehensive study based on a specific methodology can lead to a strict and effective selection of emulsion breaker additives in relation with the oil composition.

Experimental study of the solid–liquid interface dynamics and chemical segregation in concentrated semiconductor alloy Bridgman growth

Three Ga 1− x In x Sb samples with compositions x = 0.04 , 0.1 and 0.2 have been solidified by the vertical Bridgman method. The position and shape of the solid–liquid interface has been marked by electrical pulses all along the growth and revealed by subsequent metallography. The axial and radial segregation of the In has been measured by chemical analysis of the solid. The results show that the effect of the chemical segregation, coupled to thermosolutal convection, leads to huge deformations of the solid–liquid interface, with interface curvature even larger than the sample diameter. A sharp increase of the chemical composition is simultaneously observed in the most concentrated samples and interface destabilization occurs quickly. Interpretation of these results involves a strong coupling of the thermal, hydrodynamical and chemical fields.

Mechanical Stability of Microkeratome-Assisted Intracorneal KeratoprosthesisImplantation

To develop a laboratory model to study intracorneal keratoprosthesis implantation. A combination microkeratome and artificial anterior chamber system was used to create a hinged lamellar keratectomy on 13 human corneas. After reflecting the flap, the posterior stroma was trephined at either 2.5 or 3.0 mm. A model keratoprosthesis was positioned in the bed. The flap was sutured closed. Intrachamber pressure was increased, and wound leak pressure was recorded. The anterior corneal lamella was trephined at either 3.0 or 3.5 mm to expose the keratoprosthesis. Leak pressure was again determined. After keratoprosthesis placement and prior to anterior trephination, all 13 corneas were watertight at maximum attainable intrachamber pressures. With posterior/anterior trephination combinations of 2.5/3.0 mm, 2.5/3.5 mm, or 3.0/3.5 mm, mean +/- SD wound leak pressure occurred at 95 +/- 12 mm Hg, 32 +/- 7 mm Hg, or 59 +/- 12 mm Hg, respectively (P<.01). With a posterior trephination of 2.5 mm, there is significant keratoprosthesis-cornea interface destabilization between a 3.0- and 3.5-mm anterior trephination. For an anterior trephination of 3.5 mm, interface destabilization improves by increasing the posterior trephination to 3.0 mm. An intracorneal keratoprosthesis may be implanted using microkeratome assistance. Our laboratory model provides a useful method for examining a range of posterior and anterior trephination diameters and their effects on the mechanical stability of intracorneal keratoprosthesis placement.

The effects of chemical composition of adsorbed molecular layers on lithium electrode/polymer electrolyte interface stabilization

Previous work in our laboratory has shown that the adsorption of self-assembled molecular layers on a polymer electrolyte surface can make the interface between the electrolyte and a lithium metal electrode more stable and can hinder the growth of the type of passivation layer that inhibits ion movement between the electrode and electrolyte. This work is concerned with the effect that the molecular composition of the adsorbed layer has on interface stability. Several compounds of the form R(CH 2CH 2O)H have been found to adsorb on the surface of the polymer electrolyte and stabilize the interface. It was found that the R group could be a long, straight hydrocarbon tail capable of forming an ordered self-assembled structure or a branched hydrocarbon or a siloxane with bulky hydrocarbon groups that form a brush-type structure. Molecules of the form ROH and RCOOH, where R was a long, straight-chain hydrocarbon, would adsorb but appeared to have much disorder in their hydrocarbon chains. It is proposed that this disorder results from a low adsorption density that results in regions where the lithium metal can come in direct contact with the polymer electrolyte causing reactions that promote interface destabilization.