Interface Mobility Research Articles

Cahn-Hilliard mobility of fluid-fluid interfaces from molecular dynamics

The Cahn-Hilliard equation is often used to model the temporospatial evolution of multiphase fluid systems including droplets, bubbles, aerosols, and liquid films. This equation requires knowledge of the fluid-fluid interfacial mobility γ, a parameter that can be difficult to obtain experimentally. In this work, a method to obtain γ from nonequilibrium molecular dynamics is presented. γ is obtained for liquid-liquid and liquid-vapor interfaces by perturbing them from their equilibrium phase fraction spatial distributions, using molecular dynamics simulations to observe their relaxation toward equilibrium, and fitting the Cahn-Hilliard model to the transient molecular simulations at each time step. γ is then compared to a different measure of interfacial mobility, the molecular interfacial mobility M. It is found that γ is proportional to the product of M, the interface thickness, and the ratio of thermal energy to interfacial energy.

Stability analysis of electroconvection with a solid-liquid interface via the lattice Boltzmann method

Electroconvection in dielectric liquid is extended from single phase to solid-liquid interaction. Stability criteria and bifurcation are numerically predicted by the lattice Boltzmann method. Effects of interface position, permittivity and mobility ratios, and electric conductivity are considered.

Analytical Modeling of the Mixed-Mode Growth and Dissolution of Precipitates in a Finite System

In this paper, a novel analytical modeling of the growth and dissolution of precipitates in substitutional alloys is presented. This model uses an existing solution for the shape-preserved growth of ellipsoidal precipitates in the mixed-mode regime, which takes into account the interfacial mobility of the precipitate. The dissolution model is developed by neglecting the transient term in the mass conservation equation, keeping the convective term. It is shown that such an approach yields the so-called reversed-growth approximation. A time discretization procedure is proposed to take into account the evolution of the solute concentration in the matrix as the phase transformation progresses. The model is applied to calculate the evolution of the radius of spherical -Al2Cu precipitates in an Al rich matrix at two different temperatures, for which growth or dissolution occurs. A comparison of the model is made, with the results obtained using the numerical solver DICTRA. The very good agreement obtained for cases where the interfacial mobility is very high indicates that the time discretization procedure is accurate.

On the effect of the approach velocity on the coalescence of fluid particles

The effect of the constant relative approach velocity of two Newtonian fluid particles on their coalescence is investigated via a film drainage model. The interfaces are deformable and allowed to have any degree of mobility. The thinning equation is derived based on the lubrication theory, and the tangential velocity of the interface is determined via the Boundary Integral Method (BIM). The details for the treatment of the inherent singularity in BIM are provided. As the approach velocity increases, regardless of the strength of the van der Waals forces and the value of the viscosity ratio (or the degree of the interfacial mobility), three types of behavior of the coalescence time are identified: the linear slow, the dimpled and the multiple-rim drainage regimes. In the first two regions the coalescence time decreases with the approach velocity, eventually passes through a minimum and starts to increase in the third region. The slope in the linear region and the critical velocities separating the regimes are used to quantify the behavior, and given as functions of the Hamaker constant and the viscosity ratio. The results are shown to be in good agreement with several recent experimental studies in the literature.

Precipitation Kinetics and Evaluation of the Interfacial Mobility of Precipitates in an AlSi7Cu3.5Mg0.15 Cast Alloy with Zr and V Additions

Recent environmental restrictions constrained car manufacturers to promote cast aluminum alloys working at high temperatures (180 °C–300 °C). The development of new alloys permits the fabrication of higher-strength components in engine downsizing. Those technologies increase internal loadings and specific power and stretch current materials to their limits. Transition metals in aluminum alloys are good candidates to improve physical, mechanical, and thermodynamic properties with the aim of increasing service life of parts. This study is focused on the modified AlSi7Cu3.5Mg0.15 alloy where Mn, Zr, and V have been added as alloying elements for high-temperature applications. The characterization of the cast alloy in this study helps to evaluate and understand its performance according to their physical state: As-cast, as-quenched, or artificially aged. The precipitation kinetics of the AlSi7Cu3.5Mg0.15 (Mn, Zr, V) alloy has been characterized by differential scanning calorimetry (DSC), transmission electron microscopy (TEM) observations, and micro-hardness testing. The Kissinger analysis was applied to extract activation energies from non-isothermal DSC runs conducted at different stationary heating rates. Finally, first-order evaluations of the interfacial mobility of precipitates were obtained.

Quantifying uncertainty in the process-structure relationship for Al–Cu solidification

Phase field method (PFM) is a simulation tool to predict microstructural evolution during solidification and is helpful to establish the process-structure relationship for alloys. The robustness of the relationship however is affected by model-form and parameter uncertainties in PFM. In this paper, the uncertainty associated with the thermodynamic and process parameters of PFM is studied and quantified. Surrogate modeling is used to interpolate four quantities of interests (QoIs), including dendritic perimeter, area, primary arm length, and solute segregation, as functions of thermodynamic and process parameters. A sparse grid approach is applied to mitigate the curse-of-dimensionality computational burden in uncertainty quantification. Polynomial chaos expansion is employed to obtain the probability density functions of the QoIs. The effect of parameter uncertainty on the Al–Cu dendritic growth during solidification simulation are investigated. The results show that the dendritic morphology varies significantly with respect to the interface mobility and the initial temperature.

Interfaces and Phase Suppression in Biological Systems

We consider how interfaces can influence the early stages of a new phase forming in an existing phase, e.g., diseased or cancerous cells attempting to form in a region of healthy cells, or misfolded protein molecules forming amyloid deposits. We suggest a model where diseased cells can be suppressed by suitable interplay between the kinetics of disease formation, apoptosis and replacement cell creation. Maximising the energetic cost of the interface between cancerous and healthy tissue may be important for inhibiting metastasis, as small clusters attempt to escape from the surface of the parent tumour. Anything that can stabilise the interface between the tumour and healthy tissue, and so inhibit escaping cells, is useful Interfaces associated with neurodegenerative illness form when layers of misfolded protein molecules are deposited around cells. Particular nucleation sites might catalyse or 'seed' misfolding of proteins, or stabilise misfolded conformations, to begin formation of insoluble protein layers. Therapeutic drugs could suppress dangerous deposits of misfolded proteins by binding to, and hence blocking, these sites. Numbers of potential nucleation sites might be reduced if stressed cells are eliminated through apoptosis: it is likely better to remove a few stressed cells than have them remain as nucleation sites for deposits which can grow and damage many cells. Finding compromises that diminish potential nucleation sites without excessive destruction of functioning cells will become more difficult as reserves of healthy cells diminish with increasing age. There is a natural arrow of time at work, and an inverse relation with cancer. Particular combinations of interfacial mobility and deposition geometry may result in amyloid plaques that vary in density and which are thus more or less resistant to being removed by natural processes or medical interventions.

Collective Motion in the Interfacial and Interior Regions of Supported Polymer Films and Its Relation to Relaxation.

To understand the role of collective motion in the often large changes in interfacial molecular mobility observed in polymer films, we investigate the extent of collective motion in the interfacial regions of a thin supported polymer film and within the film interior by molecular dynamics simulation. Contrary to commonly stated expectations, we find that the extent of collective motion, as quantified by string-like molecular exchange motion, is similar in magnitude in the polymer-air interfacial layer as the film interior and distinct from the bulk material. This finding is consistent with Adam-Gibbs description of the segmental dynamics within mesoscopic film regions, where the extent of collective motion is related to the configurational entropy of the film as a whole rather than a locally defined extent of collective motion or configurational entropy.

Superionic UO2: A model anharmonic crystalline material.

Crystalline materials at elevated temperatures and pressures can exhibit properties more reminiscent of simple liquids than ideal crystalline materials. Superionic crystalline materials having a liquidlike conductivity σ are particularly interesting for battery, fuel cell, and other energy applications, and we study UO2 as a prototypical superionic material since this material is widely studied given its commercial importance as reactor fuel. Using molecular dynamics, we first investigate basic thermodynamic and structural properties. We then quantify structural relaxation, dynamic heterogeneity, and average ion mobility. We find that the non-Arrhenius diffusion and structural relaxation time of this prototypical superionic material can be described in terms of a generalized activated transport model ("string model") in which the activation energy varies with the average string length. Our transport data can also be described equally well by an Adam-Gibbs model in which the excess entropy density of the crystalline material is estimated from specific heat and thermal expansion data, consistent with the average scale of stringlike collective motion scaling inversely with the excess entropy of the crystal. Strong differences in the temperature dependence of the interfacial mobility from nonionic materials are observed, and we suggest that this difference is due to the relatively high cohesive interaction of ionic materials. In summary, the study of superionic UO2 provides insight into the role of cooperative motion in enhancing ion mobility in ionic materials and offers design principles for the development of new superionic materials for use in diverse energy applications.

Application of the Mixed-Mode Model for Numerical Simulation of Pearlitic Transformation

The multiscale model of pearlitic transformation, including the finite element solution of the second Fick’s equation with moving boundary in the microscale and Fourier equation in the macroscale, is presented in this paper. According to the mixed-mode approach, both the volume diffusion and the interface mobility were considered. Model describes sidewise and frontal growth of cementite and ferrite plates in a single grain of the austenite. Assessment of the possibility of the developed model application to determine the key parameters in terms of strength properties of pearlitic steel, i.e., pearlite grain size and colony size as well as interlamellar spacing for various cooling conditions, was the main aim of this work. The model was validated and verified on the basis of experimental tests performed for two eutectoid steels. In practice, developed model can support design process for a technology of high-strength rods and long products manufacturing. Rails were selected as a case study in this work, and therefore, numerical simulations of accelerated cooling of the rail head were performed and the relationship between the parameters of heat treatment, parameters of the structure and properties of the finished product was determined on the basis of obtained results.

Study of Surfactant–Polymer Flooding in High-Temperature and High-Salinity Carbonate Rocks

Secondary waterflood processes have low oil recovery in carbonate reservoirs as a result of their oil-wetness and heterogeneity. Surfactants in combination with polymer solutions can improve the oil recovery significantly from these reservoirs through ultralow interfacial tension (IFT), mobility control, and wettability alteration. However, application of surfactant–polymer (SP) flooding in high-salinity and high-temperature carbonate reservoirs is constrained by the thermal stability of polymers at elevated temperatures, compatibility of surfactants with a high concentration of divalent cations present in formation brines, and geochemical interactions with carbonate minerals. This research is focused on understanding surfactant and polymer interactions with formation brines containing high concentrations of divalent cations and thermal stability and transport of polymers in carbonate rocks at a high temperature (80 °C). Surfactant phase behavior experiments (with and without polymer) were performed to id...

Evaluation of the Growth Kinetics of θ′ and θ-Al2Cu Precipitates in a Binary Al-3.5 Wt Pct Cu Alloy

Heat treatment for precipitation hardening is known to have a significant effect on the nano/microstructure of cast aluminum alloys, and thereby affecting its properties. In this study, precipitation kinetics after solutionizing and water quenching have been characterized by differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and microhardness evaluations under different aging conditions. The Kissinger methodology was applied along with the Lee–Kim–Starink–Zahra kinetic equation to determine the kinetic parameters from the DSC runs at constant heating rates, assuming that the precipitates have an ellipsoidal shape. The TEM results showed evidence of semi-coherent θ′ precipitation in accordance with the microhardness evolution during isothermal aging at 190 °C and the kinetic analysis from the DSC data. The TEM bright field images of the specimens aged at two different times were used to record the size and number density of the precipitates. The activation energies for the precipitation kinetics of θ and θ′ were 330 and 114 kJ/mol, respectively. Finally, the values for the interfacial mobility were determined using the kinetic parameters derived from the DSC results and the TEM observations.

Versatile electrification of two-dimensional nanomaterials in water

The recent emergence of nanofluidics has highlighted the exceptional properties of graphene and its boron-nitride counterpart as confining nanomaterials for water and ion transport. Surprisingly, ionic transport experiments have unveiled a consequent electrification of the water/carbon surfaces, with a contrasting response for its water/boron-nitride homologue. In this paper, we report free energy calculations based on ab initio molecular dynamics simulations of hydroxide OH− ions in water near graphene and hexagonal boron nitride (h-BN) layers. Our results disclose that both surfaces get charged through hydroxide adsorption, but two strongly different mechanisms are evidenced. The hydroxide species shows weak physisorption on the graphene surface while it exhibits also strong chemisorption on the h-BN surface. Interestingly OH− is shown to keep very fast lateral dynamics and interfacial mobility within the physisorbed layer on graphene. Taking into account the large ionic surface conductivity, an analytic transport model allows to reproduce quantitatively the experimental data.

High-throughput calculations in the context of alloy design

Abstract

Kinetic theory of nucleation in multicomponent systems: An application of the thermodynamic extremum principle

Nucleation kinetics in a multicomponent supersaturated solid solution is examined. Attachment rate of atoms to a nucleus of a size close to the critical one is determined combining a thermodynamic extremum principle and the Fokker-Planck equation. Two limiting cases are examined; when bulk diffusion controls the nucleation kinetics and when the process is limited by the interfacial mobility. The mixed regime is also treated. Moreover, the growth law in multicomponent alloys is derived in the general case, when both mechanisms are considered. Additionally, the attachment rate is derived, in the classical framework, from a new macroscopic growth equations and the fundamental role of the interfacial mobility is examined. These new general expressions, for the attachment rates and the growth laws, determined either applying the thermodynamic extremum principle or derived from the classical formalism are found to be consistent.

Modeling droplet coalescence kinetics in microfluidic devices using population balances

The coalescence kinetics of oil-in-water emulsions in a wide range of properties and flow microfluidic conditions is quantified. The conditions were chosen in order to mimic situations found in industrial processes involving liquid-liquid dispersions. With that aim, a numerical scheme based on population balance equations is proposed, applied, and validated by comparison to microfluidic experiments reported in the literature. A coalescence efficiency model accounting for colloidal and hydrodynamic interactions, and interface mobility is incorporated using the Smoluchowski collision kernel. The latter assures the accurate estimation of the droplet size evolution which governs the interfacial area and rate of mass transfer. Besides, the combined effect of interfacial tension and oil viscosity on the coalescence kinetics is properly quantified with one single fitting parameter. From the kinetics, the estimated coalescence time increases as the shear rate and volume fraction of the dispersed phase diminish. The close agreement of our results with the experimental findings substantiates the accuracy and wider application of the methodology here described as a diagnostic tool beneficial to industrial process design and control.

Fabrication of Ag2O/Ag decorated ZnAl-layered double hydroxide with enhanced visible light photocatalytic activity for tetracycline degradation

The photocatalytic performance of layered double hydroxides (LDH) is usually confined to the slow interface mobility and high recombination rate of photogenerated electron-hole pairs in material. To overcome the low photocatalytic efficiency, novel Ag2O/Ag decorated LDH (LDH-Ag2O/Ag) was successfully synthesized by depositing Ag2O on the surface of LDH and then converted to Ag° nanoparticles in the right position after heat treatment. The as-synthesized LDH-Ag2O/Ag composites were characterized by Powder X-ray diffraction (XRD), Scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV–visible diffuse reflectance spectra (UV–vis DRS), photoluminescence spectra (PL) and transient photocurrent (TPC) analysis. Compared with virgin LDH, the photocatalytic activities of LDH-Ag2O/Ag composites were enhanced significantly. The optimum photocatalytic efficiency of LDH-Ag10 (0.0184 min−1) was nearly 46 times higher than that of virgin LDH (0.0004 min−1). The result of active species trapping experiments indicated that •OH, h+, and •O2− have an effect on the TC degradation, where •OH played the predominant role during the photocatalytic process. The possible photocatalytic mechanisms involving the charge transfer pathway and reactive species generation during the process of TC degradation were also discussed. The improved photocatalytic activity of LDH-Ag2O/Ag could be attributed to the synergetic effect between LDH and Ag2O/Ag that extended visible light range and reduced photogenerated charge carriers recombination.

Role of unsaturated hydrocarbon lubricant on the friction behavior of amorphous carbon films from reactive molecular dynamics study

Compositing amorphous carbon (a-C) film with fluid lubricant could successfully improve the friction properties and prolong the service life and reliability of protected components. However, the inevitable existence of unsaturated molecules in base oil, especially its potential effect on the friction behavior and related mechanism are still not fully understood. In this paper, the friction behavior of amorphous carbon (a-C) composited with unsaturated hydrocarbon lubricant, C5H10 as one of linear alpha olefins (AO), was explored by reactive molecular dynamics simulation, and the role of C5H10 content on friction property and interfacial structure was mainly analyzed. Results revealed that at the fixed contact pressure, the unsaturated C5H10 bound with a-C in the form of weak intermolecular interactions rather than chemical bonding and there was also no C5H10 dissociation observed; the friction coefficient was strongly dependent on the small unsaturated hydrocarbon, which decreased obviously with C5H10 content. In particular, compared to the dry condition, the existence of C5H10 molecules at the friction interface could significantly reduce the friction coefficient by 99.2% maximally. By the systematical analysis of interfacial hybridization structure and AO mobility, it indicated that the synergistic mechanism for the low friction originated from the self-passivation of a-C surface and AO hydrodynamic lubrication. Our results not only disclose the effect of unsaturated hydrocarbon molecules in bas oil on friction behavior and define the underlying lubrication mechanism, but also suggest a strategy for the design of both fluid lubricant and friction interface to realize the long-lifetime application.

Interfacial interactions, crystallization and molecular mobility in nanocomposites of Poly(lactic acid) filled with new hybrid inclusions based on graphene oxide and silica nanoparticles

Polymer composites based on poly(lactic acid) (PLA) and three fillers, graphene oxide (GO), silica nanoparticles, and a nanohybrid consisting of silica doped GO, were prepared and investigated. A combination of complimentary techniques, namely thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), polarized light microscopy (PLM), differential scanning calorimetry (DSC), and broadband dielectric spectroscopy (BDS), were employed to study the polymer-filler interaction, crystallization and molecular mobility. For the low filler content of 1 wt%, all inclusions act as nucleating agents, increasing the rate of PLA crystallization. Among all fillers, including the mixture of pristine silica and GO, the nanohybrid imposes the stronger effects on crystallization rate, the latter being highly desirable in industrial processing of PLA. The crystalline fraction (∼30%) was found to be the dominant factor in molecular mobility, is agreement with previous investigations on PLA based systems. Based on the degree of polymer/filler interaction (FTIR) and the suppression in calorimetric/dielectric strength of the glass transition (DSC/BDS), the interfacial rigid amorphous fraction (RAF) could be estimated. Interestingly, RAF exhibits its largest value for the composite based on the hybrid filler, in qualitative agreement between the three techniques. The latter fraction, representative of the polymer adsorption onto the nanofillers, is considered important within many applications (thin polymeric films, polymer nanocomposites, etc), furthermore, it is widely believed that RAF dominates the observed improvements of the properties in the nanocomposites.

Temperature-dependent stability of θ′-Al2Cu precipitates investigated with phase field simulations and experiments

The upper limit of service temperature for many Al-Cu alloys is determined by the thermal stability of strengthening θ′ (Al2Cu) precipitates. Above a certain temperature, θ′ precipitates will undergo morphological evolution and transform into the detrimental, equilibrium θ phase, leading to a rapid drop in strength. Certain alloying elements have recently been reported to increase the thermal stability of θ′ precipitates, by mechanisms that are yet unclear. Herein, we investigate the effect of modified interfacial energy and solute chemical mobility on the thermal stability of θ′ via high-throughput phase field study. We identify a critical θ′ aspect ratio to predict the onset of θ formation. Using this criterion, we predict the time required for θ′ to θ phase transformation as a function of temperature, Cu diffusivity, and the interfacial energy of θ′ precipitates. The predicted times compare favorably with reported times for θ formation under similar experimental conditions. These phase field simulations predict that a moderate reduction in Cu mobilityis adequate to stabilize the as-aged microstructure up to 300 °C, while substantial reductions to both interfacial energy and Cu mobility are needed to achieve similar stability at 400 °C. Experimental microstructural evolution results in commercial (319) and thermally stabilized (RR350) cast aluminum alloys are presented to complement the simulations.