Implementing a Registry Federation for Materials Science Data Discovery.

As a result of a number of national initiatives, we are seeing rapid growth in the data important to materials science that are available over the web. Consequently, it is becoming increasingly difficult for researchers to learn what data are available and how to access them. To address this problem, the Research Data Alliance (RDA) Working Group for International Materials Science Registries (IMRR) was established to bring together materials science and information technology experts to develop an international federation of registries that can be used for global discovery of data resources for materials science. A resource registry collects high-level metadata descriptions of resources such as data repositories, archives, websites, and services that are useful for data-driven research. By making the collection searchable, it aids scientists in industry, universities, and government laboratories to discover data relevant to their research and work interests. We present the results of our successful piloting of a registry federation for materials science data discovery. In particular, we out a blueprint for creating such a federation that is capable of amassing a global view of all available materials science data, and we enumerate the requirements for the standards that make the registries interoperable within the federation. These standards include a protocol for exchanging resource descriptions and a standard metadata schema for encoding those descriptions. We summarize how we leveraged an existing standard (OAI-PMH) for metadata exchange. Finally, we review the registry software developed to realize the federation and describe the user experience.

PFHub: The Phase-Field Community Hub.

Scientific communities struggle with the challenge of effectively and efficiently sharing content and data. An online portal provides a valuable space for scientific communities to discuss challenges and collate scientific results. Examples of such portals include the Micromagnetic Modeling Group (µMAG [1]), the Interatomic Potentials Repository (IPR [2, 3]) and on a larger scale the NIH Genetic Sequence Database (GenBank [4]). In this work, we present a description of a generic web portal that leverages existing online services to provide a framework that may be adopted by other small scientific communities. The first deployment of the PFHub framework supports phase-field practitioners and code developers participating in an effort to improve quality assurance for phase-field codes.

Microstructure-based knowledge systems for capturing process-structure evolution linkages.

This paper reviews and advances a data science framework for capturing and communicating critical information regarding the evolution of material structure in spatiotemporal multiscale simulations. This approach is called the MKS (Materials Knowledge Systems) framework, and was previously applied successfully for capturing mainly the microstructure-property linkages in spatial multiscale simulations. This paper generalizes this framework by allowing the introduction of different basis functions, and explores their potential benefits in establishing the desired process-structure-property (PSP) linkages. These new developments are demonstrated using a Cahn-Hilliard simulation as an example case study, where structure evolution was predicted three orders of magnitude faster than an optimized numerical integration algorithm. This study suggests that the MKS localization framework provides an alternate method to learn the underlying embedded physics in a numerical model expressed through Green's function based influence kernels rather than differential equations, and potentially offers significant computational advantages in problems where numerical integration schemes are challenging to optimize. With this extension, we have now established a comprehensive framework for capturing PSP linkages for multiscale materials modeling and simulations in both space and time.

Topological defects in two-dimensional orientation-field models for grain growth.

Standard two-dimensional orientation-field-based phase-field models rely on a continuous scalar field to represent crystallographic orientation. The corresponding order parameter space is the unit circle, which is not simply connected. This topological property has important consequences for the resulting multigrain structures: (i) trijunctions may be singular; (ii) for each pair of grains there exist two different grain boundary solutions that cannot continuously transform to one another; (iii) if both solutions appear along a grain boundary, a topologically stable, singular point defect must exist between them. While (i) can be interpreted in the classical picture of grain boundaries, (ii) and therefore (iii) cannot. In addition, singularities cause difficulties, such as lattice pinning in numerical simulations. To overcome these problems, we propose two formulations of the model. The first is based on a three-component unit vector field, while in the second we utilize a two-component vector field with an additional potential. In both cases, the additional degree of freedom introduced makes the order parameter space simply connected, which removes the topological stability of these defects.

Blast Quantification Using Hopkinson Pressure Bars.

Near-field blast load measurement presents an issue to many sensor types as they must endure very aggressive environments and be able to measure pressures up to many hundreds of megapascals. In this respect the simplicity of the Hopkinson pressure bar has a major advantage in that while the measurement end of the Hopkinson bar can endure and be exposed to harsh conditions, the strain gauge mounted to the bar can be affixed some distance away. This allows protective housings to be utilized which protect the strain gauge but do not interfere with the measurement acquisition. The use of an array of pressure bars allows the pressure-time histories at discrete known points to be measured. This article also describes the interpolation routine used to derive pressure-time histories at un-instrumented locations on the plane of interest. Currently the technique has been used to measure loading from high explosives in free air and buried shallowly in various soils.

Diffuse Interface Methods for Modeling Drug-Eluting Stent Coatings.

An overview of diffuse interface models specific to drug-eluting stent coatings is presented. Microscale heterogeneities, both in the coating and use environment, dictate the performance of these coatings. Using diffuse interface methods, these heterogeneities can be explicitly incorporated into the model equations with relative ease. This enables one to predict the complex microstructures that evolve during coating fabrication and subsequent impact on drug release. Examples are provided that illustrate the wide range of phenomena that can be addressed with diffuse interface models including: crystallization, constrained phase separation, hydrolytic degradation, and heterogeneous binding. Challenges associated with the lack of material property data and numerical solution of the model equations are also highlighted. Finally, in light of these potential drawbacks, the potential to utilize diffuse interface models to help guide product and process development is discussed.

The strong influence of internal stresses on the nucleation of a nanosized, deeply undercooled melt at a solid-solid phase interface.

The effect of elastic energy on nucleation and disappearance of a nanometer size intermediate melt (IM) region at a solid-solid (S1S2) phase interface at temperatures 120 K below the melting temperature is studied using a phase-field approach. Results are obtained for broad range of the ratios of S1S2 to solid-melt interface energies, k(E), and widths, k(δ). It is found that internal stresses only slightly promote barrierless IM nucleation but qualitatively alter the system behavior, allowing for the appearance of the IM when k(E) < 2 (thermodynamically impossible without mechanics) and elimination of what we termed the IM-free gap. Remarkably, when mechanics is included within this framework, there is a drastic (16 times for HMX energetic crystals) reduction in the activation energy of IM critical nucleus. After this inclusion, a kinetic nucleation criterion is met, and thermally activated melting occurs under conditions consistent with experiments for HMX, elucidating what had been to date mysterious behavior. Similar effects are expected to occur for other material systems where S1S2 phase transformations via IM take place, including electronic, geological, pharmaceutical, ferroelectric, colloidal, and superhard materials.

Phase-field crystal model with a vapor phase.

Phase-field crystal (PFC) models are able to resolve atomic length scale features of materials during temporal evolution over diffusive time scales. Traditional PFC models contain solid and liquid phases, however many important materials processing phenomena involve a vapor phase as well. In this work, we add a vapor phase to an existing PFC model and show realistic interfacial phenomena near the triple point temperature. For example, the PFC model exhibits density oscillations at liquid-vapor interfaces that compare favorably to data available for interfaces in metallic systems from both experiment and molecular-dynamics simulations. We also quantify the anisotropic solid-vapor surface energy for a two-dimensional PFC hexagonal crystal and find well-defined step energies from measurements on the faceted interfaces. Additionally, the strain field beneath a stepped interface is characterized and shown to qualitatively reproduce predictions from continuum models, simulations, and experimental data. Finally, we examine the dynamic case of step-flow growth of a crystal into a supersaturated vapor phase. The ability to model such a wide range of surface and bulk defects makes this PFC model a useful tool to study processing techniques such as chemical vapor deposition or vapor-liquid-solid growth of nanowires.

Predicting microstructure development during casting of drug-eluting coatings.

We have devised a novel diffuse interface formulation to model the development of chemical and physical inhomogeneities, i.e. microstructure, during the process of casting drug-eluting coatings. These inhomogeneities, which depend on the coating constituents and manufacturing conditions, can have a profound affect on the rate and extent of drug release, and therefore the ability of coated medical devices to function successfully. By deriving the model equations in a time-dependent reference frame, we find that it is computationally viable to probe a wide, physically relevant range of material and process quantities. To illustrate the application of the model, we have evaluated the impact of manufacturing solvent, coating thickness and evaporation rate on microstructure development. Our results suggest that modifying these process conditions can have a strong and nearly discontinuous effect on coating microstructure, and therefore on drug release. Further, we demonstrate that the model can be applied to processes that involve the incremental application of the coating in layers or passes. This new model formulation, which can also be used to predict the kinetics of drug release, provides a tool to elucidate and quantify the relationships between process variables, microstructure and performance. Establishing these relationships can reduce empiricism in materials selection and process design, providing a facile and efficient means to tailor the underlying microstructure and achieve a desired drug-release behavior.

The effect of substrate material on silver nanoparticle antimicrobial efficacy.

With the advent of nanotechnology, silver nanoparticles increasingly are being used in coatings, especially in medical device applications, to capitalize on their antimicrobial properties. The attractiveness of nanoparticulate silver systems is the expected increased antimicrobial efficacy relative to their bulk counterparts, which may be attributed to an increased silver ion (Ag+) solubility, and hence availability, that arises from capillarity effects in small, nanometer-sized particles. However, a change of the material upon which the antimicrobial nanoparticulate silver is deposited (herein called "substrate") may affect the availability of Ag+ ions and the intended efficacy of the device. We utilize both theory and experiment to determine the effect of substrate on ion release from silver particles in electrochemical environments and find that substrate surface charge, chemical reactivity or affinity of the surface for Ag+ ions, and wettability of the surface all affect availability of Ag+ ions, and hence antimicrobial efficacy. It is also observed that with time of exposure to deionized water, Ag+ ion release increases to a maximum value at 5 min before decreasing to undetectable levels, which is attributed to coarsening of the nanoparticles, and which subsequently reduces the solubility and availability of Ag+ ions. This coarsening phenomenon is also predicted by the theoretical considerations and has been confirmed experimentally by transmission electron microscopy.

Modeling the early stages of reactive wetting.

Recent experimental studies of molten metal droplets wetting high-temperature reactive substrates have established that the majority of triple-line motion occurs when inertial effects are dominant. In light of these studies, this paper investigates wetting and spreading on reactive substrates when inertial effects are dominant using a thermodynamically derived diffuse interface model of a binary three-phase material. The liquid-vapor transition is modeled using a van der Waals diffuse interface approach, while the solid-fluid transition is modeled using a phase field approach. The results from the simulations demonstrate an O(t(-1/2)) spreading rate during the inertial regime and oscillations in the triple-line position when the metal droplet transitions from inertial to diffusive spreading. It is found that the spreading extent is reduced by enhancing dissolution by manipulating the initial liquid composition. The results from the model exhibit good qualitative and quantitative agreement with a number of recent experimental studies of high-temperature droplet spreading, particularly experiments of copper droplets spreading on silicon substrates. Analysis of the numerical data from the model suggests that the extent and rate of spreading are regulated by the spreading coefficient calculated from a force balance based on a plausible definition of the instantaneous interface energies. A number of contemporary publications have discussed the likely dissipation mechanism in spreading droplets. Thus, we examine the dissipation mechanism using the entropy-production field and determine that dissipation primarily occurs in the locality of the triple-line region during the inertial stage but extends along the solid-liquid interface region during the diffusive stage.

Grain boundaries exhibit the dynamics of glass-forming liquids.

Polycrystalline materials are composites of crystalline particles or "grains" separated by thin "amorphous" grain boundaries (GBs). Although GBs have been exhaustively investigated at low temperatures, at which these regions are relatively ordered, much less is known about them at higher temperatures, where they exhibit significant mobility and structural disorder and characterization methods are limited. The time and spatial scales accessible to molecular dynamics (MD) simulation are appropriate for investigating the dynamical and structural properties of GBs at elevated temperatures, and we exploit MD to explore basic aspects of GB dynamics as a function of temperature. It has long been hypothesized that GBs have features in common with glass-forming liquids based on the processing characteristics of polycrystalline materials. We find remarkable support for this suggestion, as evidenced by string-like collective atomic motion and transient caging of atomic motion, and a non-Arrhenius GB mobility describing the average rate of large-scale GB displacement.

Modeling solvent evaporation during the manufacture of controlled drug-release coatings and the impact on release kinetics.

To improve functionality and performance, controlled drug-release coatings comprised of drug and polymer are integrated with traditional medical devices, e.g., drug eluting stents. Depending on manufacturing conditions, these coatings can exhibit complex microstructures. Previously, a thermodynamically consistent model was developed for microstructure evolution in these systems to establish relationships between process variables, microstructure, and the subsequent release kinetics. Calculations based on the model were, in general, consistent with experimental findings. However, because of assumptions regarding the evaporation of solvent during fabrication, the model was unable to capture variations through the coating thickness that are observed experimentally. Here, a straightforward method is introduced to incorporate solvent evaporation explicitly into the model. Calculations are used to probe the impact of solvent evaporation rate and drug loading on the microstructure that forms during manufacturing and subsequent drug release kinetics. The predicted structures and release kinetics are found to be consistent with experimental observations. Further, the calculations demonstrate that solvent evaporation rate can be as critical to device performance as the amount of drug within the coating. For example, changes of a factor of five in the amount of drug released were observed by modifying the rate of solvent evaporation during manufacturing.

Modeling microstructure development and release kinetics in controlled drug release coatings.

In recent years, controlled release coatings, comprised of drug-polymer composites, have been integrated with medical devices, improving device functionality and performance. However, relationships between material properties, manufacturing environment, composite (micro)structure, and subsequent release kinetics are not well established. We apply a thermodynamically consistent model to probe the influence of drug-polymer chemistry (phobicity), drug loading, and evaporation rate on microstructure development during fabrication. For these structures, we compute release profiles for exposure to polymer-insoluble media and media in which the polymer readily dissolves. We find that with increasing drug-polymer phobicity, structural heterogeneities form at lower loadings and more rapid rates. The heterogeneities remain isolated and compact at low loadings and become interconnected as the drug to polymer ratio approaches 1.0. Release into polymer-insoluble media was dramatically enhanced by heterogeneities, resulting in up to a fourfold increase in drug release. In polymer-soluble media, however, heterogeneities diminished release. Although reductions of only 30% were typically observed, the absolute changes were much larger than observed in polymer-insoluble media. Our results suggest that improved comprehension and quantification of the physico-chemical properties in controlled release systems will enable the microstructure to be tailored to achieve desired responses that are insensitive to manufacturing variations.

Diffuse-interface theory for structure formation and release behavior in controlled drug release systems.

A common method of controlling drug release has been to incorporate the drug into a polymer matrix, thereby creating a diffusion barrier that slows the rate of drug release. It has been demonstrated that the internal microstructure of these drug-polymer composites can significantly impact the drug release rate. However, the effect of processing conditions during manufacture on the composite structure and the subsequent effects on release behavior are not well understood. We have developed a diffuse-interface theory for microstructure evolution that is based on interactions between drug, polymer and solvent species, all of which may be present in either crystalline or amorphous states. Because the theory can be applied to almost any specific combination of material species and over a wide range of environmental conditions, it can be used to elucidate and quantify the relationships between processing, microstructure and release response in controlled drug release systems. Calculations based on the theory have now demonstrated that, for a characteristic delivery system, variations in microstructure arising due to changes in either drug loading or processing time, i.e. evaporation rate, could have a significant impact on both the bulk release kinetics and the uniformity of release across the system. In fact, we observed that changes in process time alone can induce differences in bulk release of almost a factor of two and typical non-uniformities of +/-30% during the initial periods of release. Because these substantial variations may have deleterious clinical ramifications, it is critical that both the system microstructure and the control of that microstructure are considered to ensure the device will be both safe and effective in clinical use.

Controlling the accuracy of unconditionally stable algorithms in the Cahn-Hilliard equation.

Given an unconditionally stable algorithm for solving the Cahn-Hilliard equation, we present a general calculation for an analytic time step Deltatau in terms of an algorithmic time step Deltat. By studying the accumulative multistep error in Fourier space and controlling the error with arbitrary accuracy, we determine an improved driving scheme Deltat=At(2/3) and confirm the numerical results observed in a previous study [Cheng and Rutenberg, Phys. Rev. E 72, 055701(R) (2005)].

Phase field theory of heterogeneous crystal nucleation.

The phase field approach is used to model heterogeneous crystal nucleation in an undercooled pure liquid in contact with a foreign wall. We discuss various choices for the boundary condition at the wall and determine the properties of critical nuclei, including their free energy of formation and the contact angle as a function of undercooling. For particular choices of boundary conditions, we may realize either an analog of the classical spherical cap model or decidedly nonclassical behavior, where the contact angle decreases from its value taken at the melting point towards complete wetting at a critical undercooling, an analogue of the surface spinodal of liquid-wall interfaces.

Solution of a field theory model of frontal photopolymerization.

Frontal photopolymerization (FPP) provides a versatile method for the rapid fabrication of solid polymer network materials by exposing photosensitive molecules to light. Dimensional control of structures created by this process is crucial in applications ranging from microfluidics and coatings to dentistry, and the availability of a predictive mathematical model of FPP is needed to achieve such control. Previous work has relied on numerical solutions in validating the model against experiments because of the intractability of the governing nonlinear equations. The present paper provides solutions to these equations in the general case in which the optical attenuation decreases (photobleaching) or increases (photodarkening) with photopolymerization. These solutions are of mathematical and physical interest because they support traveling waves of polymerization that propagate logarithmically or linearly in time, depending on the evolution of optical attenuation of the photopolymerized material.

Growth and form of spherulites.

Many structural materials (metal alloys, polymers, minerals, etc.) are formed by quenching liquids into crystalline solids. This highly nonequilibrium process often leads to polycrystalline growth patterns that are broadly termed "spherulites" because of their large-scale average spherical shape. Despite the prevalence and practical importance of spherulite formation, only rather qualitative concepts of this phenomenon exist. It is established that phase field methods naturally account for diffusional instabilities that are responsible for dendritic single-crystal growth. However, a generalization of this model is required to describe spherulitic growth patterns, and in the present paper we propose a minimal model of this fundamental crystal growth process. Our calculations indicate that the diversity of spherulitic growth morphologies arises from a competition between the ordering effect of discrete local crystallographic symmetries and the randomization of the local crystallographic orientation that accompanies crystal grain nucleation at the growth front [growth front nucleation (GFN)]. This randomization in the orientation accounts for the isotropy of spherulitic growth at large length scales and long times. In practice, many mechanisms can give rise to GFN, and the present work describes and explores three physically prevalent sources of disorder that lead to this kind of growth. While previous phase field modeling elucidated two of these mechanisms--disorder created by particulate impurities or other static disorder or by the dynamic heterogeneities that spontaneously form in supercooled liquids (even pure ones)--the present paper considers an additional mechanism, crystalline branching induced by a misorientation-dependent grain boundary energy, which can significantly affect spherulite morphology. We find the entire range of observed spherulite morphologies can be reproduced by this generalized phase field model of polycrystalline growth.