Annual Review of Materials Research - Volume 38, 2008

Large-scale molecular dynamics simulations are increasingly applied to the study of wetting dynamics. They have the advantage of being able not only to model the macroscopic world with reasonable accuracy, but also to reveal details at the microscopic level. Here we review the principles of modeling, using molecular dynamics, and comment on the specific methods used in our laboratory in comparison with others. By way of example, we explain how we extract physical parameters from simulations of partially wetting liquids on homogeneous substrates in a variety of geometries. We then show how the results have been applied to build our understanding of the underlying mechanisms.

This review critically examines the experimental aspects of dynamic wetting, considering extant data for different geometries. Both partial wetting and complete wetting are considered for cases that include droplets, fibers, tubes, and plates. The wetting of porous materials is also considered, along with the new field of micro- and nanofluidics.

Wetting of single-crystal surfaces of metallic solids and oxides by liquid metals and alloys is anisotropic because of the anisotropy of the energies of solid surfaces and of their interfaces with liquids. To provide a basis for a discussion of wetting anisotropy, we review first the literature on surface and interfacial energies in metal-metal and metal-ceramic systems. We then summarize the relevant literature on the anisotropy of the energies of solid surfaces and of solid-liquid interfaces. These energies are then used in conjunction with the Young equation to estimate the expected anisotropy of wetting. Some serious discrepancies are found between these estimates and the experimental anisotropy of wetting. We show that under certain experimental conditions the wetting angles that have been determined are not Young contact angles.

We discuss in this review how the roughness of a solid impacts its wettability. We see in particular that both the apparent contact angle and the contact angle hysteresis can be dramatically affected by the presence of roughness. Owing to the development of refined methods for setting very well-controlled micro- or nanotextures on a solid, these effects are being exploited to induce novel wetting properties, such as spontaneous filmification, superhydrophobicity, superoleophobicity, and interfacial slip, that could not be achieved without roughness.

Surfaces exhibiting complex topographies, such as those encountered in biology, give rise to an enormously rich variety of interfacial morphologies of a liquid to which they are exposed. In the present article, we elaborate on some basic mechanisms involved in the statics and dynamics of such morphologies, focusing on a few simple paradigm topographies. We demonstrate that different liquid interface morphologies on the same sample frequently coexist. To exemplify the impact of the dynamics on the final droplet morphology, we discuss the shape instability of filamentous liquid structures in wedge geometries. We finally show that some side effects that may dominate on a larger scale, such as contact line pinning and contact angle hysteresis, seem to play a minor role on the microscopic scale under study. This establishes the validity of simple theoretical concepts of wetting as a starting point for describing liquids at substrate surfaces of high complexity.

This review discusses Monte Carlo simulations of simple models for fluids and binary mixtures (lattice gas, lattice models for binary polymer blends, Asakura-Oosawa model of colloid-polymer mixtures) in confined geometries. The geometries considered are thin films confined by either symmetric or competing walls, as well as wedges and pyramid-shaped cones, including “closed” geometries such as double wedges and bipyramids. Testing of theoretical concepts on the phase behavior in these geometries is emphasized: There is a delicate interplay between wetting phenomena and phase separation into vapor and liquid (or two fluids of different composition, respectively). Capillary condensation phenomena occur in thin films with symmetric walls, and interface localization transitions occur in cases with competing walls. Concepts on wedge filling and cone filling are also examined.

We focus on the dynamical aspects of wetting phenomena on the nanoscale for which bulk hydrodynamic equations become invalid. At the nanoscale, phenomena that are irrelevant on the micrometer scale and larger, or that can be summarily incorporated in terms of boundary conditions, become important. Among these features are long-ranged molecular interactions such as dispersion forces, thermal fluctuations, hydrodynamic slip, segregation of mixtures and solutions at walls, and electrical double layers.

Wetting generally involves three phases and is controlled by the energies of the three interfaces that separate each of the three pairs of phases. Because interfacial segregation can significantly modify interfacial energies, it plays an important role in wetting phenomena. This paper is divided into two principal parts. The first part describes the effects of interfacial energy anisotropy in wetting equilibrium, the thermodynamics of segregation and its effects on interfacial energies, and the interaction of segregation and wetting. The second part reviews some experimental approaches for controlling and monitoring interfacial segregation in wetting studies and summarizes examples of wetting experiments in which such approaches have been used effectively to understand the impact of segregation effects.

This article reviews data on wetting and the work of adhesion in metal/oxide systems at high temperatures in terms of proposed mutual adsorption models. Favorable adsorption energies can induce changes, especially near the high- and low-p(O2) limits of the coexistence range that often mimic bulk oxidation or reduction reactions. However, in some systems, an intermediate range of p(O2) exists wherein all the interfaces are stoichiometric and the contact angle and work of adhesion are independent of p(O2). A revision of existing data for several pure metal/Al2O3 systems affirms predicted trends and reveals that the contact angle is 110°–130° at the plateau and drops to or below 90° at the high- and low-p(O2) limits. This article also compares the data with recent advances in the characterization and modeling of metal-ceramic interfaces at the atomic level.

The stabilization of nanoscale surficial amorphous films (SAFs) for Bi2O3 on ZnO, VOx on TiO2, SiOx on Si, and several other oxide systems provides evidence for the existence of prewetting phenomena with analogies in water and other simple systems, as well as the stabilization of intergranular amorphous films in ceramics. Experimental results show that in the subeutectic regime, the equilibrium film thickness decreases monotonically with decreasing temperature until it vanishes at a dewetting (prewetting) temperature. With increasing temperatures, nanometer-thick SAFs persist into a solid-liquid coexistence regime, in equilibrium with partial-wetting drops, with a gradual decrease in the macroscopic contact angle upon heating. The presence of an attractive dispersion force can significantly delay or inhibit the (otherwise expected) occurrence of complete wetting at higher temperatures. The equilibrium thickness of SAFs is explained from a balance between several interfacial interactions, including dispersion forces, short-range forces of structural or chemical origins, volumetric free-energy terms, and electrostatic interactions. In a generalized Cahn critical-point wetting model, these SAFs are alternatively considered to be disordered multilayer adsorbates formed from coupled prewetting and premelting transitions.

Wettability of solid metals by molten solders is reviewed. The contact angle and wetting force are tabulated for various combinations of solid metals and molten solders such as Sn-Pb base alloys, Sn-Ag base alloys, Sn-Zn base alloys, Sn-Cu base alloys, and Sn-Bi base alloys. Studies on the wetting rate are also discussed.

Experimental results on S segregation at growing Al2O3/alloy interfaces are reviewed for binary FeAl, NiAl alloys, and ternary alloys with additions of Cr, Pt, or a reactive element, such as Zr, Hf, or Y. The segregation behavior is thermodynamic in nature, but the segregation energy can change not only with alloying elements but also with oxidation time and temperature as the oxide growth process changes. Although reactive elements are capable of eliminating interfacial S segregation, they do not stop such segregation to alloy surfaces. The segregation of a reactive element at the interface further strengthens the interfacial bonding. Cosegregation of S and Cr can occur, resulting in higher levels of S at the interface. Pt usually suppresses S segregation, but this effect can be easily overwhelmed by S-Cr cosegregation. Synergisms between alloying elements and how they affect segregation, as well as the relationship between segregation and the oxidation process, are areas that demand further investigation.

 

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• 1.  During high-temperature oxidation of Al2O3-forming alloys, at T > 900°C S is the only nonmetallic impurity that consistently segregates at the growing interface; its presence weakens the interfacial strength.
• 2.  S begins to segregate to Al2O3/FeAl and NiAl interfaces only when a complete layer of α-Al2O3 is developed. No S other than on interfacial void faces is detected when the interface is that between a transition alumina and the alloy.
• 3.  With NiAl, S segregates to the interface only when the alloy is a disordered phase. Concentrations not much above background levels are found with ordered γ′ or stoichiometric β alloys.
• 4.  Small amounts of reactive elements (REs) (<0.1 at%) added in Ni- or Fe-based alloys prevent S segregation to the Al2O3/alloy interface, and the REs that segregate to interfaces further increase the interfacial strength.
• 5.  S and Cr cosegregate to the interface, causing an increase in the interfacial S concentration. Pt eliminates S segregation at alumina/β-NiPtAl interfaces and reduces it when the alloy is the γ/γ′ phase. The effect of Pt can be overwhelmed by the cosegregation of S with Cr.
• 6.  The interfacial segregation process is thermodynamic in nature. The extent of segregation depends on the chemical potential of the solute in the alloy and at the interface, and interactions between the solute and other alloying elements. The segregation energy, however, tends to vary with time owing to the dynamic nature of the oxidation process.

Combinatorial materials science offers an exciting experimental strategy for rapidly surveying a wide array of materials chemistries and process variables coupled to the screening of structure and properties. Adapting approaches used in synthetic organic chemistry for applications such as pharmaceutical sciences and chemical discovery, materials scientists have developed a variety of approaches to create libraries in the solid state in order to rapidly examine a broad range of materials characteristics; the ultimate hope is to accelerate the discovery of new materials and/or new materials properties. This article provides an overview of the different experimental strategies used in combinatorial experimentation and high-throughput screening in materials science and engineering and the challenges to analyzing the information obtained from such experiments. Particular focus is placed on the use of informatics to convert the data from high-throughput experimentation to high-throughput knowledge discovery. The review also raises the broader issue of future needs in combinatorial materials science, such as making this area an experimental platform for multiscale modeling, and the need for a stronger materials-theory-driven approach to combinatorial experimentation.

Over centuries and millennia, our ancestors worldwide found the most appropriate materials for increasingly complex acoustical applications. In the temperate climate of Europe, where the instruments of the Western symphony orchestra were developed and perfected, instrument makers still primarily take advantage of the unique property combination and the aesthetic appeal of wood. In all other continents, one material dominates and is frequently chosen for the manufacture of wind, string, and percussion instruments: the grass bamboo. Here, we review from a materials science perspective bamboo's and wood's unique and highly optimized structure and properties. Using material property charts plotting acoustic properties such as the speed of sound, the characteristic impedance, the sound radiation coefficient, and the loss coefficient against one another, we analyze and explain why bamboo and specific wood species are ideally suited for the manufacture of xylophone bars and chimes, flutes and organs, violins and zithers, violin bows, and even strings.

Patterning ferroelectric domains to engineer devices is a relatively recent phenomenon. It is practiced on nonlinear optical materials to make micrometer-sized devices and on perovskite-based thin crystals to progress toward memory devices. Such patterning forms the basis of lithographic processing of heterogeneous nanostructures on thin-film oxides and polymers. The fundamental aspects of polarization switching relevant to patterning are summarized, after which the most common methods of patterning ferroelectric compounds are reviewed, with an emphasis on poling mechanisms for each case. Issues related to the stability of domain patterns and limitations on the smallest domain size are discussed. Finally, lithography based on ferroelectric patterning is demonstrated for a number of complex systems.

The crystal chemistry of complex perovskite dielectric oxides is reviewed, with an emphasis on structures derived from ordering of the cations on the octahedral B-sites. New classes of perovskites, designed to exhibit 1:2 or 1:3 B-site order for application as low-dielectric-loss microwave ceramics, are identified, and their synthesis, structure, and properties are described. Through the use of B-site chemistries based on Li, Nb, Ta, Ti, and W, members of four new families with 1:2 order, A(β^I1/3β^II2/3)O3, and three new families with 1:3 order, A(β^I1/4β^II3/4)O3, were successfully prepared. The formation and stability of the new and previously prepared ordered perovskites are rationalized through the use of familiar crystal chemical tools such as cation size and charge difference, bond valence, tolerance factor, and new concepts related to the local charge imbalance on the A- and B-sublattices. These tools can be successfully applied to develop stability field maps for each structure and to predict other new ordered systems.

This article summarizes the general structural features and important properties of quasicrystals, a relatively new class of materials belonging to solid-state substances. Quasicrystals, which have a long-range quasiperiodic order but no three-dimensional translational periodicity, are structurally different from both conventional crystals and glassy solids. Although applications are still highly limited, quasicrystals are attracting significant attention as a structual constituent of mixed-phase materials exhibiting high strength together with certain ductility and can be applied as structural materials.

A new paradigm for ceramic composite structural components enables functionality in heat exchange, transpiration, detailed shape, and thermal strain management that significantly exceeds the prior art. The paradigm is based on the use of three-dimensional fiber reinforcement that is tailored to the specific shape, stress, and thermal requirements of a structural application and therefore generally requires innovative textile methods for each realization. Key features include the attainment of thin skins (less than 1 mm) that are nevertheless structurally robust, transpiration holes formed without cutting fibers, double curvature, compliant integral attachment to other structures that avoids thermal stress buildup, and microcomposite ceramic matrices that minimize spalling and allow the formation of smooth surfaces. All these features can be combined into structures of very varied gross shape and function, using a wide range of materials such as all-oxide systems and SiC and carbon fibers in SiC matrices. Illustrations are drawn from rocket nozzles, thermal protection systems, and gas turbine engines. The new design challenges that arise for such material/structure systems are being met by specialized computational modeling that departs significantly in the representation of materials behavior from that used in conventional finite element methods.

The widespread enthusiasm for research on bulk metallic glasses is driven by both a fundamental interest in the structure and properties of disordered materials and their unique promise for structural and functional applications. Unlike the case for crystalline materials, the disordered and nonequilibrium nature of metallic glasses causes their underlying deformation mechanisms to be poorly known. A definite correlation between mechanical behavior and the atomic/electronic structures of metallic glasses has not been established. In this article, I focus on the micromechanisms of mechanical behavior of metallic glasses and present a brief overview of the current understanding of their strength, ductility, and plasticity at the microscopic and atomic scales. The important factors that control the mechanical behavior of metallic glasses are outlined on the basis of recent theoretical and experimental findings. The outstanding issues highlighted in this review are expected to be important for future research on the mechanical properties of metallic glasses.