For many years, a fundamental problem in contact mechanics, both tribology and indentation problems, has been the inability to see what is taking place—the buried-interface problem. Over the past few years, there have been developments whereby it has become possible to perform contact mechanics experiments in situ within a transmission electron microscope. These new experiments have been enabled by both the miniaturization of sensors and actuators and improvements in their mechanical stability and force sensitivity. New information is now becoming available about the nanoscale processes of sliding, wear, and tribochemical reactions, as well as microstructural evolution during nanoindentation such as dislocation bursts and phase transformations. This article provides an overview of some of these developments, in terms of both the advances in technical instrumentation and some of the novel scientific insights.

The polyamide active layers of commercial reverse-osmosis and nanofiltration membranes are examples of nanoscale functional materials that challenge the state of the art of materials characterization. The active layer is only ∼100 nm thick, and because the active layer is formed by a process of interfacial polymerization, the structure and composition of the membrane is highly inhomogeneous. Even such basic physical and chemical properties of the membrane as the atomic density, swelling in water, distribution of charged species, and the mobility of water and ions, are poorly understood. In this article, we briefly review progress in the characterization of polyamide separation membranes using transmission electron microscopy, atomic force microscopy, vibrational spectroscopy, positron annihilation, nuclear magnetic resonance, and Rutherford backscattering spectrometry. Advances in the microanalysis methods applicable to these complex materials will advance fundamental understanding of the structure–property relationships of polymer membranes and further the long-term goal of synthesizing membranes with improved performance.

In complex transition-metal oxides, the interactions between the electronic spins, charges, and orbitals produce a rich variety of electronic phases. The competition and/or cooperation among these correlated-electron phases can lead to the emergence of surprising electronic phenomena and functionalities and form the basis for a new type of electronics.

Reducing CO2 emissions from the use of fossil fuel is the primary purpose of carbon dioxide capture and storage (CCS). Two basic approaches to CCS are available.1,2 In one approach, CO2 is captured directly from the industrial source, concentrated into a nearly pure form, and then pumped deep underground for long-term storage (see Figure 1). As an alternative to storage in underground geological formations, it has also been suggested that CO2 could be stored in the ocean. This could be done either by dissolving it in the mid-depth ocean (1–3 km) or by forming pools of CO2 on the sea bottom where the ocean is deeper than 3 km and, consequently, CO2 is denser than seawater. The second approach to CCS captures CO2directly from the atmosphere by enhancing natural biological processes that sequester CO2 in plants, soils, and marine sediments. All of these options for CCS have been investigated over the past decade, their potential to mitigate CO2 emissions has been evaluated,1 and several summaries are available.1,3,4

Recent experiments have revealed that the wax on the lotus leaf surface, by itself, is weakly hydrophilic, even though the lotus leaf is known to be superhydrophobic. Conventional understanding suggests that a surface of such waxy composition should not be able to support superhydrophobicity and high contact angles between a liquid and the surface. Here, we show that the unexpected superhydrophobicity is related to the presence of “reentrant texture” (that is, a multivalued surface topography) on the surface of the lotus leaf. We exploit this understanding to enable the development of superoleophobic surfaces (i.e., surfaces that repel extremely low-surface-tension liquids, such as various alkanes), where essentially no naturally oleophobic materials exist. We also develop general design parameters that enable the evaluation of the robustness of the composite interface on a particular surface. Based on these design parameters, we also rank various superhydrophobic and superoleophobic substrates discussed in the literature, with particular emphasis on surfaces developed from inherently hydrophilic or oleophilic materials.

Conjugated polymer artificial muscles fill a unique niche in the electroactive polymer portfolio. They combine high strength, low voltage, and reasonable speed with versatile fabrication and design. This article reviews the actuation mechanism in these materials and presents some of the designs that have been developed for applications such as Braille displays, catheters, and bioMEMS devices.

We review the development of time-resolved, high-resolution environmental scanning/ transmission electron microscopy [E(S)TEM] for directly probing dynamic gas–solid, liquid–solid, and gas–liquid–solid interactions at the atomic level. Unlike a regular TEM, such a microscope allows us to use high gas pressures (up to 40 mbars) in the sample region. The unique information available from experiments performed using E(S)TEM has enabled visualization of the dynamic nature of nanostructures during reactions. Such information can be directly applied to the development of advanced nanomaterials such as carbon nanotubes, silicon nanowires and processes, including the design of novel routes to polymers synthesis, and has aided in the identification of important phenomena during catalysis, chemical vapor deposition, and electrochemical deposition.

Photonic crystals are multidimensional periodic gratings, in which the light propagation is dominated by Bragg diffraction that appears to be refraction at the flat surfaces of the crystals. The refraction angle from positive to negative, perfectly or only partially obeying Snell's law, can be tailored based on photonic band theory. Negative refraction enables novel prism, collimation, and lens effects. Because photonic crystals usually consist of two transparent media, these effects occur at absorption-free frequencies, affording significant design flexibility for free-space optics. The photonic-crystal slab, a high-index membrane with a two-dimensional airhole array, must be carefully designed to avoid unwanted reflection and diffraction. Light focusing based on negative refraction forms a parallel image of a light source, facilitating optical couplers and condenser lenses for wavelength demultiplexing. A compact wavelength demultiplexer can be designed by combining the prism and lens effects.

This article introduces the use of in situ high-resolution transmission electron microscopy (HRTEM) techniques for the study and development of nanomaterials and their properties. Specifically, it shows how in situ HRTEM (and TEM) can be used to understand diverse phenomena at the nanoscale, such as the behavior of alloy phase formation in isolated nanometer-sized particles, the mechanical and transport properties of carbon nanotubes and nanowires, and the dynamic behavior of interphase boundaries at the atomic level. Current limitations and future potential advances in in situ HRTEM of nanomaterials are also discussed.

Trace contaminant detection in water represents both a grand challenge and great opportunity for materials scientists and engineers. The recent discovery that functional DNA can be obtained to bind selectively to a wide range of contaminants makes it possible to interface these molecules with nanoscale materials, such as gold nanoparticles and quantum dots, to transform the molecular reorganization between functional DNA and contaminants into physically detectable colorimetric and fluorescent signals. Micro- and nanofluidic devices have also played a critical role in lowering the detection limits of functional-DNA sensors, promoting sensor regeneration and thus improving sensor performance and allowing long-term unattended monitoring of water quality.

Coal is a critical component in the international energy portfolio, used extensively for electricity generation. Coal is also readily converted to liquid fuels and/or hydrogen for the transportation industry. However, energy extracted from coal comes at a large environmental price: coal combustion can produce large quantities of ash and CO2, as well as other pollutants. Advanced technologies can increase the efficiencies and decrease the emissions associated with burning coal and provide an opportunity for CO2 capture and sequestration. However, these advanced technologies increase the severity of plant operating conditions and thus require improved materials that can stand up to the harsh operating environments. The materials challenges offered by advanced coal conversion technologies must be solved in order to make burning coal an economically and environmentally sound choice for producing energy.

In this article, we review current research efforts to control the magnetic behavior of complex oxide thin films using electric fields. After providing fundamental definitions of magnetoelectric response, we survey materials, architectures, and mechanisms that exhibit promise for such electric-field control of magnetism. Finally, we mention ideas for future research and discuss prospects for the field.

Over the past ten years, a broad range of self-healing materials, systems that can detect when they have been damaged and heal themselves either spontaneously or with the aid of a stimulus, has emerged. Although many unique compositions and components are used to create these materials, they all employ basic chemical reactions to facilitate repair processes. Kinetically controlled ring-opening reactions and reversible metal–ligand interactions have proven useful in autonomic self-healing materials, which require no stimulus (other than the formation of damage) for operation. In contrast, nonautonomic self-healing materials, which require some type of externally applied stimulus (such as heat or light) to enable healing functions, have capitalized on chemistries that utilize either reversible covalent bonds or various types of noncovalent interactions. This review describes the underlying chemistries used in state-of-the-art self-healing materials, as well as those currently in development.

Self-assembled materials composed of β-sheet forming peptides hold promise as therapeutics and novel biomaterials. This article focuses on the design and engineering of amphiphilic peptide sequences, especially β-hairpins. Peptides can be designed to intramolecularly fold and then self-assemble on cue, affording hydrogels rich in β-sheet structure. Hydrogels having distinct material properties can be designed at the molecular level by modulating either the peptide's sequence or the environmental stimulus used to trigger folding and assembly, leading to gelation.

Gels, soft polymeric or composite materials that have a high fraction of water, are often found as structural materials and actuators in nature but have so far not found many uses when fabricated synthetically. We first examine some natural systems such as jellyfish, sea anemones, starfish, legumes, and human tissue, all having interesting ways of moving or otherwise reacting to the surrounding environment. Then we discuss swelling and cross-linking of hydrogels, followed by a look at actuation by electrically, thermally, and chemically stimulated gels, noting that electrical stimulation needs a chemical intermediary to show substantial actuation (comparable to human muscle, for instance). Electroactive gels have great potential as sensors and actuators but their actual uses are mainly restricted to passive drug delivery and matrices for sensors. For most applications as artificial muscles, electrically driven actuators are too weak, but chemically driven actuators look very promising. Better ways of coupling electrical energy to chemically driven gels are needed.

Fundamental processes of wear include the rupture of single chemical bonds and the displacement of atoms or small clusters by mechanical action. Experimental studies of such processes have become feasible with the development of scanning probe microscopy. The small volume affected in these experiments overlaps with the size scale of large atomistic simulations, making a direct comparison possible. The complexity of real-world wear processes is reduced in most nanometer-scale experiments, for example, by probing surfaces of single crystals or by establishing and maintaining carefully controlled environments, including ultraclean conditions. The studies address the onset and topography of wear, the formation of debris structures, the interplay of mechanical and chemical action, the role of ultrathin films, the role of crystal defects in wear processes, and temporal and thermal effects.

Three-dimensional x-ray diffraction (3DXRD) allows nondestructive characterization of grains, orientations, and stresses in bulk microstructures and, therefore, enables in situ studies of the structural dynamics during processing. The method is described briefly, and its potential for providing new data valuable for validation of various models of microstructural evolution is discussed. Examples of 3DXRD measurements related to recrystallization and to solid-state phase transformations in metals are described. 3DXRD measurements have led to new modeling activity predicting the evolution of metallic microstructures with much more detail than hitherto possible. Among these modeling activities are three-dimensional (3D) geometric modeling, 3D molecular dynamics modeling, 3D phase-field modeling, two-dimensional (2D) cellular automata, and 2D Monte Carlo simulations.

An overview of recent work on the connection between electrical and molecular properties of semiconducting polymers for thin-film transistors (TFTs) is presented. A description of the molecular packing and microstructure of amorphous to semicrystalline semiconducting polymers is presented. The features of basic models for electrical transport in TFTs are discussed. These studies indicate that defect states and traps are as important as ordered domains for understanding transport in semiconducting polymers. Advanced methods, such as electric force microscopy, useful for measuring the characteristics of defect states and charge traps, are briefly reviewed.

The field of engineered materials with designed properties is expected to continue to grow in the future, and metamaterials are instrumental in allowing this freedom of design. Metamaterials, particularly acoustic, are still in the stage of infancy. Acoustic metamaterials are being explored theoretically, but there has been little headway on the experimental front. The design, development, and characterization of acoustic metamaterials will offer many opportunities in materials science. In this article, we review the basic physics of different kinds of acoustic periodic structures with special emphasis on locally resonant acoustic metamaterials. We first survey phononic crystals and then discuss localized resonances in intrinsic and inertial resonating structures of acoustic metamaterials. Finally, we present the ongoing efforts in realizing acoustic metamaterials with negative materials properties and discuss the implications of acoustic metamaterials.

The concept of optical protein-based memory has been of interest since the early 1970s. Yet, no commercially available protein-based memory devices exist. This review presents an analysis of the main challenges associated with the practical implementation of such devices. In addition, the discussion includes details on the potential of using the unparalleled properties of photochromic proteins by creating an optical data storage disk drive with unmatched features and, particularly, record-high data densities and rates.