Cell biology at interfaces

A brief review is presented of the significant developments in the understanding of the processes involved in cell adhesion both to other cells and to substrates. The relationship between general cellular behaviour and cell adhesion is a result of the importance of the cytoplasmic cytoskeleton to most cellular processes. Interaction between a substrate and the cell is mediated through intramembranous proteins, such as the integrins. The intramembranous proteins, in turn, influence the assembly of the microfilamentous structures in the cytoplasm. Changes in the state of the microfilaments are accompanied by modifications in the behaviour of both microtubules and intermediate filaments. The expression of different types of cytoskeletal configuration result from differing types of cell-cell or cell-substratum encounters. This leads to significant changes in resultant cellular behaviour. It is argued that an understanding of changes that result from cell-biomaterial interactions, at the ultrastructural level, is necessary in order to assess the biocompatability of implant materials.     

Spontaneous emission in microcavities

The transverse distributions of intensity inside and outside an optical cavity produced by spontaneous emission from atoms inside the cavity are reviewed. The mode properties inside a closed passive one-dimensional cavity are summarised and the spatial distributions of emitted light for a passive cavity with partially transmitting mirrors are discussed. A qualitative theory describes the effects of an amplifying medium inside the cavity, covering the regimes both below and above the lasing threshold. Expressions are derived for the angular spread of the output light in the presence of gain. A theory based on rigorous general relations between gain and noise provides more complete results for the spontaneous emission noise from an amplifying slab within an optical cavity.     

Intergranular films at Au-sapphire interfaces

The existence of nanometer-thick amorphous equilibrium films at metal-ceramic interfaces has been experimentally verified for the Au–Al₂O₃ system. The films were formed using a novel experimental approach, in which thin sputtered films of Au were dewetted on a sapphire substrate which was previously partially wetted with drops of anorthite glass (CaO–2SiO₂–Al₂O₃). High-resolution transmission electron microscopy and qualitative analytical transmission electron microscopy were used to confirm the existence of the amorphous films. In addition, positive and relatively large Hamaker constants were calculated for the Au-film-Al₂O₃ interface, which indicates the existence of an attractive van der Waals force which stabilizes the film, similar to equilibrium films at grain boundaries in ceramics. A ∼ 1 nm thick surface film was also detected on the (0001) surface of sapphire substrates partially wetted by anorthite glass. The refractive index required to stabilize the surface films, via a positive Hamaker constant, is explored.     

Emergent phenomena at oxide interfaces

Recent technical advances in the atomic-scale synthesis of oxide heterostructures have provided a fertile new ground for creating novel states at their interfaces. Different symmetry constraints can be used to design structures exhibiting phenomena not found in the bulk constituents. A characteristic feature is the reconstruction of the charge, spin and orbital states at interfaces on the nanometre scale. Examples such as interface superconductivity, magneto-electric coupling, and the quantum Hall effect in oxide heterostructures are representative of the scientific and technological opportunities in this rapidly emerging field.     

Cylindrical-wave reflection and antireflection at media interfaces

A transfer-matrix method is used to study cylindrical-wave reflection at cylindrical media interfaces. We find that the reflection can be total at an interface of small radius but is only partial if the radius is large. This feature is useful for designing optical sources and receivers of small radii. Cylindrical-wave multicylindrical-layer antireflection coatings are also studied. We find that conventional quarterwavelength layer designs cannot be used for these coatings, and new design principles are outlined.     

Metallic conduction at organic charge-transfer interfaces

The electronic properties of interfaces between two different solids can differ strikingly from those of the constituent materials. For instance, metallic conductivity-and even superconductivity-have recently been discovered at interfaces formed by insulating transition-metal oxides. Here, we investigate interfaces between crystals of conjugated organic molecules, which are large-gap undoped semiconductors, that is, essentially insulators. We find that highly conducting interfaces can be realized with resistivity ranging from 1 to 30 kohms per square, and that, for the best samples, the temperature dependence of the conductivity is metallic. The observed electrical conduction originates from a large transfer of charge between the two crystals that takes place at the interface, on a molecular scale. As the interface assembly process is simple and can be applied to crystals of virtually any conjugated molecule, the conducting interfaces described here represent the first examples of a new class of electronic systems.     

Lattice compatibility at interfaces after recrystallisation

Abstract

The nature, distribution, and implications of local, i.e. intragrain, lattice rotations have been explored in recrystallised nickel using electron backscattered diffraction. The data processing methodology for such methods is discussed in some detail. The results reveal that orientation perturbations with a periodicity of ～1 μm occur frequently within grains, and that there is a strong tendency for two grains which feature either both large perturbations or both small perturbations to be neighbours. The results are interpreted in terms of lattice compatibility, potential sites for instigation of secondary recrystallisation, and the meaningful specification of grain boundary misorientation.     

Supramolecular hydrophobic-hydrophilic nanopatterns at electrified interfaces

We have developed hydrophobic-hydrophilic nanopatterns at electrified surfaces via the self-assembly of amphiphilic molecules. For this purpose, we selected 5-hexadecyloxy isophthalic acid: this neutral amphiphile forms hydrogen-bonded rows that are commensurate with the Au(111) surface. The alkyl chains are interdigitated. The molecular organization of these nanopatterns depends strongly on the substrate potential, which reveals the hierarchical nature of the assembly. The new hydrophobic-hydrophilic nanopatterns are of special interest as templates for the formation of nanostructures of higher complexity.     

Vortex dimples at charged helium interfaces

We have investigated theoretically the profile of electrically charged helium interfaces in the presence of a superfluid vortex line perpendicular to the interface. The free surface of superfluid³He-B as well as the interface between phase-separated³He and⁴He mixtures have been considered. The superflow-induced dimple profile around the vortex line was calculated within the linear theory assuming a weak curvature of the interface. Our results show that the depth of the dimple increases strongly with an external electric field as the critical value of the electrohydrodynamic surface instability is approached. The chances of observing these charge-enhanced dimples are also discussed.     

Wetting and adhesion at Mg/MgO interfaces

The wetting of (100), (110), and (111) MgO single crystals by molten Mg was studied in a flowing Ar atmosphere at 1073 K using an improved sessile drop method. The results show that the contact angle and work of adhesion are mildly dependent on MgO substrate orientation. However, the work of separation calculated by density functional theory shows a sequence of (100) < (110) < (111). This discrepancy was explained by a model considering the possible effects of evaporation and deposition of Mg at the substrate surface on the interfacial adhesion. For Mg on the clean MgO surfaces, the adhesion consists primarily of Mg–O ionic bonds with some Mg–Mg metallic bonds. In addition, a small amount of covalent bonds are present in the metal slab adjacent to the Mg/MgO (110) and Mg/MgO (111) interfaces. The different work of separation at three interfaces depends on the strength and amounts of Mg–O and Mg–Mg bonds as well as on the surface free energy of respective planes. For the MgO surfaces pre-deposited by a monolayer of Mg atoms, the interfaces are dominated by the Mg–Mg bonds, thus mitigating the effect of substrate orientation on the wettability and adhesion.     

End-tethered chain molecules at liquid interfaces

Polymeric chains attached by one end to an interfce and immersed in a liquid environment provide a powerful tool for the modification of interfacial properties. Recent developments have advanced our understanding and control over interfacial polymer layers at the molecular level, with clear implications to tribological, biosensing and multicomponent systems design considerations.     

Adhesion mechanisms at soft polymer interfaces

Based on several significant examples, we analyse the adhesion mechanisms at soft polymer interfaces with a special emphasis first on the role of connector molecules, that is, polymer chains bound to the interface and which transmit stress through a stretching and extraction mechanism, and second on the necessary relay that must be taken by additional dissipation mechanisms acting at larger scales if one wants to reach typical fracture toughnesses in the range of a few 10 J m(-2). Examples of such bulk dissipation mechanisms will be discussed for interfaces between polymer melts and for pressure-sensitive adhesives in contact with a solid surface. We shall particularly point out the fact that the level of adhesion results from a competition between adhesive failure usually driven by both the interactions and the friction properties of the interface and bulk strong deformations which take place in the bulk of the adhesive layer. Controlling the friction properties of the interface then becomes a tool to finely tune adhesive properties.     

On stress conditions and computations at bi-material interfaces

Conditions on stresses and their first spatial derivatives across a flat bi-material interface in three-dimensional linear dynamic elasticity are obtained by replacing the displacement continuity condition with continuity conditions on in-plane strains and curvatures. The stresses on one side of interfaces and near them in multi-constituent structures are readily computed using these conditions-formulae and not solving the full problem, if the stresses on the other side have been computed having solved the full problem.     

Polariton parametric amplification in semiconductor microcavities

Modifying the photonic environment of a semiconductor quantum well by embedding it in a high-reflectivity microcavity gives rise to new fundamental optical excitations, half-quantum well excitons, half-photons. These particles, called polaritons, have a light mass, as cavity photons, meaning that they have a large De Broglie wavelength. On the other hand, polaritons, like excitons, are subject to Coulomb interaction, a feature generating strong optical nonlinearities. Such properties favour quantum degeneracy and collective phenomena related to the bosonic statistics of polaritons. We review experiments on stimulated scattering of polaritons. In particular we concentrate on the resonant excitation of polaritons somewhere on the dispersion curve and the stimulation of their scattering into the fundamental state by means of an optical probe beam. The process is called polariton parametric amplification and results in very large and ultrafast optical amplification of the probe beam. The model, based on a Hamiltonian of interacting bosons, suggests that the amplification is related to the coherence between polaritons. We demonstrate that in clearly designed samples, this coherence can be preserved almost up to room temperature, so that intersting applications of this phenomenon can be conceived. At the same time we have been able to improve dramatically the efficiency of the parametric process, making the microcavity an unprecedented optical amplifier.     

Silicide characterization at alumina–niobium interfaces

Alumina–niobium interfaces formed by liquid-film-assisted joining with copper/niobium/copper interlayers exhibited microstructures that depend on the nature of the alumina components. Characterization of these interfaces in the transmission electron microscope provided insight on the relationship between interfacial microstructure and fracture performance. Interfaces between sapphire and niobium and those between high-purity (99.9%) polycrystalline alumina and niobium were free of secondary phases. However, niobium silicides were found at interfaces between lower-purity (99.5%) alumina and niobium, identified by electron diffraction analysis as the body-centered tetragonal α-Nb₅Si₃ phase. Spatially resolved compositional analysis was conducted on silicide particles at and away from the interface.     

Impurity-induced degradation at zirconia/sapphire interfaces

Sapphire fibre surface damage caused by a polycrystalline zirconia coating has been analysed using scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). “Pitting” of the sapphire surface was partially attributed to formation of a transient liquid phase (interphase) capable of local dissolution of alumina at zirconia grain contacts. Chemical etching was used to verify that the interphase material was silicate-based and resided at triple points between zirconia grains and the fibre surface. An additional crystalline calcium hexaluminate phase (hibonite) was found on some fibres. Origin of these impurities and resulting consequences of their presence are rationalized in view of observed surface modification. This revised version was published online in November 2006 with corrections to the Cover Date.