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.     

Inhomogeneous strain and recrystallisation of a shot-peened Inconel 625 alloy after annealing

Microstructural variations in a shot-peened nickel superalloy after isothermal annealing were observed via scanning electron microscopy and electron backscatter diffraction analysis. The analysis was based on inverse pole figure and kernel average misorientation maps. Before annealing, a heavily plastically deformed layer was observed under the surface. Post annealing for 1?h, the uppermost 30–40?µm layer was discontinuously recrystallised and inhomogeneous strains remained in the below layers. After a particular duration, the inhomogeneous strains recovered but did not recrystallise. The threshold depth of recrystallisation depended on the inhomogeneous strains. Recrystallisation and growth progressed from the recrystallisation-threshold depth toward the surface. These microstructural variations explain the release mechanism of the residual stress induced by shot peening.     

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.     

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.     

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.     

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.     

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.     

Analytical transmission electron microscopy at organic interfaces

Organic materials are ubiquitous in all aspects of our daily lives. Increasingly there is a need to understand interactions between different organic phases, or between organic and inorganic materials (hybrid interfaces), in order to gain fundamental knowledge about the origin of their structural and functional properties. In order to understand the complex structure–property–processing relationships in (and between) these materials, we need tools that combine high chemical sensitivity with high spatial resolution to allow detailed interfacial characterisation. Analytical transmission electron microscopy (TEM) is a powerful and versatile technique that can fulfil both criteria. However, the application of analytical TEM to organic systems presents some unique challenges, such as low contrast between phases, and electron beam sensitivity. In this review recent analytical TEM approaches to the nanoscale characterisation of two systems will be discussed: the hybrid collagen/mineral interface in bone, and the all-organic donor/acceptor interface in OPV devices.     

Electron diffraction of precipitates at copper-cordierite interfaces

The precipitation of copper particles at the interface between a copper rod and a MgO-Al₂O₃-SiO₂ ceramic was investigated by Robinson back-scattered SEM and convergent beam micro-diffraction TEM. A 20 μm wide zone depleted of copper separated the copper rod from a 70 μ m region of copper-containing particles dispersed in the ceramic matrix. The precipitates were crystallographically identified as elemental copper by matching of convergent beam, inelastically-scattered, Kikuchi patterns.     

Low-friction flows of liquid at nanopatterned interfaces

With the important development of microfluidic systems, miniaturization of flow devices has become a real challenge. Microchannels, however, are characterized by a large surface-to-volume ratio, so that surface properties strongly affect flow resistance in submicrometre devices. We present here results showing that the concerted effect of wetting properties and surface roughness may considerably reduce friction of the fluid past the boundaries. The slippage of the fluid at the channel boundaries is shown to be greatly increased by using surfaces that are patterned on the nanometre scale. This effect occurs in the regime where the surface pattern is partially dewetted, in the spirit of the 'superhydrophobic' effects that have been discovered at macroscopic scales. Our results show for the first time that, in contrast to common belief, surface friction may be reduced by surface roughness. They also open the possibility of a controlled realization of the 'nanobubbles' that have long been suspected to play a role in interfacial slippage.     

Order-disorder behavior at thin film oxide interfaces

Order-disorder processes fundamentally determine the structure and properties of many important oxide systems for energy and computing applications. While these processes have been intensively studied in bulk materials, they are less investigated and understood for nanostructured oxides in highly non-equilibrium conditions. These systems can now be realized through a range of deposition techniques and probed at exceptional spatial and chemical resolution, leading to a greater focus on interface dynamics. Here we survey a selection of recent studies of order-disorder behavior at thin film oxide interfaces, with a particular emphasis on the emergence of order during synthesis and disorder in extreme irradiation environments. We summarize key trends and identify directions for future study in this growing research area.     

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.     

Singular intensity factors at bimaterial anisotropic interfaces

The singular intensity factors at bimaterial anisotropic interfaces in bonded joints with composite adherends are found by using a hybrid method based on numerical and elasticity solutions. The method is applicable to the solution of problems having complex geometry, loading and boundary conditions, which is the case in typical composite structures. Results are given in terms of the singular intensity factor, which is a generalization of the stress intensity factor commonly used with cracks. Both closed and open wedges, which are found, respectively, in bonded joints with or without adhesive fillets, are considered. Equivalent singular intensity factors in modes I, II and III are defined, and the results indicate that the mode III factor, which arises due to out-of-plane coupling, is negligible in all cases studied. Moreover, use of the Erdogan–Sih failure criterion indicates that the direction of crack propagation in lap joints with fillets remains constant beyond a very small region near the point of singularity, while for joints without fillets crack initiation always occurs in a direction parallel to the adhesive–adherend interface.