Interface Topography Research Articles

Ultrarapid Engineering of Biomimetic Materials and Tissues: Fabrication of Nano‐ and Microstructures by Plastic Compression

AbstractCurrently, the concept of engineered tissues depends on the ability of cultured cells to fabricate new tissue around a scaffold. This is inherently slow and expensive and has had limited success so far. We report here a new process for the cell‐independent, controlled engineering of biomimetic scaffolds by rapid removal of fluid from hyperhydrated collagen gel (or other) constructs, using plastic compression (PC). PC fabrication produces dense, cellular, mechanically strong native collagen structures with controllable nano‐ and microscale biomimetic structures. The huge‐scale shrinkage (> 100‐fold) provides the ability to introduce controllable mechanical properties, microlayering, and embossed interface topography without cell participation, but with high cell viability. Critically, this takes minutes rather than the conventional days and weeks. The rapidity and biomimetic potential of the PC fabrication process at the mesoscale opens a new route for the production of biomaterials and patient‐customized tissues. It also represents a new concept in ‘engineering’ tissues.

Adhesion of repair systems to concrete: influence of interfacial topography and transport phenomena

Eight types of cement slurries were applied to concrete substrates. Surface preparation involved mechanical action – sandblasting and polishing – as well as saturation regulation, both dry and saturated surface wet. The adhesion strength for the different combinations was measured by direct tensile tests. Multivariate statistical analysis led to the identification of a major effect of surface saturation on the results. Knowledge of the interfacial behaviour was obtained by the discrimination of the effects of partial interaction between slurry and aggregates and slurry and cement mortar, respectively. It clearly showed that a third factor has to be taken into account in the explanation of adhesion, namely the interfacial transition zone between cement paste and aggregates, due to its porosity and anchoring effect. This effect was quantified by topographical observation and image analysis. Penetration of cement slurry was also confirmed by observation with binocular and fluorescent microscopy.

Adhesion of repair systems to concrete: influence of interfacial topography and transport phenomena

Eight types of cement slurries were applied to concrete substrates. Surface preparation involved mechanical action – sandblasting and polishing – as well as saturation regulation, both dry and saturated surface wet. The adhesion strength for the different combinations was measured by direct tensile tests. Multivariate statistical analysis led to the identification of a major effect of surface saturation on the results. Knowledge of the interfacial behaviour was obtained by the discrimination of the effects of partial interaction between slurry and aggregates and slurry and cement mortar, respectively. It clearly showed that a third factor has to be taken into account in the explanation of adhesion, namely the interfacial transition zone between cement paste and aggregates, due to its porosity and anchoring effect. This effect was quantified by topographical observation and image analysis. Penetration of cement slurry was also confirmed by observation with binocular and fluorescent microscopy.

Effects of oxide thickness, Al 2O 3 interlayer and interface asperity on residual stresses in thermal barrier coatings

During high temperature operation, the thermally grown oxide (TGO) usually forms along the bondcoat/topcoat interface in thermal barrier coating (TBC) and was characterized as a driving force for the failure of the coating system. The effects of TGO thickness and Al 2O 3 interlayer applied as an oxygen barrier layer between the bondcoat and topcoat on the magnitude of residual stresses in TBC during cooling process were interpreted using concentric-circle model. The results were coupled with finite element method. The influences of interface asperity and interface topography on the distribution of residual stresses normal to interfaces in TBC were also discussed.

Surface modification of exchange-coupled Co/NiO x magnetic bilayer by bias sputtering

We have investigated the effect of bias voltage on sheet resistance, surface roughness and surface coverage of Co/NiO x magnetic bilayer. In addition, interface topography and corrosion resistance of the Ta/Co/Cu/Co/NiO x/Si(1 0 0) system have been studied for Co layers deposited at an optimum bias voltage. Atomic force microscopy (AFM) and four point probe sheet resistance ( R s ) measurement have been used to determine surface and electrical properties of the sputtered Co layer at different bias voltages ranging from 0 to − 80 V. The Co/NiO x bilayer exhibits a minimum surface roughness and low sheet resistance value with a maximum surface coverage at V b = − 60 V resulted in a slight increase of magnetic resistance and its sensitivity for the Co/Cu/Co/NiO x/Si(1 0 0) magnetic multilayers, as compared with the same magnetic multilayers containing unbiased Co layers. The presence of Ta protection layer improves the corrosion resistance of the multilayers by three orders of magnitude in a humid environment.

3DINVER.M: a MATLAB program to invert the gravity anomaly over a 3D horizontal density interface by Parker–Oldenburg's algorithm

A MATLAB source code 3DINVER.M is described to compute 3D geometry of a horizontal density interface from gridded gravity anomaly by Parker–Oldenburg iterative method. This procedure is based on a relationship between the Fourier transform of the gravity anomaly and the sum of the Fourier transform of the interface topography. Given the mean depth of the density interface and the density contrast between the two media, the three-dimensional geometry of the interface is iteratively calculated. The iterative process is terminated when either the RMS error between two successive approximations is lower than a pre-assigned value—used as convergence criterion, or until a pre-assigned maximum number of iterations is reached. A high-cut filter in the frequency domain has been incorporated to enhance the convergence in the iterative process. The algorithm is capable of handling large data sets requiring direct and inverse Fourier transforms effectively. The inversion of a gravity anomaly over Brittany (France) is presented to compute the Moho depth as a practical example.

Elastic wave modelling in heterogeneous media with high velocity contrasts

SUMMARY In this paper a new numerical technique is proposed for modelling elastic wave propagation in heterogeneous media with high velocity contrasts, as for instance in the presence of weathered layers or soft marine sediments. The scheme allows a sharp jump in grid spacing across the interface with high velocity contrasts. That is, the number of nodes along the interface with high velocity contrasts may be different for two meshes beside the interface. Sharp jumps of grid spacing can occur along a curved interface. Therefore, the proposed scheme can use a fine mesh in regions with low velocities and a coarser mesh in regions with higher velocities by modelling the interface topography with a piecewise linear function, so both spatial and temporal oversampling is avoided. This leads to a distinct reduction in the storage requirements and in the computational cost. The scheme is developed by formulating the problem in terms of stresses (at nodes) in regions with low S-wave velocities and in terms of velocities (at nodes) in regions with higher velocities, and then defining an explicit boundary between them. The boundary conditions on this explicit boundary with a complex geometry are implemented by introducing an integral approach to the equations on the explicit boundary. The outcome of this is that no nodes need to be added through the interpolations of the field variables. Moreover, no smoothing of the material parameters is needed on the interface with high velocity contrasts. A stress-grid scheme is developed to solve the problem inside regions with low S-wave velocities, and the (velocity) grid method is used to solve the problem inside regions with higher velocities. The algorithm is a second-order approximation for an irregular mesh in comparison to conventional finite differences. Implementation of the free-surface conditions of a complex geometrical surface is obtained for the proposed stress-grid scheme when the surface topography is approximated to be piecewise linear. Thus, the new algorithm can model elastic wave propagation even when the regions of low S-wave velocities reach the surface. Numerical examples demonstrate the efficiency of the proposed numerical technique.

Wave propagation across fluid-solid interfaces: a grid method approach

SUMMARY This paper presents a new numerical technique for modelling wave propagation in media with both fluid (acoustic) and solid (elastic) regions, as found for instance in a marine seismic case. The scheme can correctly satisfy the fluid‐solid interface conditions and accurately models the arbitrary interface topography. This work is an extension of the grid method, which is able to model wave propagation in heterogeneous solid (elastic) media with arbitrary surface topography and irregular interfaces. The scheme is developed by formulating the problem in terms of displacements in elastic regions and pressure in acoustic regions with an explicit boundary between them. The fluid‐solid interface conditions on this explicit boundary are implemented by introducing an integral approach to the fluid‐solid interface conditions. The solution in terms of pressure in acoustic regions together with this integral approach means that no extra computational cost is needed to implement the fluid‐solid interface conditions for a complex geometry, instead of resulting in additional computations as with the spectralelement, finite-element or finite-difference methods. In this paper an acoustic grid method is developed to solve the acoustic problem inside the fluid, and the (elastic) grid method, which is improved based on a parsimonious staggered-grid scheme, is used to solve the elastic problem inside the solid. The numerical dispersion and stability criteria for the acoustic grid method are discussed in detail. Comparison with an analytical solution demonstrates that the proposed scheme handles the fluid‐solid interface conditions correctly. Examples of wave propagation in a mixed fluid/solid model with both weak and strong sinusoidal fluid‐solid interfaces illustrate the suitability of the proposed scheme for modelling wave propagation across fluid‐solid interfaces with complex geometries.

Electrical Imaging: 2D Resistivity Tomography as a tool for Groundwater Studies at Mahmudia Village, West Sulaimani City, Iraqi Kurdistan Region

Electrical Imaging (EI) is a geophysical method, developed over the past several years that provide a two or
 three dimensional resistivity model of the subsurface. EI can be very effective in providing information on
 the distribution of aquifers, aquitard, impermeable rocks, soil-bedrock interface topography, fracture zone,
 fault and voids. This technique is used for the first time in Iraq in Mahmudia village, west of Sulaimani City.
 Five traverses were taken; more than 4000 readings r.l'ere recorded using Wenner configuration with roll
 along technique. The data inversion u,as carried out by RES2DINV program using both finite difference and
 finite element mathematical methods. Two aquifers of different geological properties have been detected; one
 of them within the recent sediments while the other within the limestone beds of Sinjar Formation, These
 aquifers can be regarded as an excellent water resource fol providing sufficient groundwater to the area for
 domestic and agricultural purposes.

Nonequilibrium Adhesion Patterns at Lipid Bilayer Junctions

With lipid bilayer-bilayer junctions in mind as model systems for the study of cell-cell junctions, we have examined adhesion between simple lipid membranes using fluorescence resonance energy transfer (FRET) and fluorescence interference contrast microscopy (FLIC) to map the interfacial topography. The contact can take the form of uniform adhesion, with an intermembrane separation of nanometers, or nonuniform adhesion, in which blisters hundreds of nanometers in height coexist with tight adhesion zones. We find that blisters can result as a consequence of rapid interbilayer contact induced by osmotic shock. We develop a model for the formation and stabilization of the blisters, confirming that these phenomena are governed mainly by hydrodynamic flow in the intermembrane space.

Analysis of interfacial adhesion based on electrical contact resistance measurements

An analytical approach for determining the adhesion force and adhesion energy at contact interfaces from electrical contact resistance measurements is presented for isotropic, conductive, and rough surfaces. The method is especially suitable for in situ measurement of adhesion forces in lightly loaded contacts, and because it accounts for the real contact area, it yields more accurate estimates of the adhesion energy than traditional methods based on the apparent contact area of microcantilever beams partially adhered to substrates. The advantage of obtaining adhesion measurements in the case of plastically deformed contacts is demonstrated by the wide range of the electrical contact resistance achieved with this method. The relation between adhesion force and electrical contact resistance is shown to be independent of the surface topography when the asperities deform plastically. For this situation, the present method is especially suitable for adhesion measurements in dynamic systems, where the interfacial topography changes with contact cycles due to irreversible deformation of the asperity microcontacts. Analytical results are presented for the adhesion force and adhesion energy at rough contact interfaces in terms of electrical contact resistance and surface topography parameters. The present analytical approach also provides a means of determining the interfacial surface energy of contacting bodies with known surface energies or the surface energy of two identical contacting surfaces.

Diffusionally accommodated interfacial sliding in metal-silicon systems

The kinetics and mechanism of diffusionally accommodated interfacial sliding (interfacial creep) under far-field shear and normal stresses was studied, based on diffusion-bonded Al-Si-Al sandwich specimens. A previously developed interfacial creep law [Funn and Dutta, Acta Mater 1999; 47: 149], which proposed that interfaces may slide via interface-diffusion controlled diffusional creep, was experimentally validated by carrying out a systematic parametric study. In agreement with the model, the Si-Al interfaces slid via diffusional creep ( n = 1) under the influence of an effective shear stress, which depends on the far-field shear and normal stresses, as well as the interfacial topography. Compressive stresses acting normal to the interface lowered the effective shear stress, resulting in a threshold effect, thus reducing the sliding rate. The rate of sliding was controlled by diffusional mass transport through a thin amorphous, O-rich interfacial layer, under the influence of local interfacial stress gradients, which arose due to the topological features of the interface. Instances of interfacial sliding in the absence of interfacial de-cohesion, which have been noted in composites, thin-film systems, etc., may be explained by the present mechanism, which also offers an alternative rationalization of threshold behavior during diffusional flow (besides interface-reaction control).

The boundary element method: the simulation of voltammetry at immiscible liquid/liquid interfaces

The boundary element method is presented as an efficient and powerful method for the analysis of time-dependent electrochemical processes occurring at immiscible liquid/liquid interfaces. This paper outlines the theory and numerical details required for the development and application of two-dimensional transient diffusion models for the simulation of cyclic voltammetry behaviour at a range externally polarised immiscible liquid/liquid interfaces of differing topography. The benefits of the BEM approach are discussed, including the reduction in dimensionality brought about by the formulation procedure and complete elimination of the need for domain discretisation with the time-domain convolution approach. The versatility and efficiency of the numerical procedures are examined with respect to a number of liquid/liquid interface geometries and a series of working curves established to quantify the influence of interface topography on the observed voltammetric behaviour.

A novel mathematical procedure to evaluate the effects of surface topography on the interfacial state of stress. Part II. Verification of the method for scarf interfaces

A mathematical procedure was developed to utilize the complementary energy method, by minimization, in order to obtain an approximate analytical solution to the 3D stress distributions in bonded interfaces of dissimilar materials. The stress solutions obtained predict the stress jumps at the interfaces, which cannot be captured by the current FEA methods. As a novel method, the penalty function is used to enforce the displacement boundary conditions at the interfaces. Furthermore, the mathematical procedure developed enables the integration of different interfacial topographies into the solution procedure. In order to incorporate the effects of surface topography, the interface is expressed as a general surface in Cartesian coordinates, i.e. F(x, y, z) = 0. In this paper, the scarf interface problem, i.e. y = x/2 surface is considered for verification of the method by comparison with finite element analysis (FEA) results. Comparison of the results reveals our new mathematical procedure to be a promising and efficient method for optimizing interface topographies.

How to anchor hotspots in a convecting mantle?

Laboratory experiments were performed to study the influence of density and viscosity layering on the formation and stability of plumes. Viscosity ratios ranged from 0.1 to 6400 for buoyancy ratios between 0.3 and 20, and Rayleigh numbers between 10 5 and 2.10 8. The presence of a chemically stratified boundary layer generates long-lived thermochemical plumes. These plumes first develop from the interface as classical thermal boundary layer instabilities. As they rise, they entrain by viscous coupling a thin film of the other layer and locally deform the interface into cusps. The interfacial topography and the entrainment act to further anchor the plumes, which persist until the chemical stratification disappears through entrainment, even for Rayleigh numbers around 10 8. The pattern of thermochemical plumes remains the same during an experiment, drifting only slowly through the tank. Scaled to an Earth’s mantle without plate tectonics, our results show that: (1) thermochemical plumes are expected to exist in the mantle, (2) they could easily survive hundreds of millions of years, depending on the size and magnitude of the chemical heterogeneity on which they are anchored, and (3) their drift velocity would be at most 1–2 mm/yr. They would therefore produce long-lived and relatively fixed hotspots on the lithosphere. However, the thermochemical plumes would follow any large scale motion imposed on the chemical layer. Therefore, the chemical heterogeneity acts more as a ‘floating anchor’ than as an absolute one.

SiO2 Surface and SiO2/Si Interface Topography Change by Thermal Oxidation

Using a wide atomically flat (111) Si surface, the topography change of SiO2 surface and SiO2/Si interface by thermal oxidation was investigated for various oxidation temperatures. The initial step/terrace configuration was preserved on the SiO2 surface irrespective of oxidation temperature. On the other hand, the general step/terrace configuration of the initial Si surface was succeeded by the SiO2/Si interface at temperatures lower than 950°C, while at temperatures higher than 1050°C, the configuration was destroyed at the SiO2/Si interface with increasing oxide thickness until the steps finally disappeared. Terrace surfaces, however, were steeply microscopically roughened in the initial oxidation range irrespective of the oxidation temperature.

A novel mathematical procedure to evaluate the effects of surface topography on the interfacial state of stress. Part I: Verification of the method for flat surfaces

A mathematical procedure is developed to utilize the complementary energy method, by minimization, in order to obtain an approximate analytical solution to the 3D stress distributions in bonded interfaces of dissimilar materials. The stress solutions obtained predict the stress jumps at the interfaces, which cannot be captured by current FEA methods. As a novel method, the penalty function is used to enforce the displacement boundary conditions at the interfaces. Furthermore, the mathematical procedure developed enables the integration of different interfacial topographies into the solution procedure. In order to incorporate the effects of surface topography, the interface is expressed as a general surface in Cartesian coordinates, i.e. F (x, y, z) = 0. In this paper, the flat interface problem, i.e. y = 0 surface is considered for verification of the method by comparison with the FEA method. A comparison of the results reveals our new mathematical procedure to be a promising and efficient method for optimizing interface topographies.

Nature of magnetization reversal in exchange-coupled polycrystalline NiO-Co bilayers

The nature of magnetization reversal in exchange-coupled NiO-Co polycrystalline bilayers was investigated. As-deposited bilayers exhibit a moderate value of exchange bias ${H}_{E}$ (=-0.9 mT) and a significantly enhanced coercivity ${(H}_{c}^{\mathrm{N}\mathrm{i}\mathrm{O}\ensuremath{-}\mathrm{C}\mathrm{o}}=12.4\mathrm{mT}),$ which is roughly 5 times the coercivity of a reference Co single film ${(H}_{c}^{\mathrm{Co}}=2.7\mathrm{mT}).$ Real time investigation of magnetization reversal in exchange-coupled NiO-Co bilayers shows that reversal is highly local and nonuniform in nature. It is preceded by the formation of precursors or embryos of reversed domains as the applied field reaches a critical value \ensuremath{\cong}8.8--9.0 mT. Once this critical applied field value is reached, numerous reversed domains are formed. Growths of such reversed domains occur primarily by the abrupt nucleation and the subsequent coalescence together of reversed domains; wall motion is not the dominant growth mode. Clear evidence is presented which shows that the strength of exchange bias varies at the microscopic scale across the sample. This manifests itself as different microscopic regions switching abruptly at different fields, and a given microscopic area switching at different fields in the positive and negative field directions. When the applied field is along the unidirectional anisotropy, reversal of a given strongly coupled microscopic region is aided by exchange bias, and such a region switches first; the same region undergoes reversal last when the polarity of the applied field is changed to oppose unidirectional anisotropy. Significantly, it was found that, locally, the measured value of exchange bias may vary by a factor of 3 or more from the macroscopically measured value of ${H}_{E}$ (=-0.9 mT) obtained from the shift of the M-H loop. High-resolution transmission electron microscopy (HRTEM) shows that that the local variation in ${H}_{E}$ may be explained by considering the underlying microstructure and interfacial topography of the NiO-Co interface. HRTEM results show that the NiO surface parallel to the interface may be a fully compensated (200) plane (low ${H}_{E}),$ an uncompensated plane (111) (high ${H}_{E}),$ or a partially compensated plane (moderate ${H}_{E})$ of antiferromagnetic NiO. Clear evidence of partial compensation of the moments of antiferromagnetic NiO at the interface is found. Partial compensation (and hence a reduced value of ${H}_{E})$ results from a highly faceted NiO surface at the atomic level, with different facets having different orientation (different sublattices). Finally, a magnetic effect is observed, which suggests that it may be energetically more favorable to have steps on the NiO surface that are even multiples of the unit cell dimensions of NiO ${a}_{0}{(=2\mathrm{ma}}_{0},m\ensuremath{\in}\mathrm{integer}>0)$ and energetically unfavorable to have steps that are odd multiples of the unit cell dimension ${a}_{0}[=(2m\ensuremath{-}{1)a}_{0},m\ensuremath{\in}\mathrm{integer}>0].$

Structural assembly of Cd-arachidate molecules in multilayers

The three-dimensional (vertical and lateral) structure of Cd-arachidate multilayers prepared on two different substrates, glass and silicon, is studied simultaneously by grazing incidence x-ray reflection/diffraction. The grazing incidence reflectivity studies indicate the formation of a well-ordered layered structure on both the substrates. The reflectivity simulations show that the bilayer spacing in the case of a glass substrate is 5.54 nm while that on a silicon substrate is only 5.44 nm, indicating a vertical tilt of the Cd-arachidate molecules on a glass substrate by about 9.5°. The interface roughness is found to be 0.3 nm for multilayers on a glass substrate and 0.4 nm on the silicon substrate. The interface topography, determined by studying the nonspecular scattering behavior, is found to be replicated between the various interfaces in the multilayers, independent of the type of substrate. The lateral topography of the interfaces in multilayers on a glass substrate shows a self-affine nature with no lateral length limit for the roughness fluctuations. The interfaces in multilayers on a silicon substrate, however, exhibit a saturation behavior for the roughness fluctuations. In the plane of the multilayers the Cd-arachidate molecules on a glass substrate have a centered rectangular lattice arrangement while they have a distorted hexagonal lattice arrangement on a silicon substrate.

Influence of residual stresses on fracture and delamination of thin hard coatings

The risk of fracture and delamination of residually stressed coating systems is examined. Stress concentrations are generated at the interface of coated systems where the substrate deviates from being perfectly smooth, flat and infinitely large. Using finite element calculations, such stresses induced at pores, edges or scratches are analysed for a number of representative coating/substrate systems. The effect of the interface topography, coating thickness and elastic mismatch on the interfacial stresses are investigated. Generally, thin coatings compared to the interface topography are less sensitive to residual stress induced failure. At a critical coating thickness, normal stress across the interface of a magnitude comparable to that of the residual stress level is induced, which may initiate coating delamination. The interface stress state becomes independent of the coating thickness if the coating is thicker than about three times the amplitude of the interface roughness. The interface stress scales approximately linearly with the maximum inclination of the surface profile. It is demonstrated experimentally that the high residual stress in ceramic coatings may cause local coating fracture and delamination at, e.g., the tip of an edge or at rough substrate surfaces when the coating is thick relative to the edge radius or the surface topography, respectively.