Confinement-induced instability and adhesive failure between dissimilar thin elastic films

When two thin soft elastomeric films are separated from each other, an elastic instability develops at the interface. Although similar instability develops for the case of a soft film separating from a rigid adherent, there are important differences in the two cases. For the single-film case, the wavelength of instability is independent of any material properties of the system, and it scales only with thickness of the film. For the two-film case, a co-operative instability mode develops, which is a non-linear function of the thicknesses and the elastic moduli of both films. We investigate the development of such instability by energy minimization procedures. Understanding the nature of this instability is important, as it affects the adhesive compliance of the system and thus the energy release rate in the debonding of soft interfaces.     

Shear-induced adhesive failure of a rigid slab in contact with a thin confined film

A rigid-glass prism (square or rectangular base, rectangular cross-section) is sheared off a thin film of silicone elastomer bonded to a glass plate by applying a tangential force at various distances above the prism/elastomer interface. At a given tangential force, the prism starts to slide on the elastomeric film. As the sliding velocity, thus the frictional force, is progressively increased, an elastic instability develops at the interface that results in the formation of numerous bubbles. These bubbles, the lateral dimension of which is comparable to the thickness of the film, move across the interface with speeds 1000 times faster than the overall sliding speed of the glass prism against the PDMS film. It is found that the glass prism continues to slide on the elastomeric film as long as the applied shear stress is less than a critical value. During sliding, however, a normal stress is developed at the interface that decays from the front (i.e. where the force is applied) to the rear end of the prism. When the normal stress reaches a critical value, the prism comes off the film. The critical shear stress of fracture increases with the modulus of the film, but decreases with the thickness following a square root relationship, as is the case with the removal of rigid punches from thin elastomeric films by normal pull-off forces.     

Formation of sharp metal-organic semiconductor interfaces: Ag and Sn on CuPc

A detailed investigation of the chemistry and electronic structure during the formation of the interfaces between thin films of the archetypal organic molecular semiconductor copper phthalocyanine (CuPc) and Ag or Sn deposited on it was performed using photoemission and near-edge X-ray absorption spectroscopies with synchrotron light. Our study demonstrates the formation of sharp, abrupt interfaces, a behavior which is of particular importance for applications in organic devices. Moreover, for Ag on CuPc we demonstrate that this interface is free from any reaction, whereas there is slight interface reaction for Sn/CuPc.     

Nanorheology: An investigation of the boundary condition at hydrophobic and hydrophilic interfaces

It has been shown that the flow of a simple liquid over a solid surface can violate the so-called no-slip boundary condition. We investigate the flow of polar liquids, water and glycerol, on a hydrophilic Pyrex surface and a hydrophobic surface made of a Self-Assembled Monolayer of OTS (octadecyltrichlorosilane) on Pyrex. We use a Dynamic Surface Force Apparatus (DSFA) which allows one to study the flow of a liquid film confined between two surfaces with a nanometer resolution. No-slip boundary conditions are found for both fluids on hydrophilic surfaces only. Significant slip is found on the hydrophobic surfaces, with a typical length of one hundred nanometers. Received 21 December 2001 and Received in final form 3 August 2002 RID="a" ID="a"e-mail: ccottin@dpm.univ-lyon1.fr RID="b" ID="b"Present address.     

Elastic theory of defects in toroidal crystals

We report a comprehensive analysis of the ground-state properties of axisymmetric toroidal crystals based on the elastic theory of defects on curved substrates. The ground state is analyzed as a function of the aspect ratio of the torus, which provides a non-local measure of the underlying Gaussian curvature, and the ratio of the defect core energy to the Young modulus. Several structural features are discussed, including a spectacular example of curvature-driven amorphization in the limit of the aspect ratio approaching one. The outcome of the elastic theory is then compared with the results of a numerical study of a system of point-like particles constrained on the surface of a torus and interacting via a short-range potential. Electronic supplementary material  Supplementary material in the form of a pdf file available from the journal web page at and are accessible for authorised users.     

The energetics of large Lennard-Jones clusters: transition to the hexagonal close-packed structure

The energetics of large multiply twined particles (MTPs) such as decahedra with fivefold symmetry, face-centred cubic (fcc) and hexagonal close-packed (hcp) clusters in size from 2000 to ~45000 atoms was numerically analysed. Clusters were relaxed freely under the Lennard-Jones pair potential to the energy minimum. The essential extension of size compared to previous studies and the additional shape-optimisation of hcp and fcc clusters as well as truncated decahedra appears to be of high importance in the potential energy analysis. The best-optimised decahedra were confirmed to be the most favourable structure from 2000 to ~10⁵ atoms. Only in the short size interval, above N ∼10000 atoms, the best-optimised fcc clusters and simplest Marks' decahedra could alternate, while above N ∼14000 atoms does the shape-optimised hcp structure be proved to become more favourable for single crystal particles compared to the best-optimised fcc structure. Depending on shapes and sizes, decahedra and hcp clusters can alternate in the wide size interval above N ∼ 14000 atoms and presumably form the mixed abundances of clusters belonging to the both symmetries. Finally, the upper limit for stable MTPs was estimated to be about N ∼10⁵ atoms, while above only the hcp clusters are the most favourable.     

Whole diffraction pattern-fitting of polycrystalline fcc materials based on microstructure

A method is proposed for modelling the complete diffraction pattern of fcc polycrystalline materials. The algorithm permits a simultaneous refinement of several parameters related to microstructure and lattice defects responsible for line broadening effects. Linear (dislocations) and planar (stacking faults) defects are considered in detail, together with the effect of size and shape of coherent scattering domains (crystallites). Experimentally observed profiles are modelled by Voigt functions, whose parameters are connected with those describing the dislocation field (dislocation density, outer cut-off radius, average contrast factor), twin and deformation fault probabilities, and domain size, also considering the effect of a symmetrical instrumental profile. Domain shape is assumed spherical, with a lognormal distribution of diameters; however, the approach can be generalised to different shapes and size distributions. The proposed algorithm can be extended to other crystalline structures, and can be used within the Rietveld method or as a Whole Powder Pattern Fitting (WPPF), as in the present work. Received 16 May 2000     

Measurement of adhesion energies and Young's modulus in thin polymer films using a novel axi-symmetric peel test geometry

We present a novel method of probing adhesion energies of solids, particularly polymers. This method uses the axi-symmetric deformation of a thin spincast polymer membrane brought into contact with a flat substrate to probe the work of adhesion. The use of a thin membrane minimizes uncertainty in the radius of contact, while the use of spincast films provides very smooth surfaces by means of a very simple method. The experimental profile of the deformed membrane shows good agreement with the expected logarithmic profile. The experimental setup enables the measurement of Young's modulus and the solid-solid work of adhesion for thin films. The value obtained for Young's modulus of polystyrene (PS) was found to be in agreement with other conventional measurement techniques. In addition, measurement of the work of adhesion at the PS/silicon oxide interface was possible. The apparatus is well suited to studying the dependence of Young's modulus, work of adhesion and fracture energy on membrane thickness, temperature, pulling rate, and ageing of the interface, and can readily be modified to study biologically relevant samples.     

Non-linear rheology of lamellar liquid crystals

We measure the non-linear relation between the shear stress and shear rate in the lyotropic lamellar phase of C₁₂E₅ /water system. The measured shear thinning exponent changes with the surfactant concentration. A simple rheology theory of a lamellar or smectic phase is proposed with a prediction ∼ σ^3/2 , where is the shear rate and σ is the shear stress. We consider that the shear flow passed through the defect structure causes the main dissipation. As the defect line density varies with the shear rate, the shear thinning arises. The defect density is estimated by the dynamic balance between the production and annihilation processes. The defect production is caused by the shear-induced layer undulation instability. The annihilation occurs through the shear-induced defect collision process. Further flow visualization experiment shows that the defect texture correlates strongly with the shear thinning exponent.     

Buffer effects of titanium in the case of silver / a - Al 2 O 3(0001)

The influence of thin titanium layers on the growth of silver clusters on - Al ₂ O ₃ (0001) is investigated. We demonstrate through in situ RHEED measurements that titanium can relax stress in a growth mode where two lattice parameters show up simultaneously. Above a certain thickness, the lattice parameter closest to the bulk value of titanium dominates. Depending on the amount of stress in the titanium layer, silver films can either develop 3D textures or grow in epitaxy and form 2D like films.     

Diffusion of adsorbates on random alloy surfaces

In this work the diffusion of non-interacting adsorbates on a random AB alloy surface is considered. For this purpose a simple cubic (sc), body-centered cubic (bcc) or face-centered cubic (fcc) auxiliary metal lattice is introduced. The auxiliary lattice is truncated parallel to its (100) plane in such a way that the fourfold hollow positions of the metal surface form a regular net of adsorption sites with square symmetry. The adsorption energy of each adsorption site is determined by its own environment, i.e. by the numbers of direct A or B neighbors. The Monte-Carlo method has been utilized to simulate surface diffusion of adsorbates on such energetically heterogeneous alloy surfaces and to calculate the tracer, jump and chemical diffusion coefficients. The chemical diffusion coefficient was calculated via two different approaches: the fluctuation and the Kubo-Green method. The influence of energetical heterogeneities on the surface diffusion is largely pronounced at low temperatures and low surface coverages, where most of the adatoms are trapped by deep adsorption sites. It was found that at low temperatures the sequential occupation of the different types of adsorption sites can be observed. Received: 24 October 1997 / Accepted: 17 December 1997     

Equilibrium morphologies of epitaxially strained islands

An analytical expression of the free energy consisting of the strain energy, surface energy and interfacial energy for the coherent island/substrate system, as well as the evolving relations of aspect ratio against volume of the island, and misfit of the system, which provides a broad perspective on island behavior, is obtained, and used to study the equilibrium shapes of the systems. A two-dimensional model assuming linear elastic behavior is used to analyze an isolated island with elastic properties similar to those of the substrate. The results show that in order to minimize the total free energy, a coherent island will adopt a particular shape and height-to-width aspect ratio that are a function of only the island volume. The effect of a misfit dislocation on the equilibrium shape of an island is in passing examined. These can serve as a basis for interpretation of experiments.     

Melting transition of a network model in two dimensions

The freezing transition of a network model for tensionless membranes confined to two dimensions is investigated by Monte Carlo simulations and scaling arguments. In this model, a freezing transition is induced by reducing the tether length. Translational and bond-orientational order parameters and elastic constants are determined as a function of the tether length. A finite-size scaling analysis is used to show that the crystal melts via successive dislocation and disclination unbinding transitions, in qualitative agreement with the predictions of the Kosterlitz-Thouless-Halperin-Nelson-Young theory. The hexatic phase is found to be stable over only a very small interval of tether lengths. Received 4 June 1999 and Revised in final form 1 September 1999     

The interplay between the surface band structure and possible surface reconstructions of Mo(112)

The experimental band structure of Mo(112) and the effects by temperature and adsorbate are presented. A surface resonance, identified as crossing the Fermi level at about 1/3 from to of surface Brillouin zone, was observed to be very sensitive to both contamination and temperature. We find evidence of adsorbate and temperature induced reconstruction of the Mo(112) surface. Examination of low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM) data provides evidence for an adsorbate induced reconstruction of the Mo(112) surface with periodicities consistent with the Fermi level crossing of the surface resonance. The reconstruction is found to occur at coverages as low as 0.03 Langmuirs of oxygen or carbon. The reconstruction and/or adsorbate affects the density of states and bands near the Fermi level of a ₁ symmetry. Received 3 March 1999 and Received in final form 1 October 1999     

Elastic theory of icosahedral quasicrystals - application to straight dislocations

In quasicrystals, there are not only conventional, but also phason displacement fields and associated Burgers vectors. We have calculated approximate solutions for the elastic fields induced by two-, three- and fivefold straight screw- and edge-dislocations in infinite icosahedral quasicrystals by means of a generalized perturbation method. Starting from the solution for elastic isotropy in phonon and phason spaces, corrections of higher order reflect the two-, three- and fivefold symmetry of the elastic fields surrounding screw dislocations. The fields of special edge dislocations display characteristic symmetries also, which can be seen from the contributions of all orders. Received 21 February 2001 and Received in final form 27 June 2001     

SiC film growth on Si(111) by supersonic beams of C 60

The development of electronic devices based on Silicon Carbide (SiC) has been strongly limited by the difficulties in growing high quality crystalline bulk materials and films. We have recently elaborated a new technique for the synthesis of SiC on clean Si substrates by means of supersonic beams of C₆₀: the electronic and structural properties of the film can be controlled by monitoring the beam parameters, i.e. flux and particles energy and aggregation state. SiC films were grown in Ultra High Vacuum on Si(111)-7×7, at substrates temperatures of 800 ^° C, using two different supersonic beams of C₆₀: He and H₂ have been used as seeding gases, leading to particles energy of 5 eV and 20 eV, respectively. Surface characterisation was done in situ by Auger and X-Ray photoelectron spectroscopy, as well as by low energy electron diffraction and ex situ by atomic force microscopy technique. SiC films exhibited good structural and electronic properties, with presence of defects different from the typical triangular voids. Received 20 November 2001     

Development of a ReaxFF description for gold

Atomistic simulations of the chemistry of thiol-gold-systems have been restricted by the lack of interatomic interaction models for the involved elements. The ReaxFF framework already has potentials for hydrocarbons, making it an attractive basis for extending to the complete AuSCH-system. Here, an interatomic potential for gold, based on the ReaxFF framework, is presented and compared to existing gold potentials available in the literature. Electronic supplementary material  Supplementary Online Material     

Grain boundaries and the law of corresponding cones in smectics

Focal Conic Domains (FCDs) in smectic phases often assemble according to a particular rule, experimentally discovered by G. Friedel, the law of corresponding cones (l.c.c.). This paper reports various results relating to this type of association. First we show that a l.c.c. contact between 2 focal conic domains has a vanishing energy, yielding metastable local equilibrium. Then we use some projective properties of conic sections to extend the celebrated Apollonian tiling, which describes a tilt grain boundary (TiGB) of vanishing disorientation made of toric focal conic domains, to any TiGB. Finally we present a realistic model of the energy of the TiGB, which we compare to the energy of a TiGB split into dislocations, and to the energy of a curvature wall. This model explains why FCD tilings show macroscopic zones not filled with FCDs. Received 21 June 1999 and Received in final form 10 September 1999