Effective Surface Energy Research Articles

The surface action on dimer–monomer liquid crystal system

A microscopical mechanism of the interaction between dimerized nematics of 4, n-alkyloxybenzoic acids and hydrophilic solid surfaces is developed. The liquid crystal molecules: monomers and closed dimers are considered like dipoles and quadrupoles, respectively. It is demonstrated that in the low-temperature and the high-temperature ranges of the nematic phase, the mechanisms of the microscopical interaction are different thus providing different macroscopical effects. The cell thickness and the temperature trends of the scalar order parameter, the effective surface energy and the concomitant anomalies in these trends are discussed on the basis of the proposed microscopical mechanism.

Finite Element Modeling of Particle Adhesion: A Surface Energy Formalism

The adhesion of particles is modeled with finite element analysis using an energy approach comparable with that used in the JKR formalism. The strain energy of a cylindrically symmetric system, comprising a particle adhering to a surface with a fixed contact size, is computed as a function of contact size and then added to an energy term that is linearly proportional to the contact patch area. These computations also include contributions from the potential energy of a body force comparable with that which might be applied by a centrifuge. The results show regions of stability (adhesion) where a local energy minimum exists and regions of release where separation of the particle from the surface leads to a continuous decrease in the energy of the system. The effect of the deformation of the particle is included implicitly as a result of the FEM which provides details of the strains and stresses within the system. Discussion concentrates on the physical meaning of the behaviors and the significance of JKR-like theories that use an effective surface energy to represent electrostatic and van der Waals contributions to the adhesion. Modeling the effects of surface roughness of particles and the plastic deformation of particles through an effective surface energy is considered.

Electrical driving of the surface memorization in dimerized nematics with short range smectic order

Electrical control of the surface memorization of an oriented smectic C texture in the temperature range of dimerized nematics with short range smectic C order (4-n-alkyloxybenzoic acids) is presented. It is demonstrated that by suitable choice of the electric field parameters and surface conditions it is possible to separate the thermal and electrical components of the total erasure activation energy. The significant role of the double charge electric layer in the mechanism of the surface memorization is verified. By the electrical control of the memorization, we have confirmed the electrical part of the modified Rapini-Papoular anchoring energy in the effective surface energy.

Effect of surface relaxations on the equilibrium growth morphology of crystals: platelet formation

In this paper we report results showing that surface relaxations may profoundly alter the equilibrium growth morphology of crystals. This may result in growth forms not predicted by the Wulff plot. Surface relaxations and polarization fluctuations produce extra terms in the free energy of a growing crystal nucleus which combine with the surface energy to produce an effective surface energy which depends on the size and shape of the crystal. We consider the case of the growth of nuclei of a crystal with cubic symmetry and show that if the surface relaxations are large enough then small nuclei will grow into platelets, although large nuclei still grow into large three-dimensional crystals. We show by considering materials with the perovskite structure that this effect may occur in real materials.

Dynamical generation of the primordial magnetic field by ferromagnetic domain walls

The spontaneous generation of uniform magnetic condensate in QED 3 gives rise to ferromagnetic domain walls at the electroweak phase transition. These ferromagnetic domain walls are characterized by vanishing effective surface energy density avoiding, thus, the domain wall problem. Moreover we find that the domain walls generate a magnetic field B≃10 24 Gauss at the electroweak scale which account for the seed field in the so called dynamo mechanism for the cosmological primordial magnetic field. We find that the annihilation processes of walls with size R≃10 5 km could release an energy of order 10 52 erg indicating the invisible ferromagnetic walls as possible compact sources of Gamma Ray Bursts.

Determination of energetic parameters controlling cleavage fracture in a Ti-V microalloyed ferrite-pearlite steel

In BCC metals the process of brittle fracture has been attributed to the nucleation of a microcrack followed by its propagation into the surrounding matrix. This process implies three different steps that must take place dynamically. In the first step a microcrack nucleates from the rupture of some microstructural feature (carbide, non-metallic inclusion, ...). In the second and third steps, this microcrack propagates across the particle-matrix (p-m) and matrix-matrix (m-m) boundaries, respectively. The critical cleavage fracture stress required to achieve this three step brittle process has been experimentally determined for a wide range of microstructures. This value and the microstructural and energetic parameters ({gamma}{sub pm} and {gamma}{sub mm} effective surface energies for the second and third steps, respectively) are currently being used in the models developed to predict brittle fracture processes. One of the problems which arises in the modelling process is the definition of {gamma}{sub pm} and {gamma}{sub mm} effective surface energy values. Concerning the {gamma}{sub pm} energy, extensive studies have been carried out with different steels and microstructures, usually proposing values in the range from 7 to 14 J/m{sup 2}. In contrast, {gamma}{sub mm} experimental measured data are very scarce. In this work {gamma}{sub pm} and {gamma}{sub mm}more » effective surface energies have been experimentally determined in the case of a Ti-V microalloyed steel with ferrite-pearlite microstructure. In this steel, the nucleation of cleavage fracture takes place through the rupture of coarse TiN particles (>0.5 {micro}m).« less

Field-induced reorientation of nematic liquid crystals with twofold degenerate alignment onSiOxsurfaces

We show that an electric field normal to the boundaries confining a slab of nematic liquid crystals can induce a large change of the in-plane preferred orientation (i.e., the azimuthal angle). The degree of reorientation is controlled by the surfaces which have been coated with obliquely evaporated ${\mathrm{SiO}}_{x}.$ The experimental data are compared with simulations of a theoretical model where the effects of both large and small electric fields on the nematic orientation are considered. The reorientation phenomenon caused by the electric field appears to be a threshold phenomenon, due to the symmetry of the initial nematic orientation. For fields larger than the critical one, it is possible to define an effective surface energy for the system, containing the anisotropic surface energy and the contribution from the electric field.

Temperature and Time Effects on the “Adhesion Dynamics” of Poly(butyl methacrylate) (PBMA) Surfaces

The authors present a surface force apparatus (SFA) study of the effects of time and loading-unloading rates on the adhesion of solid polymeric surfaces of poly(butylmethacrylate). They used the equilibrium JKR theory of adhesion or contact mechanics as a framework for analyzing the adhesion dynamics of two surfaces during nonequilibrium (viscoelastic, plastic) adhesion and separation. PBMA films of thickness {approximately}2 {micro}m were prepared on curved mica surfaces by casting from a solution of methyl ethyl ketone. Pull-off forces from adhesive contact were measured at different temperatures around the glass-rubber transition temperature (T{sub g} {approx} 25 C) at different loads and contact times, and hysteretic loading-unloading cycles were measured at different rates. On entering the rubber regime, the effective surface energies deduced from the pull-off forces increase dramatically, by up to 3 orders of magnitude above the equilibrium value, with increasing contact time and load. Strong entanglements across the interface, probably through reptation, increase the effective area of contact with time, giving rise to the high pull-off forces observed. Bulk viscoelastic deformations of the surface profiles accompany the time-dependent adhesion processes. The existence of at least two different relaxation (energy dissipating) processes, one at the molecular level and the other atmore » the microscopic to macroscopic level, can be inferred from these experiments. The implications of the results for understanding the adhesion, fracture strength, and crack-propagation of elastic versus viscoelastic materials are discussed.« less

The Role of Microstructure in Toughness Behaviour of Microalloyed Steels

The micromechanisms involved in cleavage propagation in the brittle and ductile-brittle behaviour of microalloyed steels are analysed. The cleavage process is divided into three different steps which must take place dynamically in order to succeed: nucleation of a microcrack in a microstructural feature, propagation of the microcrack across the particle-matrix boundary and finally, propagation across the matrix surmounting high angle boundaries. The microstructural parameters involved in the three steps are considered, together with their effective surface energy values. Depending on testing temperature, the microstructural feature controlling cleavage fracture changes. In relation to the ductile-brittle transition, the main parameter is the volume fraction of coarse microstructural units surrounded by high angle boundaries.

Fracture characteristics of SiC particle reinforced oxynitride glass using chevron-notch three-point bend specimens

The fracture toughness K1c and the effective fracture surface energy γeff, of oxynitride glass (m)/SiC(p) brittle particulate composites were measured by means of stable fracture tests, using chevron-notch three-point bend specimens. In comparison to oxide glasses, the oxynitride glass matrix is noticeably tougher: K1cm = 1.2 MPa.\(\sqrt m\) and γeff = 5 J/m2. The addition of SiC particles, 6 μm in diameter, results in a significant toughening: K1c = 2.5 MPa.\(\sqrt m\) and γeff = 9.1 J/m2 for the composite with 40 vol\% SiC. In such systems, with strong particle-matrix interfacial bonding, and where (Ep, K1cp, σrp) > (Em, K1cm, σrm), the main toughening contribution is due to a discrete pinning of the crack by particles near the crack tip in conjunction to bowing of the crack front between particles, and to a small scale bridging mechanism.

Adhesion of elastic spheres

Bradley (1932) showed that if two rigid spheres of radii R 1 and R 2 are placed in contact, they will adhere with a force 2πΔ R γ, where R is the equivalent radius R 1 R 1 /( R 1 + R 2 ) and Δγ is the surface energy or ‘work of adhesion’ (equal to γ1+γ2-γ12). Subsequently Johnson et al. (1971) (JKR theory) showed by a Griffith energy argument (assuming that contact over a circle of radius a introduces a surface energy -π a 2 Δγ) how the Hertz equations for the contact of elastic spheres are modifed by surface energy, and showed that the force needed to separate the spheres is equal to (3/2)πΔ R γ, which is independent of the elastic modulus and so appears to be universally applicable and therefore to conflict with Bradley9s answer. The discrepancy was explained by Tabor (1977), who identified a parameter 3 Δγ 2 / 3 / E * 2 / 3 \e governing the transition from the Bradley pull-off force 2π R Δ|γ to the JKR value (3/2)π R Δγ. Subsequently Muller et al. (1980) performed a complete numerical solution in terms of surface forces rather than surface energy, (combining the Lennard–Jones law of force between surfaces with the elastic equations for a half-space), and confirmed that Tabor9s parameter does indeed govern the transition. The numerical solution is repeated more accurately and in greater detail, confirming the results, but showing also that the load–;approach curves become S-shaped for values of μ greater than one, leading to jumps into and out of contact. The JKR equations describe the behaviour well for values of μ of 3 or more, but for low values of μ the simple Bradley equation better describes the behaviour under negative loads.

The Crack Opening Method in Silicon Wafer Bonding: How Useful Is It?

The crack opening method is widely used for the determination of the surface energy of two bonded wafers, which is associated with the energy required to separate bonded wafers. In the present paper the dependence of the measured surface energy of bonded silicon wafers on the time after insertion of a blade at various pressures, silanol group densities, and annealing temperatures is reported. At normal conditions (air pressure, room temperature, 30% humidity) a strong dependence of the effective surface energy on the insertion time is found for wafers which were annealed at 200, 400, and 900°C. For bonded wafers without subsequent annealing this phenomenon is not observed. Additional measurements under reduced pressure show that the debonding process appears to be related to the adsorption and chemical reaction of water molecules at sites of broken bonds at the opened surfaces. We conclude that a meaningful comparison of measured surface energies requires detailed information on the measurement conditions.

On the contribution of G. R. Irwin to fracture mechanics

This is a brief survey of the scientific biography and works of George Rankine Irwin (born on February 26, 1907 in El Paso, Texas)—outstanding American scientist in the field of fracture mechanics and strength of materials, a Member of the American National Academy of Engineering, Royal Society of London, an Honorary Member and winner of prizes and awards of many American and European scientific societies. G. R. Irwin attended and graduated from Knox College and the University of Illinois where he won his Ph.D. degree. From 1936 till 1967, he worked in the US Naval Research Laboratory and in 1972, he became a Professor of Lehigh University and the University of Maryland where he now works. On the basis of his own experiments and the results of other researchers, G. R. Irwin suggested fundamentally new modification of the Griffith theory of brittle fracture and generalized this theory to the case of quasibrittle materials (see [2] and subsequent works). He introduced the concept of effective surface energy (1) and reformulated the Griffith theory in terms of singular stresses at the crack front and proved that his approach is equivalent to the Griffith energy approach [8]. He introduced stress intensity factors, i.e., parameters which relate the local mechanical state near the crack tip to macroscopic characteristics (such as the geometry of a body and loading) and enable one to give a very simple description of fracture processes as a continuous process of stable crack growth up to the onset of instability. His ideas very important for practical use and subsequent works devoted to their improvement and development caused a decisive influence on the progress in fracture mechanics as a new science and powerful engineering tool applicable to numerous problems connected with manufacturing and processing of materials and their failureproof operation in critical structures of various materials. By his works, Irwin initiated investigation of various aspects of the theory of fracture such as the role of plasticity, dynamics of the process, fractographic analysis of crack growth, etc. He also gave much attention to the development of new test methods for the determination of the characteristics of crackgrowth resistance of structural materials and their application. In recognition of Irwin's fundamental contribution to fracture mechanics, its fundamentals are now called the Griffith-Irwin theory (concept). A list of selected works by G. R. Irwin is given at the end of the present paper.

Langmuir-Blodgett film and nematic interface

The orientation induced by a Langmuir-Blodgett film on a Nematic Liquid Crystal (NLC) is theoretically analyzed. We show that the effective surface energy is due to different contributions connected with steric and van der Waals interactions between the nematic and the solid substrate. The analysis shows that the Langmuir-Blodgett film orientation depends on the surface density of the molecules. The initial homeotropic orientation may become unstable giving rise to a tilted film. The average orientation of the nematic molecules is also analyzed. We show that, in the event in which the steric interaction Nematic-Langmuir-Blodgett film is very large with respect to the dispersion interaction Nematic-Substrate, the nematic orientation coincides with the one of the film. On the contrary, when the two interactions are comparable, the orientation of the two media may differ. In particular, we analyze how the stable orientation depends on the surface molecular density of the film.

Surface energy of solids.

We discuss the concept of the surface energy of a solid. Our point of view is that the surface enrgy, as classically defined, is but one attribute of a more general distance-dependent interaction between bodies. We argue that the mechanics of adhesion and fracture of solids can be correctly modeled by the incorporation of classical surface energy into a conventional elastic model. As evidence of this, we reconsider a classical model of Obreimoff for the cleaving of mica. Our analysis shows that the introduction of a short-range distance-dependent interaction leads to the system being governed by an effective surface energy which is 2.25 times larger than the surface energy as classically defined.

Fabrication and Characterization of Titanium Carbide-Graphite Composite Materials. 1st Report. Mechanical Properties and Microstructure of Titanium Carbide and Its Composites with Addition of a Small Amount of Graphite.

Titanium carbide and its composites incorporated with 1, 3, 5 and 10 wt% of graphite were consolidated by hot-pressing at temperatures ranging from 1800°to 2100°C. Although significant grain growth occurred in the monolithic TiC at higher hot-pressing temperatures, the graphite addition inhibited the grain growth well in the composites. Room-temperature bending strength, σ, of the monolithic TiC correlated with the average grain diameter, d, through the Hall-Petch relation (σ∝d-1/2). Such a correlation may be attributed to intrinsically existing surface flaws with size dominated by average grain size. Fracture toughness of the monolithic TiC increased linearly with the average grain diameter, in accordance with an increase in effective surface energy. The graphite addition resulted in strengthening of the TiC through the inhibition of both grain growth and crack propagation ; however, excess addition of graphite (more than 3 wt%) caused strength reduction in the composites due to graphite agglomeration which acts as a fracture origin. Fracture toughness of the TiC also increased with graphite addition through deflection and bridging of the cracks by graphite particles.

Anchoring energy and easy direction of non uniform surfaces

The effect of a periodic distribution of easy directions on the surface macroscopic properties of nematic samples are considered. Our analysis shows that the effective surface energy of the sample contains an interference term coming from the two different easy axis. This allows to introduce an effective uniform easy direction, depending on the two easy directions and containing an interference term too, as suggested a few years ago by some researchers

Determination of effective surface energy of dynamic failure

A method is presented for determining the effective surface energy under dynamic loading. It is based on combining the effect of a valuewise different static tensile load and similar tensile pulses on identical specimens. It is shown that the method is insensitive to the sharpness of the originally made notch. This facilitates the form of the moving crack front to be controlled and eliminates the need to set up a fatigue crack. Graphs of variation in time of the SIF and length and velocity of the crack are given.

The anisotropic nature of local crack stability in BCC crystals

In a semi-brittle material, dislocation emission from the crack tip and from near tip sources controls the local elastic-plastic tip stress field. As such, this local dislocation emission, which is connected to the intrinsic constraint of crystal plasticity, might also lead to anisotropic features of both the effective surface energy, γ eff , and the local driving force, G local . The major objective of the current study is to elucidate some of the local anisotropic aspects involved in the cleavage fracture mode of body centered cubic (BCC) crystals. Special emphasis is given to the effective surface energy recognized here as an anisotropic entity expressed by [γ eff]φ, λ (here φ, λ designate the specific cleavage plane and the crack growth direction, respectively). These local views become possible through an extensive research activity in hydrogen induced subcritical growth of Fe-3%Si crystals. The experimental program included internal and external hydrogen interactions for different crystal orientations. Attention was given to the study of internal flaw/slow flaw growth in addition to sharp crack extension in prefatigued single edge notched specimens. A detailed phenomenological and theoretical analysis which is related to the local fracture parameters is described, with some application to the current hydrogen interaction problem as well. This case of quasi-static slow crack growth on a cleavage plane enabled the identification of the anisotropic features mainly by: extensive crack tip morphology investigation, the shapes of growing crack fronts or voids, a modified elastic-plastic dislocation simulation at the crack tip and further distinction between the microcrack initiation and propagation stages. Analysis of the above elements as based on experimental results indicates that the crack tip stability and the crack front shape are related to the dislocations and/or slip plane arrangements with anisotropic control of crack initiation and arrest. It is emphasized that crack growth itself need not be anisotropic and, in fact, may be more isotropic.

A thermodynamic approach to the influence of plastic deformation on crack instability

It is argued that the role of plastic deformation in crack growth has been widely misinterpreted. In particular the notion that the work of plastic deformation contributes to the energy balance in such a way as to give rise to an increased effective surface energy is shown to be physically incorrect. A fundamental reappraisal of the role of plastic deformation on the instability of cracks is undertaken and a thermodynamic formulation of the problem of the energetics of crack extension is developed. This is capable of treating both moving cracks and the dislocations producing plastic deformation in a unified manner and provides a powerful tool for the study of combined crack extension and plastic deformation. A rigorous definition of plastic work is also given and it is shown that the crack extension force is the only physically meaningful parameter which can be used in the formulation of a crack instability criterion. The work of plastic deformation is not a part of the crack extension force and should not be included in the criterion for crack instability. It is found that one of the important effects of plastic deformation on crack instability is the ‘shielding’ effect or the reduction it causes in the magnitude of the stress at the crack tip resulting in a smaller crack extension force. Other possible effects of plastic deformation on crack propagation are briefly discussed.