Van der Waals Attraction in Multilayer Structures-II

The vdW attraction in multilayer structures is reconsidered on the basis of Green's function techniques and finite boundary conditions. No difficulties related to branch points are encountered in the course of integration. Electric and magnetic modes are considered. By applying Floquet's theorem an exact expression for the vdW energy between periodic multilayers is obtained. The relations of the present method to the surface mode hypothesis and to the reaction field approach are shown. Quantitative conclusions pertaining to the dependence of the vdW energy on the separation d are drawn.     

Van Der Waals Attraction Between Macroscopic Bodies

A general dispersion formula is derived, which represents the dispersion energy between two bodies A and B by their macroscopic screened fluctuation fields. These fluctuation fields are calculated exactly in the case of half-spaces and spheres. In the case of half-spaces the Lifshitz dispersion formula is obtained. The dispersion energy between spheres is found to vary as 1/d for separations d small compared with the radii and to be proportional to 1/d ⁶ for separations d large compared with the radii. The effect of layers adsorbed on the surfaces of the spheres on the dispersion energy is calculated. The dielectric properties of the adsorbed layers predominate over those of the bulk material for separations d smaller than the layer thickness.     

Van Der Waals Attraction Between Macroscopic Bodies

A general dispersion formula is derived, which represents the dispersion energy between two bodies A and B by their macroscopic screened fluctuation fields. These fluctuation fields are calculated exactly in the case of half-spaces and spheres. In the case of half-spaces the Lifshitz dispersion formula is obtained. The dispersion energy between spheres is found to vary as 1/d for separations d small compared with the radii and to be proportional to 1/d⁶ for separations d large compared with the radii. The effect of layers adsorbed on the surfaces of the spheres on the dispersion energy is calculated. The dielectric properties of the adsorbed layers predominate over those of the bulk material for separations d smaller than the layer thickness.     

高亲水性纳米二氧化硅颗粒稳定范德华乳液

利用未修饰的高亲水性纳米二氧化硅颗粒(SiO2 NPs)在其等电点附近制备了范德华乳液.结果表明,在等电点(pH 2.7)下,SiO2 NPs借助高速均质与油/水界面剧烈混合,并受范德华引力的驱动逼近界面,在SiO2 NPs质量分数0.5％、水相体积分数50％的条件下稳定为平均粒径244μm的O/W型范德华乳液.增加S...     

Van der Waals epitaxy and characterization of hexagonal boron nitride nanosheets on graphene

Graphene is highly sensitive to environmental influences, and thus, it is worthwhile to deposit protective layers on graphene without impairing its excellent properties. Hexagonal boron nitride (h-BN), a well-known dielectric material, may afford the necessary protection. In this research, we demonstrated the van der Waals epitaxy of h-BN nanosheets on mechanically exfoliated graphene by chemical vapor deposition, using borazine as the precursor to h-BN. The h-BN nanosheets had a triangular morphology on a narrow graphene belt but a polygonal morphology on a larger graphene film. The h-BN nanosheets on graphene were highly crystalline, except for various in-plane lattice orientations. Interestingly, the h-BN nanosheets preferred to grow on graphene than on SiO₂/Si under the chosen experimental conditions, and this selective growth spoke of potential promise for application to the preparation of graphene/h-BN superlattice structures fabricated on SiO₂/Si.     

Equation of State of Explosion Products on the Basis of a Modified Van der Waals Model

A semi-empirical model is proposed for the equation of state of high explosives in a range of pressures and temperatures typical of detonation processes. A possibility of formation of solid phases (e.g., graphite or diamond) in the gas is implied. The model can be used to calculate all thermodynamic quantities for arbitrary molecular compositions and to calculate the thermodynamically equilibrium molecular (and phase) composition. An iterative scheme of calculations is proposed. The model contains several empirical functions whose form can be changed without violating the overall calculation scheme. A particular set of these functions is considered as an illustration. Some results calculated for a number of high explosives containing four elements (C, H, N, and O) are presented. The calculated results are compared with available experimental data. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 1, pp. 87–99, January–February, 2006.     

用Visual Foxpro语言编程求解范德华方程的根

范德华方程求解真实气体的体积,传统的求解方程的根的方法是试差法,花费大量的时间,且精度达不到要求,改用Visual Foxpro语言进行程序设计以后,难题迎刃而解。     

Improvement of the Van Der Waals equation of state

The Van Der Waals equation of state has been modified in order to improve its accuracy. The introduction of a temperature dependence of the attraction term allows reproducing accurately pure-compound vapour pressures. The modification of the mixing rules, with the infinite-pressure excess-free-energy term expressed by a NRTL-like model, allows reproducing accurately phase equilibria of polar and apolar systems. Finally, the introduction of a volume correction improves markedly the calculated liquid densities, without changing the phase-equilibrium conditions. The improved equation is accurate enough to be applied for design purposes.     

Microwave Bandgap in Multilayer Ceramic Structures

A multilayer ceramic structure with a photonic bandgap (MC-PBG) was fabricated by a method of tape casting combined with screen printing. The MC-PBG structure is a two-dimensional array with either rectangular or hexagonal metal coils in a ceramic matrix. The metal coils are connected to the base metal layer in the ceramic substrate to form a monolithic body. The surface-wave dispersion properties of these MC-PBG structures were measured. A stop band, which is significantly influenced by the symmetrical characteristics of the inductor–capacitor ( LC ) arrays, was found in both the structures in the frequency range of 2.0–3.5 GHz. Because of their effective surface-wave suppression, MC-PBG structures can be used as high-performance antenna substrates to enhance the broadside gain of patch antenna devices.     

Enhancement of Silica Aerosol Coagulation by Van Der Waals Forces

The interaction energy between small silica particles has been calculated for a range of diameters between 2 and 30 nm. The enhancement of the coagulation rate has been calculated for these particles. The enhancement factor ranges from a maximum of about 2.62 for a 2-nm diameter pair down to 1.96 for a 30-nm diameter pair. Calculations of the aerosol size distribution indicate that a nearly self-preserving form is attained after the cessation of nucleation.     

Van der Waals parameters,refractive indices and dispersion equations of spectrin,actin and other mammalian proteins

Van der Waals' parameters and dispersion equations of some mammalian proteins including actin, spectrin, haemoglobin, myoglobin, fibrinogen, ribonuclease A, and a calf serum protein are determined from refractive indices of their solutions and the solvents. The results indicate that van der Waals' parameters of these proteins, and also those of bovine serum albumin, fall within a relatively narrow range of values; which slightly overlap those of sugars and are above those of phospholipids.     

London-van der Waals adhesiveness of rough particles

Van der Waals forces often dominate interactions and adhesion between fine particles and, in turn, decisively influence the bulk behaviour of powders. However, so far there is no effective means to characterize the adhesive behaviour of such particles. A complication is that most powder particles have rough surfaces, and it is the asperities on the surfaces that touch, confounding the actual surface that is in contact. Conventional approaches using surface energy provide limited information regarding adhesion, and pull-off forces measured through atomic force microscope (AFM) are highly variable and difficult to interpret. In this paper we develop a model which combines the Rumpf-Rabinovich and the JKR-DMT theories to account simultaneously for the effects of surface roughness and deformation on adhesion. This is applied to a ‘characteristic asperity’ which may be easily obtained from AFM measurements. The concept of adhesiveness, a material property reflecting the influences of elastic deformability, surface roughness, and interfacial surface energy, is introduced as an efficient and quantitative measure of the adhering tendency of a powder. Furthermore, a novel concept of specific adhesiveness is proposed as a convenient tool for characterizing and benchmarking solid materials. This paper provides an example to illustrate the use of the proposed theories.     

Calculation of van der Waals forces in adhering systems

The existing theories of van der Waals attractive forces between two media separated by a vacuum or a third medium can be adapted to the case of true adhesion by introducing an adjustable parameter z₀. The consistency of the new model leads to the introduction of an effective dielectric constant. A study of the influence of the frequency range of the dielectric spectrum on van der Waals forces shows that when there is adhesion or when the intermediate layer is thin, only the u.v. component of the dielectric spectrum plays a role.The invariance of van der Waals forces to changes in the i.r. component of the dielectric spectrum allows the work function of electrons and accordingly the always and simultaneously present electrostatic adhesion forces, which are of the same order of magnitude, to be varied without changing the magnitude of the van der Waals adhesion forces.     

van der Waals interaction energies between non-planar bodies

The van der Waals interaction energies between non-planar geometries are obtained without the assumption that the distance between two non-planar bodies is much smaller than radii of the non-planar bodies. Based on atomto-body van der Waals energies, we calculate body-to-body van der Waals interaction energies for several non-planar geometries. Using the continuum approach, we discuss the van der Waals interactions in two-dimensional carbon nanotubes and C₆₀ molecules. Professor Hyun-Ku Rhee on the occasion of his retirement from Seoul National University     

The adhesion of spherical particles: Contributions of Van Der Waals and electrostatic interactions

The forces needed to detach monodisperse spherical polystyrene particles having radii between approximately 1 μm and 6 μm from a polyester substrate were determined using electrostatic detachment. It was found that the removal force varied linearly with particle radius, as predicted by the JKR theory (K. L. Johnson, K. Kendall, and A. D. Roberts, Proc. R. Soc. London, Ser. A 324 , 301 (1971)). In addition, the work of adhesion, estimated from JKR theory, was found to be approximately 0.01 J/m 2 . This is a reasonable value for a system such as this. These results are, however, inconsistent with the predictions of models that assume that particle adhesion is dominated by electrostatic forces due to either a uniform charge distribution over the surface of the particle or localized charged patches.     

Equation of State and Transport Coefficients of Argon,Based on a Modified Van der Waals Model up to Pressures of 100 GPa

A modified Van der Waals model of the equation of state and a model of transport coefficients (of viscosity, thermal conductivity, diffusion, and electrical conductivity) based on this modified Van der Waals model are described. The approach is used to describe the characteristics of liquid and gaseous argon. As a whole, model calculations are in reasonable agreement with results of various experiments up to pressures of the order of 100 GPa.     

Electronic-Vibrational Spectroscopy of Large van der Waals Molecules

In this paper we focus on the electronic-vibrational spectra of tetracene · A₁ (A = Ar, Kr, Xe) heteroclusters, which provide information on the intermolecular nuclear motion of a rare-gas atom on the microsurface of a large aromatic molecule. Calculations of the energy levels and nuclear Franck-Con-don factors for the excitation of the parallel and perpendicular intermolecular vibrations in the electronic S₀ → S₁ excitation of heteroclusters were performed. The experimental energetic and intensity data for the perpendicular intermolecular nuclear excitation are well accounted for by our calculations. This study provides some information regarding the potential surfaces of large heteroclusters in electronically excited states, which are important for the elucidation of the spectroscopy, nuclear motion, and isomerization dynamics of large aromatic molecule · (rare gas)n heteroclusters.     

Light Emission from Highly Reflective Porous Silicon Multilayer Structures

Porous silicon photoluminescence and electroluminescence can be controlled by periodically modulating the material porosity to form high quality multilayer stacks and microcavities. Important issues not yet fully addressed are (a) the precise role played by this microstructuring, given that the luminescence is distributed throughout the entire structure and that the low porosity layers are highly absorbing at short wavelengths, and (b) whether the quality of such microcavities could be sufficient to support lasing. Using both experimental and theoretical techniques, the emission and reflection properties of different porous silicon single and multilayer structures have been investigated in order to understand further and exploit the nature of light propagation within them.