Bending elasticity of surfactant films: the role of the hydrophobic tails

We study the tail contributions to the elastic constants of an amphiphilic layer. For dense systems the terms arising from the attractive van der Waals interactions prevail, whereas the translational entropy of the liquid layer is essential at lower density. Both the membrane rigidity kappa and the Gaussian bending elastic constant kappa; strongly vary with the density of the hydrocarbon chains; for dense systems, they are of the order 100kT with a ratio kappa;/kappa = -2 / 3. At lower density, partial cancellation of interaction and entropic contributions leads to elastic constants of the order of kT, with a ratio close to the value -2 observed for microemulsions.     

Van der Waals interaction of membranes

The van der Waals interaction of hydrocarbon films in water is calculated using the exact expression derived by Parsegian and Ninham. The results are compared to an often used approximation, which fails for this system due to the large difference between the static dielectric constants of hydrocarbons and water. For large membrane separations the correct value of the van der Waals interaction is about a hundred times larger than the approximate result. The consequences for predicting unbinding transitions and interpreting experiments are discussed.Dedicated to Prof. Herbert Wagner on the occasion of his 60th birthday     

Study of the anharmonic properties of copper halides

Summary We have investigated the anharmonic properties of cooper halides by means of a three-body interaction potential extended to include the van der Waals effects. For this purpose, we have computed the thirdorder elastic constants and the pressure derivatives of second-order elastic constants after deriving their correct expressions from the present potential following the lines of Sharma and Verma. Our results have, generally, followed a systematic trend and given a satisfactory prediction of the available experimental data on the anharmonic properties of copper halides. This potential has as scope of its application to describe the dynamical and dielectric properties of these halides. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Van der waals constants of rare gases in an interpolation model

The possibility is investigated of determining the dispersion constants for pairs of rare-gas atoms directly from the homoatomic and heteroatomic interaction potentials. A simple interpolation model is proposed which makes it possible to find the constant C₆ from the known values of the potentials close to the minimum point. Calculations of the van der Waals constants C₆ performed using the potentials obtained by different authors demonstrated the good accuracy of the model and a definite correlation between the accuracy of the calculated van der Waals constants and the closeness to experiment of the pair potential employed in the model. Kabardino-Balkarian State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 70–73, August, 1997.     

The generalised Huggins-Mayer form of born repulsive potentials for NaCl-type alkali halides

The generalised Huggins-Mayer form of Born repulsive potentials for NaCl-type alkali halides have been re-evaluated using more reliable, recently published thermodynamic data and van der Waals energy coefficients. As with older versions of these potentials excellent agreement between model and experimental values of interionic distance and cohesive energy is achieved. Further, the earlier shortcomings in the failure of the stability condition and predicted elastic constants are significantly reduced. For the majority of salts the new short-range interactions have stronger van der Waals attractions and slider repulsions. The model gives average crystal radii for the alkali ions about ~0.3 Å larger than traditional free ion radii and ~0.08 Å larger than Tosi-Fumi crystal radii. The predicted halogen crystal radii are correspondingly smaller by the same amounts.     

论范德瓦尔斯a和b参数与温度的关系

范德瓦尔斯方程中a、b参数是否与温度相关,不但不同文献中的说法互不相同,而且有同一文献前后的结论相互矛盾.本文分析了这个令人迷惑的问题.在热力学中a和b参量被处理为与温度无关,它仅仅在临界点附近有效并可以把范德瓦尔斯方程表述为对应态定律;在更加广泛的温度区间a、b参量和温度有关,但是范德瓦尔斯方程却丧失了其独特性.统计物理文献把a和b参量处理成为位力展开的一种拟合方程,发现一般情况下两个参数都依赖于温度,仅仅在特殊情况下和温度无关,以说明对应态定律成立的条件.热力学和统计物理关于a和b参量是否依赖于温度相互矛盾的表述,源于这两个理论中范德瓦尔斯状态方程适用范围不同.     

Calculated pressure effects on spectral lines for long-range interatomic potentials: Rb and Cs with heavy rare gases

We have calculated the impact broadening and shift of the first and second doublets of Rb and Cs. The interaction potential of the collision pair is assumed to be a long-range van der Waals potential. The van der Waals constants involved are provided by recent theoretical calculations. We obtain good agreement between the calculated and experimental values for both the P_{$\text{1}\text{2}$} and P_{$\text{3}\text{2}$} component. Comparison of our calculated values is also made with those of Granier, Granier and Schuller.     

The Molecular Structure and Potential Energy Function for Ne-HF System

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Buckling of spherical shells adhering onto a rigid substrate

Deformation of a spherical shell adhering onto a rigid substrate due to van der Waals attractive interaction is investigated by means of numerical minimization (conjugate gradient method) of the sum of the elastic and adhesion energies. The conformation of the deformed shell is governed by two dimensionless parameters, i.e., C_s/epsilon and C_b/epsilon where C_s and C_b are respectively the stretching and the bending constants, and epsilon is the depth of the van der Waals potential between the shell and substrate. Four different regimes of deformation are characterized as these parameters are systematically varied: (i) small deformation regime, (ii) disk formation regime, (iii) isotropic buckling regime, and (iv) anisotropic buckling regime. By measuring the various quantities of the deformed shells, we find that both discontinuous and continuous bucking transitions occur for large and small C_s/epsilon, respectively. This behavior of the buckling transition is analogous to van der Waals liquids or gels, and we have numerically determined the associated critical point. Scaling arguments are employed to explain the adhesion induced buckling transition, i.e., from the disk formation regime to the isotropic buckling regime. We show that the buckling transition takes place when the indentation length exceeds the effective shell thickness which is determined from the elastic constants. This prediction is in good agreement with our numerical results. Moreover, the ratio between the indentation length and its thickness at the transition point provides a constant number (2–3) independent of the shell size. This universal number is observed in various experimental systems ranging from nanoscale to macroscale. In particular, our results agree well with the recent compression experiment using microcapsules.     

Many body forces and elastic constants of rare gas crystals

The effect of many body Van der Waals forces on the elastic constants of solid rare gases is calculated. These results are combined with contributions from the two body potential and zero point energy. The violation of the Cauchy relation is studied.     

Calculation of a one dimensional van der waals liquid-gas transition by means of a Ginzburg-Landau expansion

The grand canonical partition function written as a Feynman path integral over the order parameter is calculated numerically within the framework of a Ginzburg-Landau theory for a one dimensional van der Waals system. The equation of state is calculated. The results show that the transition is of quasi first order.     

范德瓦尔斯磁性材料CrOCl变磁性相变的磁力显微镜研究

近年来,二维范德瓦尔斯磁性材料因为在自旋电子学的应用前景而吸引了广泛的关注.CrOCl是一种范德瓦尔斯磁性材料,理论预言其单层具有高达160 K的居里温度,因此吸引了广泛的关注。为了更好的理解这一材料的磁性,我们利用磁力显微镜研究了CrOCl变磁性相变中磁畴结构随磁场的变化。实验发现,在2K下CrOCl样品表面出现随磁场变化的方格条纹,给出了变磁性相比中反铁磁相和铁磁相竞争的图样,并通过二维快速傅里叶变换证实了CrOCl磁性的各向异性。我们的结果为后续研究CrOCl薄层的磁性提供了参考依据。     

Bemerkung zur Bestimmung der van der Waals-Konstanten aus Streuquerschnitten

The attempt has been made by several authors to clear up the discrepancy between van der Waals constants determined from scattering cross sections for thermal energies and theoretical values using measured oscillator strengths. The present paper discusses three sources of error not accounted for properly in the earlier work. The appropriate corrections discussed in this paper lead to a satisfactory agreement between experiment and theory.     

A simple model for the surface energy of ionic crystals

The surface energy of ionic materials is empirically related to bulk properties (elastic constants, electronic dielectric constant and optical band gap) through an analysis of the cleavage force. This is evaluated at small and large separations of the two crystal halves from phonon dispersion curves and from van der Waals interactions, respectively, and these two limiting behaviours are connected by a scaling hypothesis introduced for metals by Kohn and Yaniv. The experimental data that are available for a few ionic crystals seem to satisfy the suggested relation, with an empirical universal parameter which has roughly the same value as determined for metals.     

二维范德瓦尔斯异质结构的制备与物性研究

二维范德瓦尔斯材料(可简称二维材料)已发展成为备受瞩目的材料大家族,而由其衍生的二维范德瓦尔斯异质结构的集成、性能及应用是现今凝聚态物理和材料科学领域的研究热点之一.二维范德瓦尔斯异质结构为探索丰富多彩的物理效应和新奇的物理现象,以及构建新型的自旋电子学器件提供了灵活而广阔的平台.本文从二维材料的转移技术着手,介绍二维范德瓦尔斯异质结构的构筑、性能及应用.首先,依据湿法转移和干法转移的分类,详细介绍二维范德瓦尔斯异质结构的制备技术,内容包括转移技术的通用设备、常用转移方法的具体操作步骤、三维操纵二维材料的方法、异质界面清洁.随后介绍二维范德瓦尔斯异质结构的性能和应用,重点介绍二维磁性范德瓦尔斯异质结构,并列举在二维范德瓦尔斯磁隧道结和摩尔超晶格领域的应用.因此,二维材料转移技术的发展和优化将进一步助力二维范德瓦尔斯异质结构在基础科学研究和实际应用上取得突破性的成果.     

二维范德瓦尔斯异质结构的制备与物性研究

二维范德瓦尔斯材料(可简称二维材料)已发展成为备受瞩目的材料大家族,而由其衍生的二维范德瓦尔斯异质结构的集成、性能及应用是现今凝聚态物理和材料科学领域的研究热点之一.二维范德瓦尔斯异质结构为探索丰富多彩的物理效应和新奇的物理现象,以及构建新型的自旋电子学器件提供了灵活而广阔的平台.本文从二维材料的转移技术着手,介绍二维范德瓦尔斯异质结构的构筑、性能及应用.首先,依据湿法转移和干法转移的分类,详细介绍二维范德瓦尔斯异质结构的制备技术,内容包括转移技术的通用设备、常用转移方法的具体操作步骤、三维操纵二维材料的方法、异质界面清洁.随后介绍二维范德瓦尔斯异质结构的性能和应用,重点介绍二维磁性范德瓦尔斯异质结构,并列举在二维范德瓦尔斯磁隧道结和摩尔超晶格领域的应用.因此,二维材料转移技术的发展和优化将进一步助力二维范德瓦尔斯异质结构在基础科学研究和实际应用上取得突破性的成果.     

Van der Waals forces in electrolyte solutions

In this article we study van der Waals forces in electrolyte solutions from a local point of view. It is shown for arbitrary geometry that the classical limit of the force density exerted on the ions, as found in the macroscopic theory of van der Waals forces, is identical with the force density calculated from electrostatics and thermodynamic fluctuation theory. Thus neither retardation, nor the Lorentz force affect the average force density.     

Many-body theory for multipole polarizabilities and dispersion forces in helium-helium interactions

Linked cluster many-body perturbation theory has been developed for the calculation of the dynamic quadrupole polarizabilities of the helium atom in its ground state. The polarizabilities have been used to calculate the London dispersion force constants for He-He interactions. The results for the polarizabilities and the van der Waals constants compare very well with other standard values.     

Lithium dipole properties and van der Waals constants using a pseudo spectral one-centre method

A pseudo spectral one-centre method is used to evaluate the moments of the dipole oscillator strength distribution for the ground-state Li atom and the van der Waals constants for the ground-state Li-Li, Li-He and Li-H interactions. Various combinations of C.I. type Li atom ground and excited ² P ⁰-state wavefunctions are used to construct the required pseudo spectral dipole oscillator strengths and excitation energies. The analysis of all the pseudo-state data yields recommended values for the Li dipole properties and van der Waals constants that are probably more accurate than those obtained previously. The calculations illustrate some of the advantages of the pseudo spectral approach for the evaluation of atomic and molecular properties and interaction energies.     

热力耦合作用下双层碳纳米管的扭转屈曲

考虑碳纳米管周边弹性介质和层间范德瓦耳斯力的作用,利用连续介质力学的壳体理论,建立了热力耦合作用下碳纳米管屈曲问题的控制方程,给出了相应的临界屈曲扭矩的解析解.数值模拟结果表明,在低温和室温环境下,碳纳米管的临界屈曲载荷随着温度变化量的增加而提高;在高温环境下,碳纳米管的临界屈曲载荷随着温度变化量的增加而降低. 关键词： 碳纳米管 屈曲 热力耦合