Repulsive and attractive Casimir forces between magnetodielectric slabs

The Casimir force between two parallel magnetodielectric slabs is investigated by means of Casimir–Lifshitz Theory. For two magnetodielectric slabs, one is permittivity-negative, while the other is permeability-negative in the real frequency space. Numerical results show that when the separation between these two slabs is small (or large), the Casimir force is repulsive, while for the intermediate separation, the Casimir force is attractive. As a consequence, there are two equilibria with zero Casimir force, and a repulsive–attractive–repulsive transition takes place with increasing the separation. Therefore, if the separation between two interacting slabs is manipulated in the small (or large) separation region, it is possible to overcome the stiction in micromechanical and nanomechanical systems.     

The physics of fine powders: plugging and surface instabilities

The behavior of granular matter depends greatly on the size of its elementary components. Besides the well studied field of granulates made up of large sized particles which ignore the interaction of the particles with the fluid or gas environment, the physics of a collection of tiny particles such as fine or superfine powders concerns a majority of industrial applications. This paper briefly outlines several basic behavior of powders showing that new features come into play when the particle interaction with the surrounding gas is taken into account. It starts from two key mechanisms: the first arises when the typical particle velocity is in the order of the free fall velocity of that particle, which simply means that fluid drag comes into play. The second consists in considering the powder cakes as a porous material. Combining these two basic mechanisms with well-known granulate properties such as avalanching or heaping, leads to previously ignored sets of plugging effects or surface instabilities resulting from what we call the ‘volcano effect’. Furthermore, we show that, up to a certain extent, the physics of fine powders interacting with gas, may mimic the physics of wetting liquids. To cite this article: J. Duran, C. R. Physique 3 (2002) 217–227.     

Double layers and interparticle forces in colloid science and biology: analytic results for the effect of ionic dispersion forces

We study analytically two of the central problems of colloid science: the structure of the double layer surrounding a charged particle and the forces between two such particles. Traditionally, these have been understood using a combination of electrostatic forces acting on the ions and the entropy of the same ions. Here we derive explicit formulas showing how the often dominant dispersion forces between ions and the surfaces can dramatically change the structure of the double layer and the forces.     

Relationship between the magnetic field distribution and attractive force of single-domain YBCO bulk under different field cooling processes

The levitation forces under different field cooling states were measured at 77K by changing the field cooling distance Z_{fc} between a YBCO bulk and a permanent magnet. It is found that the relationship between the absolute maximum attractive force (Z_{maf}) and the corresponding gap distance (Z_{maf}) to F_{maf} can be well described by exponential laws as a function of Z_{fc}, which allow us to predict these values according to Z_{fc}. It is also found that the distance between the Z_{0fa} (gap distance corresponding to the zero force) and Z_{maf} in the ascending process is a constant value, which is closely related to the constant reduction factor of the axial component of flux density along the axial line of the magnet if Z_{maf}-Z_{0fa} is a constant value. These results are very interesting for fundamental research and helpful in practical designing and applications.     

The difference between Newtonian and relativistic forces

This paper discusses a new turn in the 148-year old electrodynamic force law controversy between the 1822 Ampère force law of the Newtonian electrodynamics and Grassmann's 1845 law which has become the electrodynamic force law of relativistic electromagnetism. Faced with the infallibility of Ampère's empirical law, defenders of relativity theory now argue that Ampère's law is equivalent to the relativistic law. This paper demonstrates that, far from being equivalent, the laws require two different mechanics of solid bodies, disagree on internally generated stresses, and predict different force distributions.     

Superparamagnetic effects and cation distribution in industrially produced hexaferrite fine powders

Fine powder samples of substituted Barium M-type haxaferrites with composition BaFe_12?2xCo_xTi_xO₁₉ (0.72相似文献   

The negative electromagnetic attractive forces arising from kinetic energy of conduction electrons in double-layer metallic nanowire arrays

The coupled-nanowire plasmatic resonances and very strong negative electromagnetic force between the double-layer metallic nanowire arrays is investigated theoretically. The negative electromagnetic force indicates attractive interaction between the metallic nanowires with the air-gap cavity local resonance effect. Plasmon resonances lead to extremely large localized ?eld, thereby resulting in large mutual coupling forces between the double-layer metallic nanowire arrays. The light coupling into metallic microcavity can stimulate collective electron oscillations of plasmatic resonance, and cause the contraction of the negative pressure in metallic cavity wall. The electromagnetic field of the plasmatic resonance mode is mainly localized inside the air-gap region between the two wires.     

Study of the relationship between particle sizes of polymer powders and their radiative properties

The reflectance of a surface considered as smooth relative to the wavelength mainly consists of a specular component while for a rough surface, the reflectance is composed of both specular and diffuse (or volume) components, their relative importance depending on the nature of the reflecting material. The present samples consist of polypropylene powders and the influence of particle size on total and diffuse reflectance is discussed over two wavelength ranges: 0.25–2.5 and 2.5–15.5 μm. The results of this study indicate firstly, that in the first wavelength range, the areas of the absorbance peaks increase while they decrease for the other wavelength range with particle size and secondly, that the reflectance values calculated for the wavelength range 0.25–2.5 μm, with respect to a blackbody at 5800 K, vary with particle size.     

Annealing behaviour of implanted nitrogen in bulk and evaporated Ni

The diffusion coefficients of nitrogen in N-implanted polycrystalline Ni have been deduced. Both bulk and Ni-evaporated samples implanted with nitrogen were annealed at 150–500° C. The nitrogen profiles were probed using the nuclear resonance broadening technique. The value of 0.99±0.12 eV for the activation energy and (3.0 _–2.8 ⁺⁴⁰ )×10^–6 cm²/s for the frequency factor were obtained for implanted N in bulk Ni. The solubilities for both the bulk and evaporated Ni samples are given. In evaporated Ni nitrogen migration is enhanced due to the defects arising during evaporation.     

On the theoretical relationship between the Monin-Obukhov stability parameter and the bulk Richardson number

Summary Nomograms were constructed to determine relations among different stability parameters in the surface layer. The variables interrelated were the Monin-Obukhov stability parameter, ξ=z/L, the bulk Richardson number, Ri_b, and the PasquillA-F stability classes. Also, the nomograms were used to estimate the Monin-Obukhov length variation, with respect to changes in surface roughness and atmospheric stability.     

Competition between local potentials and attractive particle-particle interactions in superlattices

Naturally occuring or man-made systems displaying periodic spatial modulations of their properties on a nanoscale constitute superlattices. Such modulated structures are important both as prototypes of simple nanotechnological devices and as particular examples of emerging spatial inhomogeneity in interacting many-electron systems. Here we investigate the effect different types of modulation of the system parameters have on the ground-state energy and the charge-density distribution of the system. The superlattices are described by the inhomogeneous attractive Hubbard model, and the calculations are performed by density-functional and density-matrix renormalization group techniques. We find that modulations in local electric potentials are much more effective in shaping the system’s properties than modulations in the attractive on-site interaction. This is the same conclusion we previously [M.F. Silva, N.A. Lima, A.L. Malvezzi, K. Capelle, Phys. Rev. B 71 (2005) 125130.] obtained for repulsive interactions, suggesting that it is not an artifact of a specific state, but a general property of modulated structures.     

The relationship between magnetic behaviour and local structure around Fe ions in Fe-doped TiO2 rutile

The magnetic and structural characterization of Ti_1−xFe_xO₂ (x=0.025, 0.05, 0.07, 0.125, and 0.15) samples prepared by mechano-synthesis using TiO₂ and Fe₂O₃ as starting materials are reported. XANES measurements performed at the Fe K-edge show that Fe ions are in 3+ oxidation state in the 7 at% Fe-doped sample and in a mixture of 2+ and 3+ oxidation states in the other samples. EXAFS results show the incorporation of Fe ions substituting Ti ones in the rutile TiO₂ structure. They also reveal a strong correlation between the number of oxygen nearest neighbours and the Fe²⁺ fraction, i.e the number of oxygen near neighbours decreases when the Fe²⁺ fraction increases. All samples present ferromagnetic-like behaviour at room temperature. We found a clear dependence between saturation magnetization and coercivity with the fraction of Fe²⁺ and/or the number of Fe near neighbour oxygen vacancies.     

On the relationship between the gradient and the bulk Richardson number for the atmospheric surface layer

Summary Semi-empirical formulations which have been proposed to describe the wind and potential temperature profiles are used to derive relationships between the gradient Richardson number, Ri, the finite-difference layer Richardson number, Ri_b, the surface layer Richardson number, Ri_s, and the bulk Richardson number,B, through the atmospheric surface layer. The theoretical analysis for stable conditions indicates that Ri (z ₃)=Ri_b, wherez ₃=(z ₂−z ₁)/ln(z ₂/z ₁), andz ₂;z ₁=upper and lower levels at which temperature and wind speed are specified. It is also found that, during stable conditions, the wind profile power law exponent,p, is computed at the heightz ₃, instead of the widely used geometric mean height,z _m, between top (z ₂) and bottom (z ₁) of the layer considered.     

The relationship between ferritin and haemosiderin

Under conditions of iron overload haemosiderin may replace ferritin as a major iron store. Much of this haemosiderin is contained within secondary lysosomes, known as sidersomes, where it is thought to have arisen by breakdown of ferritin. We report on⁵⁷Fe Mössbauer studies of ferritin and haemosiderin prepared from iron loaded rat livers and measured over a wide temperature range. Our results indicate significantly different superparamagnetic blocking temperatures consistent with the model whereby haemosiderin is derived from ferritin by lysosomal degradation.     

How crossing affects the analytic relationship between low energy ππ amplitudes and their asymptotic behaviour

Different physical assumptions about the asymptotic behaviour of ππ amplitudes are realised in the different number of substractions involved in fixed t dispersion relations for the various amplitudes and their inverses. The fact that each new dispersion relation must be consistent with s - t crossing leads to a number of conditions relating low energy ππ amplitudes to their high energy behaviour. These are discussed in detail. Such relationships supplement finite energy sum rules with which they are compared. The dispersive sum rules, crossing conditions, and finite energy sum rules we discuss are applied to recent phenomenological solutions to Roy's equations and shown not to narrow the presently accepted range of threshold parameters. These results are in marked contrast to the conclusions of other recent studies. To complete the study of finite energy sum rules we consider the behaviour of the isospin zero t-channel amplitude and estimate the asymptotic ππ total cross-section. We present evidence to suggest that the pomeron is late-developing in meson-meson scattering.     

The correlations between the saturated and dry P-wave velocity of rocks

Sometimes engineers need to estimate the wet-rock P-wave velocity from the dry-rock P-wave velocity. An estimation equation embracing all rock classes will be useful for the rock engineers. To investigate the predictability of wet-rock P-wave velocity from the dry-rock P-wave velocity, P-wave velocity measurements were performed on 41 different rock types, 11 of which were igneous, 15 of which were sedimentary and 15 of which was metamorphic. In addition to the dry- and wet-rock P-wave velocity measurements, the P-wave velocity changing as a function of saturation degree was studied. Moreover, dry-rock S-wave velocity measurements were conducted. The test results were modeled using Gassmann’s and Wood’s theory and it was seen that the measured data did not fit the theories. The unconformity is due to the fact that the theories are valid for high-porosity unconsolidated sediments at low frequencies. Gassmann’s equation was modified for the rocks except high-porosity unconsolidated sediments.

The dry- and wet-rock P-wave velocity values were evaluated using regression analysis. A strong linear correlation between the dry- and wet-rock P-wave velocities was found. Regression analyses were repeated for the rock classes and it was shown that correlation coefficients were increased. Concluding remark is that the derived equations can be used for the prediction of wet-rock P-wave velocity from the dry-rock P-wave velocity.