Pressure tensor in wedge-shaped cavity of solid with dispersion forces

The pressure tensor in a wedge-shaped cavity of a solid with dispersion forces is calculated. All four (three diagonal and one nondiagonal) components of the pressure tensor are calculated as functions of their positions inside the cavity. It is shown that these components satisfy the mechanical equilibrium condition. The dependence of the pressure tensor on the opening angle of the wedge-shaped cavity is determined. Disjoining pressure of vacuum film for its two alternative definitions is calculated.     

The asymptotics of distribution functions in fluid-filled finite slits

Asymptotic formulas have been derived for distribution functions in cylindrical slits with finite cross-section radii. The problem has been solved based on the asymptotic principle of interbody interactions within the framework of dispersion forces. Symmetric and asymmetric slits between two identical cylinders and between a cylinder and a planar infinite solid surface have been considered. Calculations have been performed for a symmetric slit with a width equal to two cylinder radii.     

Light-Force Technique for Measuring Cluster Polarizabilities

We report on a light-force technique to make absolute measurements of cluster polarizabilities in the gas phase. The technique relies on several important experimental components and methods, including a front-ablation cluster source to produce cold clusters, a position-sensitive time-of-flight spectrometer, glass collimating slits, and a reliable technique for timing a particular slice of the cluster beam. The cluster beam is collimated with a pair of slits and the ideal locations of the slits to maximize the observable light force is discussed. To date, the polarizability of C₆₀ at the fundamental wavelength of a Nd:YAG laser (1.064µm) has been measured.     

Correction to the paper “the condition of mechanical equilibrium on the surface of a nonuniform thin film” [1]

The correction is introduced to the definition of the disjoining pressure of wedge-shaped film formulated in the authors’ previous publication. In refined definition, the nondiagonal pattern of pressure tensor even in cylindrical coordinates is taken into account and the disjoining pressure is introduced by analogy with the case of the transition zone of wetting meniscus. The alternative procedure of the definition of the disjoining pressure through the diagonal pressure tensor in the middle of the wedge is also considered.     

Three-Dimensional Aspect of the Surface Tension: An Approach Based on the Total Pressure Tensor

The conditions of mechanical equilibrium were considered, and the generalized notion of the surface tension at an arbitrarily curved surface layer was analyzed on the basis of the total nondiagonal pressure tensor including the external fields and anisotropic even in the bulk phases. It was shown that the transverse surface tension can be eliminated using the selection of a dividing surface; however, in the general case, this surface does not exhibit the properties of the tension surface. On the whole, three-dimensional and nondiagonal character of the tensors of excess surface stresses determined by the integration over the volume and the cross section of the surface layer is retained at any selection of the dividing surface.     

Monte Carlo Simulations of Polymers in Nanoslits

Summary: Monte Carlo computer simulations have been performed for model polymers confined in slits of thickness comparable to the transverse diameter of the chains. The density of polymer within the slits is allowed to vary with the slit thickness in such a way that the content of the slits is always in equilibrium with a large reservoir of bulk polymer. The calculations reveal the presence of polymer‐mediated attractive or repulsive interactions between the slit plates, oscillating with the slit thickness in good agreement with experimental results.

The base cell used in the simulations.     

Determination of potential-energy surface of molecules in an applied field on the basis of virial relations

The quantum-mechanical virial theorem (in diagonal and nondiagonal forms) for molecules in an external, weak, uniform electric field is used for obtaining the analytical expression of the potentialenergy surface in the Hartree–Fock–Roothaan one-determinantal approximation. The polarizability tensor components of the H₂O molecule are computed on this basis. The dependence of basing the atomic orbitals upon the intensity of the applied field lead to good coincidence of results with the data of near Hartree–Fock calculations.     

Molecular arrangements and conformations of liquid unbranched alkanes in narrow slits

Realistic, atomistic models of liquid tridecane in broad slits (>3 nm) and in narrow slits of thickness 1,2 nm and 1,0 nm have been obtained using the Monte Carlo technique. The setup of the models is such that the molecules in the slits are in equilibrium with the bulk liquid. The surfaces of the plates are modelled as two-dimensional arrays of hexagonally packed units having the same size and interaction parameters of a methylene group. The regions adjacent to the plates in slits with thickness > 3 nm are characterized by a well defined tendency to form partially ordered layer structures, while molecules at a distance from the plates larger than 1,5 nm are unperturbed. The simultaneous presence of two plates increases the tendency to form layer structures when their distance is 1,2 nm, while this tendency is almost totally destroyed when the slit is squeezed down to a thickness of 1,0 nm. This is also associated with a 10% decrease of the density in the latter slit.     

Pressure enhancement in carbon nanopores: a major confinement effect

Phenomena that occur only at high pressures in bulk phases are often observed in nanopores, suggesting that the pressure in such confined phases is large. We report a molecular simulation study of the pressure tensor of an argon nanophase within slit-shaped carbon pores and show that the tangential pressure is positive and large, while the normal pressure can be positive or negative depending on pore width. We also show that small changes in the bulk pressure have a large effect on the tangential pressure, suggesting that it should be possible to control the latter over wide ranges in laboratory experiments.     

Low-temperature anomalies of two-photon absorption in In2O3 nanocrystals incorporated into PMMA matrixes

By using a method of two-photon absorption, it has been observed that monodisperse In2O3 nanocrystals (NCs) with sizes equal to approximately 14-24 nm show significant increase of two-photon absorption at low temperatures. When T = 40 K, the two-photon absorption coefficients for low-sized samples drastically increase, whereas such enhancement for the samples with averaged sizes above approximately 24 nm is not significant. The nondiagonal tensor components give one-order-less two-photon absorption values. A principal role in the observed dependences is played by interface nanolayers, demonstrating substantial contribution of nanoconfined effects. This phenomenon could only be observed in highly monodisperse NCs. Increase of the size distribution substantially suppresses the two-photon absorption observed. For the relatively large nanocrystals, the effects are comparable with those of the bulklike crystals. At the same time, the drastic increase at low temperatures may indicate a substantial role of the phonon subsystem.     

Estimation of the liquid-vapor spinodal from interfacial properties obtained from molecular dynamics and lattice Boltzmann simulations

Interfacial pressure and density profiles are calculated from molecular dynamics and lattice Boltzmann simulations of a liquid film in equilibrium with its vapor. The set of local values of tangential pressure and density along an interface exhibits a van der Waals-type loop; starting from the stable vapor bulk phase one passes through metastable and unstable states to the stable liquid bulk phase. The minimum and maximum values of the profile of tangential pressure are related to the liquid and vapor spinodal states, respectively. The spinodal pressures turn out to be linearly related to the extreme values of the tangential pressure in the interface. The comparison with equations of state shows good agreement with the simulation results of the spinodals. In addition the properties of the metastable region are obtained. Based on this investigation a method is proposed for the estimation of the liquid spinodal from experimentally obtained interfacial properties. Estimations for water and helium are presented.     

Theory of the excitation spectrum of nondiagonal disordered systems

The spectrum of a two-component solid solution with a nondiagonal disorder is studied in the framework of the average T matrix method. For a one-dimensional system in the nearest-neighbor approximation the criteria for the system parameters are given such that at an in-band resonance, one or two “impurity bands” may be realized, and the corresponding model calculation is performed. In the single-site approximation an expression of the self-energy part of a nondiagonal disordered system Green's function is found taking into account multiple occupancy corrections. The possibility of using it to describe a disordered system excitation spectrum and the calculation of state density moments are discussed.     

Polymolecular adsorption and capillary condensation in narrow slit pores

Capillary condensation and polymolecular adsorption in narrow slits has been calculated, where the fields of surface forces overlap one another. The calculations were carried out on the basis of macroscopic theory of dispersion forces and the isotherms of lone adsorption layers at the free surface. It has been shown that under the effect of mutual attraction through a gap, polymolecular adsorption films lose their stability long before their thickness has approached the half-width of a flat slit. This results in hysteresis of the capillary condensation in an ensemble of plane-parallel slits.

In the case of systems having strong adsorbate-adsorbate interaction, there has been detected the existence of the lower limit of sizes of slit pores, wherein the capillary meniscus can coexist with adsorption films. With a slit width smaller than the critical one, the meniscus is likely to form a finite contact angle with “dry” surfaces of a slit. Thus an explanation has been given of the lower limit of the capillary condensation in an ensemble of flat-surface, slit pores. In the case of strong adsorbate-adsorbent interaction, the coexistence of meniscus with adsorption films within the scope of the approach used is possible in slits of any width.

The value of corrections for the surface forces effect to be entered in the calculations of slit pores dimensions has been analyzed on the basis of the capillary condensation data obtained.

In wedge-shaped slits there also exists, besides lower limit the upper limit of capillary hysteresis.     

Numerical observation of preferred directionality in ion ejection from stretched rectilinear ion traps

We report on numerical investigations of directionality of ion ejection in stretched rectilinear ion traps (RIT). Three 4-electrode trap geometries have been investigated. In all cases, one pair of electrodes has slits at their center and the other pair has no slits. The studied traps include the RIT-S, in which the mass analyzer electrodes are symmetrically positioned around the central axis; the RIT-X, in which the mass analyzer has a stretch in the direction of the electrodes which have slits (labeled as x-direction); and the RIT-Y, in which the mass analyzer has a stretch in the direction of the electrodes which have no slits (labeled as y-direction).Our analysis has been carried out on two-dimensional (2D) fields at the centre of an infinitely long mass analyzer. The boundary element method (BEM) has been used for field computations. The trajectory of ion motion has been generated using Runge Kutta fourth order integration.Three sets of simulations have been carried out on each of the RIT-S, the RIT-X and the RIT-Y to check for directionality of ion ejection. In the first, we numerically obtain the stability region on the potential (U_dc– V_rf) axes. In the second we generate an escape velocity plot with U_dc=0 for different values of V_rf. In the third, we simulate the mass selective boundary ejection experiment on a single ion.In the symmetric RIT-S, as expected, all three simulations show that there is an equal probability of ion reaching the trap boundary in either of the x- or y-directions. For the stretched traps, however, the results are dramatically different. For the RIT-X, all three simulations suggest that ion destabilization at the stability boundary occurs in the x-direction. Similarly, for the RIT-Y, ions preferentially get destabilized in the y-direction. That is, ions reaching the trap boundary overwhelmingly prefer the stretch direction.     

Study of the hydrostatic pressure dependence of the Raman spectrum of single-walled carbon nanotubes and nanospheres

We have investigated the behavior of single-walled carbon nanotubes and nanospheres (C(60)) under high hydrostatic pressure using Raman spectroscopy over the pressure range 0.2-10 GPa using a diamond anvil cell. Different liquid mixtures were used as pressure transmission fluids (PTF). Comparing the pressure dependence of the Raman peak positions for the nanotubes and the nanospheres in different PTF leads to the observation of a number of new phenomena. The observed shift in Raman peak position of both radial and tangential modes as a function of applied pressure and their dependence on the PTF chemical composition can be rationalized in terms of adsorption of molecular species from the of PTF on to the surface of the carbon nanotubes and/or nanospheres. The peak shifts are fully reversible and take place at a comparatively modest pressure (2-3 GPa) that is far below pressures that might be required to collapse the nanoparticles. Surface adsorption of molecular species on the nanotube or nanospheres provides a far more plausible rational for the observed phenomena than ideas based on the notion of tube collapse that have been put forward in the recent literature.     

Polymer-induced depletion interaction between weakly attractive plates

Lattice Monte Carlo simulations have been employed to calculate depletion interaction of excluded volume chains in a weakly attractive slit, particularly in the region around the critical point of adsorption. The simulations were performed under full equilibrium conditions where a dilute solution in a slit was in contact with the reservoir. The free energy of confinement deltaA, the force f, and the relative pressurepI/pE on the slit walls were calculated as a function of slit width D and the attraction strength epsilon. The depletion region in the pressure profile pI/pE vs D is reduced by an increase in the attraction potential epsilon in a manner resembling the influence of polymer concentration. At the critical point of adsorption epsilonc the depletion interaction vanishes both in the pressure pI/pE and in the intraslit concentration profile phiI(x). The parameters used to assess the stability of colloidal dispersions such as the depletion potential W(D) (an integral of the net pressure deltap) reach a unique value at the critical condition. A monotonic repulsive profilepI vs D was found for chains trapped in the slit at restricted equilibrium. The mean dimensions (R2) of chains compressed in attractive slits feature a distinct minimum at intermediate slit widths.     

Study of continuous two-dimensional thermal field-flow fractionation of polymers

Two-dimensional thermal field-flow fractionation (2D-ThFFF) is a new instrumental technique devised for continuous fractionation of soluble macromolecules and particles. The sample mixture is introduced into a disc-shaped channel and the separated sample components are collected continuously from the channel outlets. The method is based on a two-dimensional fractionation mechanism with radial and tangential flow components in the channel. The effects of flow components and thermal gradient on the fractionation were studied in the separation of polystyrene samples of different molecular masses using cyclohexane or a binary solvent consisting of 25% ethylbenzene and 75% cyclohexane as carrier. The continuous separation of polystyrene samples was improved with increasing thermal gradient and with the use of slow radial and tangential flow rates. The technique can be applied to preparative continuous separation of macromolecules.     

On the different types of states of a one dimensional system of electrons in the Hartree-Fock method

We consider the different types of ground states of one-dimensional lattice gas of electrons. Interactions between electrons on the same lattice point and on neighboring lattice points are taken into account. The treatment is based on Hartree-Fock approximation for an unbounded system. Regions of the parameter space of the electron-electron interaction are found corresponding to the existence of mixed solutions (i.e., diagonal and nondiagonal long-range order are present stimultaneously). It is shown that the ground state of the system always corresponds to one of the pure types of order (only diagonal or only nondiagonal long-range order) and that diagonal order is present in the ground state. Conditions for the stability of the pure solutions against various perturbations are found.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 25, No. 5, pp. 513–520, September–October, 1989.The authors thank V. Ya. Krivnov for useful discussions.     

Role of electron correlation in hyperfine interaction

The spin density near the nucleus of a magnetic atom is examined in the first order with respect to the nondiagonal matrix element for the electron interaction with the corresponding excitation level. A system containing one electron outside the closed shells is discussed.     

Gradient Expansions and Beyond for Stress Tensor and Tangential Pressure Deficit through a Planar Liquid-Vapour Interface

Abstract

After a discussion of gradient expansions for physical quantities through a planar liquid-vapour interface, two proposals are made as to possible generalizations of the tangential pressure deficit through the interface. One expresses the deficit in terms of force-force correlation functions and the other in terms of the direct correlation function in the presence of the interface. Finally, a very practical approach generalizes Fowler's formula for the surface tension, to obtain an explicit, though now evidently approximate expression for the tangential pressure deficit in the limit when the thickness of the interface tends to zero.