bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of charge distribution on the thermophysical and dynamical properties of polar linear molecules
Authors: Calero Sofı́a , Garzón Benito , Lago Santiago
Published: June 2003

Gibbs ensemble Monte Carlo and molecular dynamics simulations were used to study thermophysical and dynamical properties of Kihara fluids consisting of linear molecules with dipolar symmetry. Two models differing in the electrostatic part of the intermolecular potential have been considered. The first one is an ideal dipole (ID) model where electrostatic interactions are modeled as point dipoles placed on the molecular center of mass, and the second one is a discrete charge (DC) model with single positive and negative charges placed at opposite ends of the molecules. The magnitude of the charges and the distance between them were chosen to reproduce the dipole moment of the ID model. In addition, an effective ionic strength for the DC model has been defined. Simulations were performed at several densities and temperatures in a wide range of molecular lengths and at three dipole moments. For all the systems, vapor–liquid equilibrium, thermodynamic, and structural properties, autocorrelation functions, correlation times, and transport properties such as diffusion, shear viscosity, and thermal conductivity have been obtained and analyzed. The results of the present study are in agreement with those found in previous works and they confirm that, although differences between the DC model and the ID model are small for the lower molecular lengths, they become more pronounced at higher molecular lengths. Finally, the influence of the effective ionic strength on the different properties of the system is discussed.
Heavy (or large) ions in a fluid in an electric field: The fundamental solution of the Fokker–Planck equation and related questions
Authors: Ferrari Leonardo
Published: June 2003

The fundamental solution of the Fokker–Planck equation for heavy (or large) ions in a fluid in a generally time-varying electric field is presented. The solution procedure makes use of a convenient transformation of the variables (position r and velocity v) which is suggested by the physics of the problem, and which reduces the solution of the complete Fokker–Planck equation to the well-known solution of this equation in the field-free case. The result obtained shows that, at any time t, the “local” ion velocity distribution f(r,v,t) (apart from the normalization factor taking into account the total number of the ions) differs from the product of the ion number density n(r,t) by the “global” ion velocity distribution Φ(v,t), for a corrective factor C(r,v,t) which is generally different from unity also in the absence of the electric field. The fundamental solution of the Fokker–Planck equation is then widely discussed, in the long-time limit, in the cases of a static—and of an alternate—electric field acting on the fluid. From this discussion, the physical meaning and the importance of C(r,v,t) emerge in a particularly evident manner.
Morphology of asymmetric diblock copolymer thin films
Authors: Podariu Iulia , Chakrabarti Amitabha
Published: June 2003

We numerically study morphology of thin diblock copolymer films as a function of the relative size or the “composition” f of the shorter block of the copolymer chains. We probe various values of f, spanning the whole bulk phase diagram of asymmetric diblock copolymers. Finite size of the film and wetting properties of the substrate modify the film morphology by shifting the effective value of the composition f in the film. A dramatic change in film morphology takes place for a small change in external parameters if one chooses the composition f to be at the borderline between two different bulk phases.
Continuous transition between Brillouin–Wigner and Rayleigh–Schrödinger perturbation theory, generalized Bloch equation, and Hilbert space multireference coupled cluster
Authors: Pittner Jiřı́
Published: June 2003

A continuous transition between the Rayleigh–Schrödinger and Brillouin–Wigner perturbation theories is constructed and the Bloch equation for the corresponding wave operator is derived. Subsequently it is applied to the Hilbert space multireference coupled cluster theory and used to investigate relationships between several versions of multireference coupled cluster methods. Finally, based on those continuous transitions, new size extensivity corrections for the Brillouin–Wigner coupled cluster method are suggested. Numerical tests of size-extensivity and separability of a supermolecule to closed- and open-shell fragments are also presented. Equivalence of some of the multireference coupled cluster methods with single and double excitations to full configuration interaction for two-electron systems is investigated, both theoretically and numerically.
Voltammogram spikes interpreted as envelopes of spikes resulting from electrode crystals of various sizes: Application to the UPD of Cu on Au(111)
Authors: Medved’ Igor , Huckaby Dale A.
Published: June 2003

We study and explain shapes of voltammogram spikes, observed during underpotential deposition (UPD) on electrode surfaces, as averaged envelopes of mutually shifted spikes associated with first-order phase transitions that occur in crystalline domains of various sizes that are formed on the electrode surface. This concept, already used in our previous work for two-phase systems and symmetric voltammogram spike shapes, is here substantially generalized to systems with multiple-phase coexistence and asymmetric spike shapes, using the rigorous statistical mechanical techniques of Borgs and Kotecký. Rather than mere numerical plots, we extract explicit functions that accurately describe the spike shapes. For the sake of clarity, we present our analysis and apply our results to fit the voltammogram of the UPD of Cu on Au(111) in sulfuric acid medium. This voltammogram shows two distinct spikes with a broad foot region near the spike at higher potentials. As was done in earlier treatments, we explain each of the two spikes as a result of a first-order transition. Here, though, the spikes are obtained as envelopes of closely spaced spikes resulting from crystals of various sizes. In contrast to earlier studies, however, we also explain the foot region in the same way. The foot’s shape, despite its large width and small height, can be equally well obtained as an envelope of shifted crystal spikes that are broader and smaller than those giving rise to the two distinct spikes. We achieve very good agreement with experiment.
Stretching a double-stranded DNA: Nature of the B-form to the S-form transition
Authors: Lai Pik-Yin , Zhou Zi-cong
Published: June 2003

The abrupt extension of the contour length and the self-unwinding of the double helix in the transition from the B-form to S-form of a double-stranded DNA under a stretching force is investigated in the framework of the model with basepair interactions and bending [Phys. Rev. Lett. 22, 4560 (1999)]. In the region where thermal fluctuations can be neglected the classical mechanical approach is employed and equations governing the detail structure of the DNA are derived with some analytical results obtained. The transition from the B-form to S-form can be understood in terms of an effective potential with a barrier separating these two states and resulting in a first-order transition. The double helix of the DNA is almost fully unwound across the transition. Detail structural configurations, such as the loci of the two strands, relative extension, linear extension coefficient, and the threshold stretching force are calculated. The mean torque release as the dsDNA untwist across the transition is also estimated. These results are in agreement with various experimental data.
The stability of the hydroxylated (0001) surface of α-Al2O3
Authors: Łodziana Z. , Nørskov J. K. , Stoltze P.
Published: June 2003

Self-consistent density functional calculations of the hydroxylated (0001) corundum surfaces are presented. It is demonstrated that the hydroxylated surfaces are the most stable under most, but not all, conditions. Hydroxylation significantly lowers the surface free energy of α-alumina. The stability of the hydrated surface resolves the discrepancies between the morphology of the α-alumina (0001) surface observed under ultra-high vacuum, and at ambient conditions. A method for the calculation of the equilibrium surface stoichiometry is proposed. The proposed approach provides a valuable connection between theoretical calculations and experiments with metal oxides.
Electrostatic interactions in dissipative particle dynamics—simulation of polyelectrolytes and anionic surfactants
Authors: Groot R. D.
Published: June 2003

Electrostatic interactions have been incorporated in dissipative particle dynamics (DPD) simulation. The electrostatic field is solved locally on a grid. Within this formalism, local inhomogeneities in the electrostatic permittivity can be treated without any problem. Key issues like the screening of the potential near a charged surface and the Stillinger–Lovett moment conditions are satisfied. This implies that the method captures the essential features of electrostatic interaction. For the direct simulation of mixed surfactants near oil–water interfaces, or for the simulation of Coulombic polymer–surfactant interactions, this method has all the advantages of DPD over full atomistic molecular dynamics (MD). DPD has proven to be faster than MD by many orders of magnitude, depending on the precise scaling factor chosen for the simulation. This brings phenomena of microseconds in reach of routine simulation, while maintaining a fairly accurate representation of the structure of the molecules. As an example of this simulation tool, the interaction between a cationic polyelectrolyte and anionic surfactant is discussed. Without a surfactant, the polyelectrolyte shows a fractal dimensionality that is in line with the theoretical and experimental values cited in literature, it behaves as a fairly stiff rod, df∼1.1. When salt is replaced by anionic surfactant, the polymer wraps around one or more discrete surfactant micelles, in line with the current understanding of these systems, and scaling invariance in the correlation function is broken.
Free ion formation in K(np)/SF6 collisions at low-to-intermediate n: Velocity dependence of the number and lifetime of the product ions
Authors: Suess L. , Parthasarathy R. , Dunning F. B.
Published: June 2003

The rates for free ion production through electron transfer in K(np)/SF6 collisions are measured as a function of Rydberg atom velocity for low-to-intermediate values of n, together with the lifetime of the product SF6− ions against autodetachment. The data show that postattachment interactions between the K+–SF6− product ion pair become increasingly important as n, and the Rydberg atom velocity, are decreased. These result in the conversion of internal energy from the SF6− ion into translational energy of the ion pair and in a dramatic increase in SF6− ion lifetimes. Analysis of the data using a Monte Carlo collision model suggests that the energy transfer is sizable, and is more than sufficient to stabilize the product SF6− ions against autodetachment.
Dual-level direct dynamics studies for the reactions of CH3OCH3 and CF3OCH3 with the OH radical
Authors: Wu Jia-yan , Liu Jing-yao , Li Ze-sheng , Sun Chia-chung
Published: June 2003

The reactions of CH3OCH3+OH (R1) and CF3OCH3+OH (R2) via two hydrogen abstraction channels are investigated theoretically using the dual-level direct dynamics approach. The minimum energy path calculation is carried out at the MP2/6-311G(d,p) level, and energetic information is further refined by the G3 theory. For each reaction hydrogen abstraction is favored for the out-of-plane hydrogen, while the abstraction from the in-plane hydrogen is a minor channel. Hydrogen-bonded complexes are present on the reactants and products sides of the primary channel, indicating that the reactions may proceed via an indirect mechanism. By means of variational transition state theory with interpolated single-point energies method the dynamic results of all channels are obtained, and the small-curvature tunneling is included. The total rate constants calculated from the sum of the individual rate constants are in good agreement with the experimental data and are fitted to be k1=3.33×10−20 T2.91 exp(−409.7/T) and k2=1.23×10−24 T3.93exp(−188.2/T) cm3 molecule−1 s−1 over the temperature range 230–2000 K. The calculation indicates the CH3OCH3+OH reaction may proceed much easier than the CF3OCH3+OH reaction and fluorine substitution decreases the reactivity of the C–H bond.
Methyl and t-butyl group reorientation in planar aromatic solids: Low-frequency nuclear magnetic resonance relaxometry and x-ray diffraction
Authors: Beckmann Peter A. , Buser Carolyn A. , Gullifer Kathleen , Mallory Frank B. , Mallory Clelia W. , Rossi Gene M. , Rheingold Arnold L.
Published: June 2003

We have synthesized 3-t-butylchrysene and measured the Larmor frequency ω/2π (= 8.50, 22.5, and 53.0 MHz) and temperature T (110–310 K) dependence of the proton spin–lattice relaxation rate R in the polycrystalline solid [low-frequency solid state nuclear magnetic resonance (NMR) relaxometry]. We have also determined the molecular and crystal structure in a single crystal of 3-t-butylchrysene using x-ray diffraction, which indicates the presence of a unique t-butyl group environment. The spin-1/2 protons relax as a result of the spin–spin dipolar interactions being modulated by the superimposed reorientation of the t-butyl groups and their constituent methyl groups. The reorientation is successfully modeled by the simplest motion; that of random hopping describable by Poisson statistics. The x-ray data indicate near mirror-plane symmetry that places one methyl group nearly in the aromatic plane and the other two almost equally above and below the plane. The NMR relaxometry data indicate that the nearly in-plane methyl group and the entire t-butyl group reorient with a barrier of 24.2 ± 0.9 kJ mol−1, and the two out-of-plane methyl groups reorient with a barrier of 14.2±0.6 kJ mol−1. Following a brief review of methyl group rotation in simple ethyl-, and isopropyl-substituted one- and two-ring aromatic van der Waals molecular solids, the barriers for the out-of-plane methyl groups and the t-butyl group in 3-t-butylchrysene are compared with those barriers in three related molecular solids whose crystal structure is known: 4-methyl-2,6-di-t-butylphenol, 1,4-di-t-butylbenzene, and polymorph A of 2,6-di-t-butyl- naphthalene. A trend is observed in the reorientational barriers for the t-butyl and the out-of-plane methyl groups across this series of four compounds: as the t-butyl barriers decrease, the out-of-plane methyl barriers increase.
Viscoelasticity of fluids with steeply repulsive potentials
Authors: Rickayzen G. , Powles J. G. , Heyes D. M.
Published: June 2003

We consider the infinite frequency moduli and time correlation functions of fluids composed of particles that interact through a steeply repulsive potential of the general analytic form, φ(r)=kBT exp[−αf(r)], where α is a measure of the steepness or stiffness of the potential. Although these potentials have different analytical forms, in the steeply repulsive limit of α→∞, the derived properties become almost identical and are only dependent on the value of α and other basic variables. All the infinite frequency moduli which we study are proportional to α and the interaction part of the pressure is only weakly dependent on α. For the force and other configurational property time functions C(t), time t can be replaced by αt, i.e., C(t)=1−T*(αt*)2+O[(αt*)4], where T*=kBT/ε, is the reduced temperature, kB is Boltzmann’s constant, where ε is a characteristic energy for the potential, and t* is a reduced time. We proved this in earlier publications for an inverse power, r−n potential (where α≡n), and show here this more general relationship. The effective hard-sphere diameter by the Barker–Henderson equation, and an alternative prescription derived here, give to first order in α−1 the same formula for the effective hard-sphere diameter for these potentials. We have carried out molecular-dynamics simulations that confirm the equivalence in the steeply repulsive limit of both the static and dynamical properties of two such potentials, which have an inverse power r−n and exponential potential exp(−κr) form. We consider that the theory for the infinite frequency shear rigidity modulus presented here could be usefully applied to predict the infinite frequency storage modulus of colloidal liquids.
Behavior of single nanoparticle/homopolymer chain in ordered structures of diblock copolymers
Authors: Wang Qiang , Nealey Paul F. , de Pablo Juan J.
Published: June 2003

We have performed Monte Carlo simulations to study the behavior of a single nanoparticle and a single homopolymer chain in ordered structures of diblock copolymers. Our results show that the microphase-separated copolymers can be used to position nanoparticles or homopolymers according to their interactions with the two blocks. This is consistent with recent experimental observations. We also observe that, while short neutral homopolymers segregate at the A–B interfaces, longer neutral homopolymers exhibit an apparent preference for the cylinders over the matrix, mainly due to geometrical considerations associated with the curvature of A–B interfaces.
Theoretical study about adsorption of atomic oxygen on unmodified and I-modified Ag(100) surface
Authors: Wang Yun , Jia Lingling , Dai Weilin , Wang Wenning , Fan Kangnian
Published: June 2003

The adsorption of atomic oxygen on the unmodified and I-modified Ag(100) surface has been studied with the density functional theory method. The various mechanisms of iodine atom modification to affect the adsorption of atomic oxygen have been investigated. The most stable adsorption site for both oxygen and iodine atom on the silver surface is the fourfold hollow site. The modification of iodine atom changes the electronic properties of the substrate and strengthens the bond between the oxygen atom and the surface. The distance from the subsurface oxygen to the surface on the I-modified silver surface is smaller than that on the clean surface. These all benefit the enhancement of the selectivity and the reactivity of the partial oxidation of methanol. The formation of the subsurface oxygen at both the clean and I-modified surface is also investigated. The introduction of the iodine atoms has little effect on the penetrative process. The energy barrier for the penetration of the surface oxygen atoms is about 2.1 eV, which is less than the adsorption energy.
State resolved inelastic scattering of N2 from Ru(0001)
Authors: Mortensen H. , Jensen E. , Diekhöner L. , Baurichter A. , Luntz A. C. , Petrunin V. V.
Published: June 2003

Detailed measurements of state resolved inelastic scattering of N2 from Ru(0001) are reported for a wide range of initial energies (0–3 eV) and angles of incidence. The ion time-of-flight resonantly enhanced multiphoton ionization (REMPI) detection scheme developed here and used with cw molecular beams simultaneously measures the internal quantum state and translational energy normal to the sample surface. Doppler broadening of the REMPI spectrum of scattered particles yields the dispersion in scattering out of plane. The results are qualitatively similar to inelastic N2 scattering studies for a wide variety of other metal surfaces; i.e., no observable vibrational excitation, weak rotational excitation described as a Boltzmann distribution, strong surface excitation depending upon the incident normal energy, and an anticorrelation between rotational and surface excitation. The absence of any vibrational excitation at E≈3 eV is inconsistent with adiabatic model dynamics based on the ab initio potential-energy surface. It is, however, consistent with a strong nonadiabatic damping of vibration to electron-hole pairs in the region of the barrier. This same suggestion was previously found necessary to rationalize unusual dissociative adsorption and associative desorption of N2 on Ru(0001).
Brownian dynamics simulations of polyelectrolyte solutions with divalent counterions
Authors: Chang Rakwoo , Yethiraj Arun
Published: June 2003

Brownian dynamics simulations are performed for salt-free polyelectrolyte solutions with divalent counterions. The polymer molecules are modeled as freely jointed charged chains and the counterions are incorporated explicitly. The conformational properties, static structure, and dynamic properties of salt-free polyelectrolyte solutions show interesting behavior that can be attributed to the correlations induced by the counterions. The size of polyelectrolyte chains and the counterion self-diffusion coefficient show a nonmonotonic concentration dependence. There is a sharp peak in the polyion pair correlation functions at short distances and an upturn in the partial static structure factors at low wave vectors. In semidilute solutions, the polyions contract in the presence of divalent counterions, when compared to solutions with monovalent counterions. This contraction is accompanied by the peak in the static structure moving to lower wave vectors. The self-diffusion of polyions is faster with divalent counterions than with monovalent counterions, whereas the collective diffusion at long length scale slows down with divalent counterions compared to the monovalent case. These results are consistent with experiments and suggest that primitive models with strong electrostatic correlations can explain many of the experimental observations.
Isotropic and anisotropic collision-induced light scattering by gaseous sulfur hexafluoride at the frequency region of the ν1 vibrational Raman line
Authors: Le Duff Y. , Godet J.-L. , Bancewicz T. , Nowicka K.
Published: June 2003

Experimental binary isotropic and anisotropic Stokes spectra of the collision-induced light scattered by gaseous sulfur hexafluoride are measured at the frequency region of the ν1 vibrational Raman line. They are compared to theoretical intensities due to dipole–multipole interactions. Taking into account the results of a previous study on the interaction-induced intensities in the Rayleigh wings of gaseous sulfur hexafluoride, an experimental value of the derivative of the dipole–octopole polarizability associated with the ν1 vibrational mode is provided and compared to the result of a recent ab initio calculation.
Spectroscopy and dynamics of methylamine. II. Rotational and vibrational structures of CH3NH2 and CH3ND2 in cationic D states
Authors: Baek Sun Jong , Choi Kyo-Won , Choi Young S. , Kim Sang Kyu
Published: June 2003

Accurate and precise ionization energies of methylamines (CH3NH2 and CH3ND2) are determined to be 9.0422±0.0012 and 9.0532±0.0012 eV, respectively, by (1+1′) two-photon mass-analyzed threshold ionization (MATI) spectroscopy. From selective ionizations from specified intermediate quantum states, fundamental frequencies of amino-wagging and CH3-rocking modes of CH3NH2+ (CH3ND2+) in D0 states are determined to be 738 (573) and 1013 (1024) cm−1, respectively. The frequency of the amino wag is largely blueshifted from that of the neutral S1 state, while the CH3-rocking frequency is little shifted from that of S1. Internal rotational constants associated with the nearly free internal rotation of the top (amino group) with respect to the frame (methyl group) about the C–N axis are accurately determined, from which the geometries of methylamine ions are revealed. Barrier heights for torsional motion in CH3ND2+ are determined to be 25±5 and 34±5 cm−1 at the origin and first ND2-wagging bands, respectively. Mode-resolved spectroscopy using the MATI signal for resolving overlapped spectral features in the intermediate state is employed for clarifying the vibrational assignment of the intermediate state. An ab initio calculation at the QCISD level is carried out, giving good agreement with the experiment.
Near-infrared spectroscopy of H3+ above the barrier to linearity
Authors: Gottfried Jennifer L. , McCall Benjamin J. , Oka Takeshi
Published: June 2003

The first H3+ transitions above the barrier to linearity have been observed in absorption in the near infrared using a highly sensitive dual-beam, double-modulation technique with bidirectional optical multipassing. A total of 22 rovibrational transitions of H3+ have been detected and assigned to the fourth and fifth overtone and combination bands (5ν21, 5ν25, 2ν1+2ν22, 3ν1+ν21, ν1+4ν22, 2ν1+3ν21, and 6ν22). These transitions, which are more than 4600 times weaker than the fundamental band, probe energy levels above 10 000 cm−1, the regime in which H3+ has enough energy to sample linear configurations. Experimentally determined energy levels above the barrier to linearity provide a critical test of ab initio calculations in this challenging regime. The comparison between experimental energy levels and theoretical energy levels from ab initio calculations in which the adiabatic and relativistic corrections are incorporated reveals the extent of higher-order effects such as nonadiabatic and radiative corrections. We compare our results with several recent theoretical calculations.
Flows past a tiny circular cylinder at high temperature ratios and slight compressible effects on the vortex shedding
Authors: Sabanca M. , Durst F.
Published: June 2003

The present study contributes to the numerical simulations of weak compressible effects for laminar flows around heated circular cylinders. Our aim in this work is to understand the physics of the flow around a heated cylinder and the reasons for the decrease in the frequency of vortex shedding in the wake behind the cylinder as the ratio of surface temperature to freestream temperature increases for moderate Re numbers. The previous achievements on this subject rely mainly on experimental investigations by using a “representative” or “film temperature” and therefore should be investigated more accurately by the numerical considerations. In that context, numerical experiments are carried out. A technique which was previously tested and validated on a variety of flow problems is utilized in the numerical experiments. The results show that the temperature is not a passive contaminant especially for flows with temperature ratios T*=Tw/T∞ greater than 1.1. The temperature boundary layer surrounding the wall introduces weak compressible effects on the velocity and on the pressure field especially for Re numbers below 100. As a consequence of this result, the local Ma number effects are shown on a Re–Nu curve for the laminar unsteady vortex shedding regime.
The modeling of turbulent reactive flows based on multiple mapping conditioning
Authors: Klimenko A. Y. , Pope S. B.
Published: June 2003

A new modeling approach—multiple mapping conditioning (MMC)—is introduced to treat mixing and reaction in turbulent flows. The model combines the advantages of the probability density function and the conditional moment closure methods and is based on a certain generalization of the mapping closure concept. An equivalent stochastic formulation of the MMC model is given. The validity of the closuring hypothesis of the model is demonstrated by a comparison with direct numerical simulation results for the three-stream mixing problem.
Electronic structure of the π-bonded Al–C2H4 complex: Characterization of the ground and low-lying excited states
Authors: Cristian Ana-Maria C. , Krylov Anna I.
Published: June 2003

The equilibrium properties of the π-bonded Al–ethylene complex in its ground state are calculated by coupled-cluster theory. Significant changes in the geometry of the ethylene molecule upon complexation (elongation of the CC bond, pyramidalization of the CH2 groups) are consistent with the formation of a chemical bond between fragments. The overall interaction is rather weak because bonding is derived from the overlap between: (i) a singly occupied p orbital of Al and the antibonding π* orbital of ethylene and (ii) a vacant Al sp hybrid and π of C2H4. Electronically excited states are studied by the equation-of-motion coupled-cluster method. The covalent nature of the interaction between fragments is reflected in the excited-state delocalization over both fragments (as opposed to the corresponding van der Waals complex). In the examined energy range (0–5.18 eV) both valence and Rydberg excited states are found. Bonding in the valence states is explained in terms of a simple molecular orbital picture. Two very intense transitions at 3.50 eV and at 3.79 eV can be used as a probe in experimental studies.
Density functional response approach for the linear and nonlinear electric properties of molecules
Authors: Sophy K. B. , Pal Sourav
Published: June 2003

This is a preliminary study toward implementation of analytic density functional response approach for molecules to obtain linear and nonlinear electric properties. The Kohn–Sham framework has been used with Gaussian basis sets. We propose a fully variational approach to obtain the response of electronic density in terms of the atomic orbital basis (contracted Gaussians). As a first step, this derivative of the Kohn–Sham operator is obtained by a finite field method using five different values of electric field. Using this, we obtain the energy derivatives up to third order using fully analytic expressions. We calculate the dipole moment, polarizability, and hyperpolarizability of the hydrogen fluoride (HF) molecule as a test case using different exchange–correlation functionals and basis sets within the present methodology. We also explore the feasibility of this response approach by studying the properties of the HF molecule for different H–F distances.
Optical spectroscopy of simple aqueous solutions under extreme conditions
Authors: Vass H. , Edington D. , Crain J.
Published: June 2003

We report the results of an extensive series of Brillouin scattering experiments on simple aqueous solutions with a view to exploring their dynamical properties over a wide range of temperatures and pressures. For all solutes studied that inhibit freezing and allow access to temperatures far below the normal supercooling limit of water, we find clear spectroscopic evidence of GHz-range viscoelastic behavior below ≈−40 °C. This is manifested by a dramatic rise in the Brillouin mode frequencies accompanied by initial broadening and subsequent narrowing of the spectral linewidths on cooling. We find similar behavior in pure compressed (to between 2 and 4 kbar) supercooled water. This suggests that the low-temperature viscoelastic dynamics of these solutions is dominated by the behavior of the aqueous component which evidently exhibits a pronounced decrease in relaxation time though the temperature range over which it occurs is inaccessible unless freezing is suppressed either by pressure or the presence of solutes. No firm conclusion can be drawn concerning the proposed second critical point at these very low temperatures though it is not required for a consistent interpretation of the data. In the high-temperature regime, where the hydrogen bond structure is disrupted, we find considerable chemical sensitivity (even among the three chloride salts NaCl, CsCl, and CaCl2) in the dynamics especially in the vicinity of the liquid–gas critical point. This is in contrast to the low-temperature case where the cooperative dynamics of the water network dominates and appears to remain intact in the presence of a wide variety of solutes.
Structure of linear polymeric chains confined between impenetrable spherical walls
Authors: Picu R. C. , Ozmusul M. S.
Published: June 2003

The bond-scale and chain-scale structure of linear polymers located close to spherical impenetrable surfaces is studied in dense systems by means of lattice Monte Carlo simulations. The role of the various types of interactions (entropic, cohesive in the bulk polymer, attraction to filler surface) and that of the chain length, polymer density, and wall curvature in defining the polymer structure is analyzed. The size effect of the spherical fillers is investigated by scaling the filler radius at constant filler volume fraction. On the bond scale, the chain ends segregate to the wall in all systems, with the effect being essentially independent of wall curvature. The bonds are preferentially oriented in the direction tangential to the wall. The distance from the wall over which these effects are observed is about one bond length in the athermal case and about two chain gyration radii in the energetic case. On the chain scale, the ellipsoidal chains undergo a “docking” transition to the spherical fillers. The ellipsoids do not deform, rather rotate with their large semiaxis in the direction tangential to the filler as their centers of mass approach the wall. This configurational entropy-controlled situation remains valid when cohesive interactions are considered in the bulk polymer and even with a hydrogen bond strong attraction of the polymer to the wall. When the wall-to-wall distance between fillers decreases below two bulk gyration radii, the chain size decreases in the direction of its large semiaxis, an effect essentially independent of the details of the energetic interactions in the system.
On the mechanism of optimal disturbances: The role of a pair of nearly parallel modes
Authors: Ben-Dov Guy , Levinski Vladimir , Cohen Jacob
Published: June 2003

In the present work the role of a pair of nearly parallel least stable modes (having opposite phases and almost identical amplitude distributions), as a key element of optimal transient growth in shear flows, is explored. The general character of this mechanism is demonstrated by four examples. The first two examples are the temporal and spatial growth of optimal disturbances in circular pipe flow. The time and distance, at which the maximum energy amplification of an initial disturbance is achieved, are well predicted analytically by considering only the pair of least stable modes. Furthermore, the dependence of the maximum energy amplification on the Reynolds number matches previous numerical results based on the analysis of many modes. In the temporal case the predicted amplification factor agrees well with these numerical results. The other two examples are concerned with a two-dimensional potential shear layer over a compliant surface, and with a two-dimensional wall-jet. In these examples, a similar growth mechanism takes place near a point where two kinds of two-dimensional modes bifurcate. In the potential shear layer, the maximum optimal growth achieved by two nearly parallel modes is solved analytically for temporal disturbances, whereas in the wall-jet case the distance, at which the maximum amplification is achieved, is well predicted for spatial disturbances. Finally, it appears that the transient growth mechanism based on the interference between two nearly parallel modes is a general case which includes the direct resonance mechanism as a limit for short times.
Galilean invariance of velocity probability density function transport equation
Authors: Tong Chenning
Published: June 2003

Model equations for turbulent flows generally must have the same Galilean transformation properties as the exact transport equations. We examine the transport equation of one-point velocity joint probability density function (JPDF) under Galilean transformations. The JPDF and its transport equation as well as the unclosed terms in the equation are shown to be Galilean invariant. For constant density flows they are also invariant under extended Galilean transformations. It is also shown that conditional statistics of any Galilean invariant variables conditional on velocity are invariant. The present work provides a basis and justification for using such conditional statistics in studying turbulent flows.
Variable scale filtered Navier–Stokes equations: A new procedure to deal with the associated commutation error
Authors: Iovieno M. , Tordella D.
Published: June 2003

A simple procedure to approximate the noncommutation terms that arise whenever it is necessary to use a variable scale filtering of the motion equations and to compensate directly the flow solutions from the commutation error is here presented. Such a situation usually concerns large eddy simulation of nonhomogeneous turbulent flows. The noncommutation of the average and differentiation operations leads to nonhomogeneous terms in the motion equations, that act as source terms of intensity which depend on the gradient of the filter scale δ and which, if neglected, induce a systematic error throughout the solution. Here the different noncommutation terms of the motion equation are determined as functions of the δ gradient and of the δ derivatives of the filtered variables. It is shown here that approximated noncommutation terms of the fourth order of accuracy, with respect to the filtering scale, can be obtained using series expansions in the filter width of approximations based on finite differences and introducing successive levels of filtering, which makes it suitable to use in conjunction with dynamic or mixed subgrid models. The procedure operates in a way which is independent of the type of filter in use and without increasing the differential order of the equations, which, on the contrary, would require additional boundary conditions. It is not necessary to introduce a mapping function of the nonuniform grid in the physical domain into a uniform grid in an infinite domain. A priori tests on the turbulent channel flow (Reτ 180 and 590) highlight the approximation capability of the present procedure. A numerical example is given, which draws attention to the nonlocal effects on the solution due to the lack of noncommutation terms in the motion equation and to the efficiency of the present procedure in reducing the commutation error on the solution.
On the Batchelor constant in decaying isotropic turbulence
Authors: Antonia R. A. , Orlandi P.
Published: June 2003

Direct numerical simulations of a passive scalar mixed in decaying homogeneous isotropic turbulence indicate that, when the Schmidt number is close to or exceeds unity, the Batchelor constant is about 3. This is smaller by nearly a factor of 2 than that inferred from stationary turbulence simulations. In the present case, Batchelor’s spectrum approximates the present data well in the viscous-convective range while Kraichnan’s spectrum provides a very good fit at large wavenumbers.
Spectra of turbulence in dilute polymer solutions
Authors: Fouxon A. , Lebedev V.
Published: June 2003

Turbulence in dilute polymer solutions when polymers are strongly stretched by the flow is investigated. We establish power-law spectra of velocity, that are not associated with a flux of a conserved quantity, in two cases. First, such spectrum is formed in the elastic waves range of high Reynolds number turbulence of polymer solutions above the coil–stretch transition. Second, such spectrum is characteristic of the elastic turbulence, where chaotic flow is excited due to elastic instabilities at small Reynolds numbers.
Super-Burnett solutions for Poiseuille flow
Authors: Xu Kun
Published: June 2003

In the slip flow regime with Kn=0.1 for the force and pressure driven Poiseuille flow, Zheng et al. [Rarefied Gas Dynamics, Vol. 23 (Whistler, Canada, 2002)] found out that the Navier–Stokes equations with slip boundary condition could give qualitative different results in the cross-stream direction from the reliable direct simulation Monte Carlo (DSMC) solution. In this Brief Communication, we are going to show that the discrepancy between the Navier–Stokes and the DSMC results can be resolved based on the simulation results of higher-order equations, such as Burnett and super-Burnett ones.
Magnetic-field effects on second-mode instability of a weakly ionized Mach 4.5 boundary layer
Authors: Cheng Felix , Zhong Xiaolin , Gogineni Sivaram , Kimmel Roger L.
Published: June 2003

This paper investigates, by numerical simulation, the effects of an imposed magnetic field on a weakly ionized Mach 4.5 boundary layer. The main emphasis of the study is on magnetohydrodynamic (MHD) effects on the second mode instability in supersonic boundary layer. The imposed magnetic fields are generated by placing two-dimensional magnetic dipoles below the flat plate surface. The gas is assumed to have a constant electrical conductivity of 100 mho/m. The magnetic Reynolds number of the flow is small so that the induced magnetic field in the flow is neglected. The governing equations of the MHD flow, which are the Navier–Stokes equations with the applied magnetic force terms, are computed by a fifth-order shock-fitting numerical scheme. A series of cases with different imposed magnetic fields have been investigated on the influences of imposed magnetic field on both the mean flow and on the second mode stability. It is found that the imposed magnetic fields significantly retard the streamwise velocity and reduce the local skin friction in the mean flow. For the case of a strong imposed magnetic field, a local separation region is generated in the mean flow with a strong adverse pressure gradient. Meanwhile, the second mode wave disturbances are found to be stabilized by the imposed magnetic fields, even for the case with strong adverse pressure gradient and a local separated flow region. This strong overall stabilization of the second mode wave is believed to be caused by the alteration of the steady base flow by the magnetic field. The results presented in this paper are the first concrete results on the interaction of second instability mode with magnetic field in a supersonic boundary layer.
Breakdown of a magnetohydrodynamic flow structure
Authors: Sreenivasan Binod
Published: June 2003

The breakdown of a swirling blob of fluid subject to a magnetic field parallel to its axis is investigated experimentally in the present work. A flow structure in an electrically conducting fluid always tends to elongate in the direction of a strong, external magnetic field. However, if the magnetic field is sufficiently weak, the poloidal recirculation that is created at the expense of the swirl flow is not damped fast enough. While a vortex may diffuse weakly along the magnetic-field lines, it begins to propagate radially outward in the form of a ring. The transition from a purely diffusive regime to a centrifugally unstable regime has been observed in the present study.
Spreading of droplets under the influence of intermolecular forces
Authors: Glasner K. B.
Published: June 2003

The motion of fluid droplets under the influence of short and long range intermolecular forces is examined using a lubrication model. Surface energies as well as the microscopic contact line structure are identified in the model. A physically constructed precursor film prevents the usual stress singularity associated with a moving contact line. In the quasistatic limit, an analysis of the energy and its dissipation yield an ordinary differential equation for the rate of spreading. Two dimensional and axisymmetric solutions are found and compared to numerical simulations. The motion of the contact line is found to be both a function of the local contact angle and the overall droplet geometry.
Dynamics of turbulence strongly influenced by buoyancy
Authors: Riley James J. , deBruynKops Stephen M.
Published: June 2003

The dynamics of quasi-horizontal motions in a stably stratified fluid have been simulated for Froude numbers of order 1, so that the flows are strongly affected by the stable density stratification, and for a range of Reynolds numbers. It is found that the horizontal scales of the motion grow continuously in time. The vertical scales decrease and the vertical shearing increases with time, maintaining the Richardson number of order 1, as suggested by Lilly [J. Atmos. Sci. 40, 749 (1983)] and Babin et al. [Theor. Comput. Fluid Dyn. 9, 223 (1997)]. Small-scale instabilities and turbulent-like motions are observed to occur in the high shearing regions, while the larger-scale motions appear to evolve somewhat independently of the Reynolds number. The results suggest that the larger-scale, quasi-horizontal motions would be a continuous source of smaller-scale turbulence until the local Reynolds number drops below a critical value, which is estimated. Finally, a Froude number based upon a vertical differential scale and used in previous scaling arguments and theories is estimated in terms of other parameters.
High-coercivity ultralight transparent magnets
Authors: Gich M. , Casas Ll. , Roig A. , Molins E. , Sort J. , Suriñach S. , Baró M. D. , Muñoz J. S. , Morellon L. , Ibarra M. R. , Nogués J.
Published: June 2003

Magnetic silica-aerogel composites have been synthesized by dispersing hard magnetic Nd2Fe14B particles in a sol during a fast sol-gel process and subsequently supercritically drying the resulting gels. The composites are found to retain most of the outstanding properties of their constituents: the large coercivity and moderate remanence of the magnetic powders and the transparency and low density of silica aerogels. Moreover, aerogels synthesized in the presence of a magnetic field exhibit the alignment of the particles, forming needle-like structures along the direction of the applied magnetic field, which results in optical and magnetic anisotropies. Due to their unique combination of properties, these types of materials may be appealing for magneto-optics and magnetic actuator applications.
Transient displacement of a Newtonian fluid by air in straight or suddenly constricted tubes
Authors: Dimakopoulos Yannis , Tsamopoulos John
Published: June 2003

We study the displacement of a viscous fluid by highly pressurized air in a straight or a suddenly constricted cylindrical tube of finite length. In contrast to previous efforts, the transient situation is examined. A long, narrower than the tube and round-ended bubble is created during the process. This is sometimes called “fingering instability” and is often encountered in several applications, but we will focus on process parameters that are relevant to the gas-assisted injection molding. For our numerical simulations we have combined the mixed finite element method with an appropriate system of elliptic partial differential equations and boundary conditions, capable of generating a boundary-fitted finite element mesh. The bubble front and the thickness of the deposited film on the tube wall are affected by the properties of the fluid being displaced and the flow conditions. Specifically, in straight tubes, the bubble keeps accelerating due to the decreasing fluid mass ahead of it. Increasing the Reynolds number decreases the film thickness and makes the bubble front steeper. When inertia becomes significant a tip-splitting instability arises. For sufficiently low Reynolds numbers, but still large applied pressures, a steady bubble shape is attained even in a straight tube and the fraction of the liquid deposited on the wall of the tube reaches the asymptotic value of 0.60, as observed by Taylor [J. Fluid Mech. 10, 161 (1961)] and Cox [J. Fluid Mech. 14, 81 (1962)]. In a constricted tube, the bubble temporarily attains a nearly constant velocity. When its front approaches the tube constriction it becomes pointed, due to the extensional flow that prevails there, but it reassumes its finger-like profile, after it goes through the constriction.
Sn-enhanced epitaxial thickness during low-temperature Ge(001) molecular-beam epitaxy
Authors: Bratland K. A. , Foo Y. L. , Desjardins P. , Greene J. E.
Published: June 2003

The incorporation of dilute Sn concentrations CSn during Ge(001) low-temperature molecular-beam epitaxy significantly increases the critical thickness h1(Ts) for the onset of epitaxial breakdown. With CSn=6×1019 cm−3, h1 increases by an order of magnitude at Ts=95 °C, while gains in h1(Ts) by factors ranging from 3.2 at 95 °C to 2.0 at 135 °C are obtained with CSn=1×1018 cm−3 (20 parts per million!). Nevertheless, the ratio of the surface width at breakdown to the in-plane correlation length remains constant, independent of Ts and CSn, showing that epitaxial breakdown for both Ge(001) and Sn-doped Ge(001) is directly related to surface roughening. We attribute the dramatic Sn-induced increases in h1(Ts) to enhancements in both the Ge surface diffusivity and the probability of interlayer mass transport. This, in turn, results in more efficient filling of interisland trenches, and thus delays epitaxial breakdown during low-temperature growth.