bluuebunny/crossref_metadata_2025 huggingface dataset:
Degenerate two-photon-absorption spectral studies of highly two-photon active organic chromophores
Authors: He Guang S. , Lin Tzu-Chau , Dai Jianming , Prasad Paras N. , Kannan Ramamurthi , Dombroskie Ann G. , Vaia Richard A. , Tan Loon-Seng
Published: March 2004

Degenerate two-photon absorption (TPA) spectral properties of five AFX chromophore solutions have been studied using a single and spectrally dispersed sub-picosecond white-light continuum beam. In a specially designed optical configuration, optical pathways inside the sample solution for different spectral components of the focused continuum beam were spatially separated from each other. Thus, the nondegenerate TPA processes coming from different spectral components can be eliminated, and the direct nonlinear absorption spectrum attributed to degenerate TPA processes can be readily obtained. Using this new technique, the complete TPA spectra for these five highly two-photon-active compounds (AF-380, AF-350, AF-295, AF-270, and AF-50) were obtained in the spectral range from 600 to 950 nm on an absolute scale of TPA cross section. The relationship between the molecular structures and their TPA spectral behaviors are discussed. In general the measured TPA spectra are not identical with the linear absorption spectra on the scale of absorbed photon(s) energy. Moreover, for some sample (such as AF-380), the TPA spectrum is totally different from the linear spectrum, which implies the difference of molecular transition pathways and selection rules for one- and two-photon excitation processes. At high excitation intensity levels (⩾15 GW/cm2), the saturation behavior of TPA transition can be observed obviously in AF-350 and AF-380 solutions that exhibit much higher nonlinear absorptivity than the other chromophores investigated.
Free energy, energy, and entropy of swelling in Cs–, Na–, and Sr–montmorillonite clays
Authors: Whitley Heather D. , Smith David E.
Published: March 2004

A Monte Carlo method for grand canonical and grand isoshear ensemble simulations has been used to characterize the free energy, energy, and entropy of clay mineral swelling. The Monte Carlo approach was found to be more efficient at simulating water content fluctuations in the highly constrained clay environment than a previously developed molecular dynamics method. Swelling thermodynamics calculated for Cs–, Na–, and Sr–montmorillonite clays indicate a strong dependence of swelling on the interlayer ion identity, in agreement with various experimental measurements. The Sr clay swells most readily, and both the Na and Sr clays prefer expanded states (two-layer hydrate or greater) when in contact with bulk water. In contrast, swelling is inhibited in the Cs clay. Differences in swelling behavior are traced directly to the tendency of the different ions to hydrate. The swelling free energies are decomposed into their energetic and entropic components, revealing an overall energetic driving force for the swelling phenomena. Entropic effects provide a smaller, mediating role in the swelling processes. The results provide a unique molecular perspective on experimentally well-characterized swelling phenomena.
Influences of hydration force and elastic strain energy on the stability of solid film in a very thin solid-on-liquid structure
Authors: Yang C. Y. , Zhao Ya-Pu
Published: March 2004

Since hydration forces become very strong at short range and are particularly important for determining the magnitude of the adhesion between two surfaces or interaction energy, the influences of the hydration force and elastic strain energy due to hydration-induced layering of liquid molecules close to a solid film surface on the stability of a solid film in a solid-on-liquid (SOL) nanostructure are studied in this paper. The liquid of this thin SOL structure is a kind of water solution. Since the surface forces play an important role in the structure, the total free energy change of SOL structures consists of the changes in the bulk elastic energy within the solid film, the surface energy at the solid–liquid interface and the solid–air interface, and highly nonlinear volumetric component associated with interfacial forces. The critical wavelength of one-dimensional undulation, the critical thickness of the solid film, and the critical thickness of the liquid layer are studied, and the stability regions of the solid film have been determined. Emphasis is placed on calculation of critical values, which are the basis of analyzing the stability of the very thin solid film.
Johari–Goldstein relaxation and crystallization of sorbitol to ordered and disordered phases
Authors: Power G. , Vij J. K.
Published: March 2004

The equilibrium permittivity εs and the dielectric relaxation spectra of supercooled liquid D-sorbitol were measured during its crystallization to orientationally disordered or ordered phases depending on the sample preparation procedure at several fixed temperatures up to a period of 6 days. The εs measurements showed that when the sample was contaminated by a minute amount of crystals, it crystallized to an ordered phase. When the liquid was not contaminated, the sample crystallized to an orientationally disordered phase. When supercooled D-sorbitol was kept close to its Tg, its dielectric spectra did not change over a period of 138.5 h. It was found that the Johari–Goldstein (JG) relaxation rate of the orientationally disordered crystalline phase is higher in comparison with that of the supercooled liquid, the spectrum broader, and the relaxation strength lower. Its glasslike transition temperature is higher than Tg of the liquid. The results on crystallization showed that the structural changes occurring at a temperature where the α relaxation emerges from the JG relaxation affects the crystallization kinetics of the liquid.
Vibrational behavior of adsorbed CO2 on single-walled carbon nanotubes
Authors: Yim Wai-Leung , Byl Oleg , Yates John T. , Johnson J. Karl
Published: March 2004

We present theoretical and experimental evidence for CO2 adsorption on different sites of single walled carbon nanotube (SWNT) bundles. We use local density approximation density functional theory (LDA-DFT) calculations to compute the adsorption energies and vibrational frequencies for CO2 adsorbed on SWNT bundles. The LDA-DFT calculations give a range of shifts for the asymmetric stretching mode from about −6 to −20 cm−1 for internally bound CO2, and a range from −4 to −16 cm−1 for externally bound CO2 at low densities. The magnitude of the shift is larger for CO2 adsorbed parallel to the SWNT surface; various perpendicular configurations yield much smaller theoretical shifts. The asymmetric stretching mode for CO2 adsorbed in groove sites and interstitial sites exhibits calculated shifts of −22.2 and −23.8 cm−1, respectively. The calculations show that vibrational mode softening is due to three effects: (1) dynamic image charges in the nanotube; (2) the confining effect of the adsorption potential; (3) dynamic dipole coupling with other adsorbate molecules. Infrared measurements indicate that two families of CO2 adsorption sites are present. One family, exhibiting a shift of about −20 cm−1 is assigned to internally bound CO2 molecules in a parallel configuration. This type of CO2 is readily displaced by Xe, a test for densely populated adsorbed species, which are expected to be present on the highest adsorption energy sites in the interior of the nanotubes. The second family exhibits a shift of about −7 cm−1 and the site location and configuration for these species is ambiguous, based on comparison with the theoretical shifts. The population of the internally bound CO2 may be enhanced by established etching procedures that open the entry ports for adsorption, namely, ozone oxidation followed by annealing in vacuum at 873 K. Xenon displacement experiments indicate that internally bound CO2 is preferentially displaced relative to the −7 cm−1 shifted species. The −7 cm−1 shifted species is assigned to CO2 adsorbed on the external surface based on results from etching and Xe displacement experiments.
Nature of long-range correlations of density fluctuations in glass-forming liquids
Authors: Bakai A. S. , Fischer E. W.
Published: March 2004

Based on a concise review of the experimental data, a theory of the structural, kinetic, and dynamic features of the Fischer cluster is put forward. This theory is based on the idea of heterophase fluctuations of glass-forming liquids. According to these ideas in a one-component liquid the solidlike and fluidlike transient associations of molecules (fluctuons) are formed. The fluctuons differ in the short-range order and, consequently, in the free energy. They form a random free energy landscape. The random field free energy functional of the Ginzburg–Landau type, which accounts for the free energy landscape, is deduced. Based on this, the phase equilibrium, critical behavior, and ordering phenomena of the heterophase liquid are considered. It is shown that in the critical region an effective attractive interaction of the fluctuons results in their gravitation and formation of correlated fractal aggregations. The aggregation consists of the fluctuons of similar short-range order. The fractal aggregation formation is a special structural state of the liquid above the glass transition temperature. The condition at which the aggregations are formed is formulated. Thus it is shown that the Fischer cluster is an array of fractal aggregations of fluctuons. The growth kinetics and dynamics of the Fischer cluster are described. The experimental data are analyzed and discussed using the theory that is put forward.
A trimer model for water
Authors: Girardi M. , Figueiredo W.
Published: March 2004

A statistical model for water is studied, where the molecules are represented by trimers in a triangular lattice. Each atom of a water molecule occupies a single site on the lattice, and the HOH bond angle is assumed to be 120°. The molecules can interact via three different potentials: the excluded volume interaction, which prevents two molecules from occupying the same atom site, an attractive potential between any two nearest-neighbor atoms belonging to different molecules (the van der Waals interaction), and the hydrogen bond interaction, which occurs only for a particular orientation and displacement of a pair of molecules. The model is investigated by means of Monte Carlo simulations in the canonical and grand canonical ensembles. The Metropolis and the entropic sampling algorithms are used to obtain the thermodynamics of the system. We find that the entropic sampling prescription is the most efficient algorithm of them, providing information about the entropy and free energy of the system in a straightforward way. The curves for the polarization, number of hydrogen bonds, specific heat, and cumulant of energy were obtained as a function of the temperature and total concentration. In addition, the entropy of the noninteracting version of the model is compared to that of the angular trimers in a square lattice and triangles in a triangular lattice.
Selective photochemistry via adiabatic passage: Degenerate product states with different lifetimes
Authors: Gong Jiangbin , Rice Stuart A.
Published: March 2004

Two-pulse selective photochemistry that exploits population transfer via adiabatic passage is considered for the case that there are degenerate product states with different lifetimes. As an example, a four-level model system with a complex symmetric Hamiltonian is constructed. Analytical and numerical studies of this model system demonstrate that extensive control over the product branching ratio can be achieved by detuning either the pump pulse or the Stokes pulse while maintaining negligible population in the intermediate state. This control approach represents a significant simplification of both the Kobrak–Rice extended stimulated Raman adiabatic passage scheme and the Chen–Shapiro–Brumer strong-field control scheme.
Molecular dynamics simulations of the Ag+ or Na+ cation with an excess electron in bulk water
Authors: Spezia Riccardo , Nicolas Cédric , Archirel Pierre , Boutin Anne
Published: March 2004

The properties of an excess electron interacting with a monovalent cation in bulk water are studied by molecular dynamics simulations. Sodium and silver cations are chosen as prototypical cases because of their very different redox properties. In both cases, mixed quantum classical molecular dynamics simulations reproduce the experimental UV-Vis spectra. In the case of silver, we observe a highly polarized neutral atom, corresponding to a dipolar excitonic state. For sodium a contact cation/electron pair is observed. Free energy curves along the cation electron coordinate are calculated using quantum Umbrella Sampling technique. The relative stability of the different chemical species is discussed.
Plume formation in strongly temperature-dependent viscosity fluids over a very hot surface
Authors: Ke Y. , Solomatov V. S.
Published: March 2004

Plume formation in a strongly temperature-dependent viscosity fluid placed on a very hot surface involves an intermediate step—small-scale convection in the thermal boundary layer. We perform numerical simulations and suggest a simple analysis of this process using the stagnant lid convection theory and Canright and Morris’ theory of Rayleigh–Taylor instability of two layers with different viscosities. We show that plume formation can approximately be predicted from the requirement that the growth of the large-scale instability becomes faster than the growth of the convecting thermal boundary layer.
Nonsingular boundary integral method for deformable drops in viscous flows
Authors: Bazhlekov Ivan B. , Anderson Patrick D. , Meijer Han E. H.
Published: March 2004

A three-dimensional boundary integral method for deformable drops in viscous flows at low Reynolds numbers is presented. The method is based on a new nonsingular contour-integral representation of the single and double layers of the free-space Green’s function. The contour integration overcomes the main difficulty with boundary-integral calculations: the singularities of the kernels. It also improves the accuracy of the calculations as well as the numerical stability. A new element of the presented method is also a higher-order interface approximation, which improves the accuracy of the interface-to-interface distance calculations and in this way makes simulations of polydispersed foam dynamics possible. Moreover, a multiple time-step integration scheme, which improves the numerical stability and thus the performance of the method, is introduced. To demonstrate the advantages of the method presented here, a number of challenging flow problems is considered: drop deformation and breakup at high viscosity ratios for zero and finite surface tension; drop-to-drop interaction in close approach, including film formation and its drainage; and formation of a foam drop and its deformation in simple shear flow, including all structural and dynamic elements of polydispersed foams.
On Marangoni convective patterns driven by an exothermic chemical reaction in two-layer systems
Authors: Bratsun D. A. , De Wit A.
Published: March 2004

This article is devoted to the investigation of Marangoni-driven pattern formation at the interface between two immiscible fluids filling a Hele-Shaw cell, each of them containing a reactant of an exothermic neutralization reaction. In such a system, convective patterns arise when one reactant diffuses through the interface to react with the other chemical species in one of the fluids. A chemo-hydrodynamical pattern appears due to Marangoni instabilities taking place because of heat and solutal driven changes of the surface tension. The mathematical model we develop consists in a set of reaction-diffusion-advection equations ruling the evolution of concentrations and temperature coupled to Navier–Stokes equation, written in a Hele-Shaw approximation. In our analysis, the time-dependent convectionless reaction-diffusion base state is first obtained and studied in detail. Next, we perform a linear stability analysis of this base state with regard to thermal and solutal Marangoni effects to determine the parameter values beyond which convection occurs. Finally, we perform numerical simulations of the fully nonlinear system and study the influence of the different parameters on pattern formation.
Effects of arbitrary directional system rotation on turbulent channel flow
Authors: Wu Haibin , Kasagi Nobuhide
Published: March 2004

A series of direct numerical simulations is carried out to study the effects of system rotation on a fully developed turbulent channel flow. The rotating axis is in an arbitrary direction to the channel walls. Three cases are considered by combining two of the orthogonal rotating vectors, i.e., streamwise, wall-normal, and spanwise rotations. The spanwise rotation effect is still dominant even when another orthogonal rotation is imposed to the system. However, the streamwise rotation induces the recovery of turbulence on the suction side, the enhancement of the secondary flow rotating in the positive streamwise direction, and the redirection of the near-wall vortical structures if this rotation is much stronger than the spanwise rotation. The wall-normal rotation reduces the slope of the linear region of the streamwise mean velocity distribution and increases the streamwise friction coefficient. In the case of combined streamwise and wall-normal rotation, turbulence is also enhanced on one side and depressed on the other side, while rotation-induced large-scale vortices also appear, extending in the absolute mean flow direction, even though no explicit spanwise rotation is imposed to the system.
Differential diffusion in a shocked helium jet
Authors: Cloutman Lawrence D.
Published: March 2004

We present direct numerical simulations of a shock tube experiment in which a cylindrical laminar jet of helium doped with biacetyl is injected into air and subjected to a weak shock wave. Computed species distributions in a planar cross section of the jet are compared to planar laser-induced fluorescence images produced by the experiment. Given the qualitative nature of the published experimental images, the agreement with spatial flow features is quite good. We find that differential diffusion of species is an important feature of this experiment, leading to a significant decoupling of the helium and the biacetyl tracer.
Vortex ring formation at the open end of a shock tube: A particle image velocimetry study
Authors: Arakeri J. H. , Das D. , Krothapalli A. , Lourenco L.
Published: March 2004

The vortex ring generated subsequent to the diffraction of a shock wave from the open end of a shock tube is studied using particle image velocimetry. We examine the early evolution of the compressible vortex ring for three-exit shock Mach numbers, 1.1, 1.2, and 1.3. For the three cases studied, the ring formation is complete at about tUb/D=2, where t is time, Ub is fluid velocity behind shock as it exits the tube and D is tube diameter. Unlike in the case of piston generated incompressible vortex rings where the piston velocity variation with time is usually trapezoidal, in the shock-generated vortex ring case the exit fluid velocity doubles from its initial value Ub before it slowly decays to zero. At the end of the ring formation, its translation speed is observed to be about 0.7 Ub. During initial formation and propagation, a jet-like flow exists behind the vortex ring. The vortex ring detachment from the tailing jet, commonly referred to as pinch-off, is briefly discussed.
Certain aspects of channel entrance flow
Authors: Asai M. , Floryan J. M.
Published: March 2004

Two-dimensional, developing laminar flow in the downstream zone of the channel entry is investigated experimentally for Reynolds number 3000⩽Re⩽6000. Measured evolution of the flow towards the fully developed state agrees with the theoretical predictions [R. M. Sadri and J. M. Floryan, “Entry flow in a channel,” Comput. Fluids 31, 133 (2002)].
Aerosol deposition in periodic shock waves
Authors: Alexeev A. , Gutfinger C.
Published: March 2004

The paper considers deposition of small aerosol particles inside a closed tube filled with a gas at resonance, when the flow is accompanied by periodic shock waves traveling back and forth along the tube. It is found that resonance gas oscillations cause vigorous aerosol deposition leading to a rapid concentration decrease. We study the effect of the Saffman lift and thermophoretic forces on particle trajectory in the boundary layer near the tube wall. The thermophoretic force arises due to gas heating by periodic shock waves, while the lift force is caused by the interaction of the shock with the viscous boundary layer. Both forces move particles toward the wall causing their deposition. The effect of turbulence on aerosol deposition is also considered. It is found that for particles less than 1 μm in diameter, the effect of turbulent fluctuations on their deposition is dominant, while for larger particles, the Saffman lift force prevails. The effect of the thermophoretic force is found to be slight. It is also found that at resonance the Saffman lift force causes instability in motion of large particles resulting in their rapid deposition. Our experiments of aerosol deposition in a resonance tube compare favorably with the results of the numerical study.
Cubatic phase for tetrapods
Authors: Blaak Ronald , Mulder Bela M. , Frenkel Daan
Published: March 2004

We investigate the phase behavior of tetrapods, hard nonconvex bodies formed by four rods connected under tetrahedral angles. We predict that, depending on the relative lengths of the rods these particles can form a uniaxial nematic phase, and more surprisingly they can exhibit a cubatic phase, a special case of the biaxial nematic phase. These predictions may be experimentally testable, as experimental realizations of tetrapods have recently become available.
Ghost effect of infinitesimal curvature in the plane Couette flow of a gas in the continuum limit
Authors: Sone Yoshio , Doi Toshiyuki
Published: March 2004

The time-independent plane Couette flow of a gas in the continuum limit is studied on the basis of kinetic theory as the limit of the cylindrical Couette flow of a rarefied gas between two rotating coaxial circular cylinders when the mean free path and the curvature (or the inverse of the radius) of the inner cylinder tend to zero simultaneously, keeping the difference of the radii of the two cylinders fixed. The fluid-dynamic-type equations and their boundary conditions governing the limiting state are derived for arbitrary circumferential speeds of rotation of the cylinders and for arbitrary temperature difference of the two cylinders. The resulting equations depend on the relative speed of decay of the two parameters and contain a term due to the infinitesimal curvature of the cylinder, as well as non-Navier–Stokes stress terms, when the curvature decays not faster than some function (generally, the square) of the mean free path. The bifurcation analysis of the plane Couette flow of a linear profile is carried out when the speeds of the walls and their temperature difference are small. The bifurcation point and the bifurcated flow fields, where the linear profile is considerably deformed by infinitesimal cross flows induced by the infinitesimal curvature, are obtained.
Natural convection in a horizontal layer of fluid with a periodic array of square cylinders in the interior
Authors: Lee Jae Ryong , Ha Man Yeong , Balachandar S. , Yoon Hyun Sik , Lee Sang San
Published: March 2004

This study of thermal convection uses the following geometry: a horizontal layer of fluid heated from below and cooled from above, with a periodic array of evenly spaced square cylinders occupying the center of the layer (whose aspect ratio is 6). Periodic boundary conditions are employed in the horizontal direction to allow for lateral freedom for the convection cells. A two-dimensional solution for unsteady natural convection is obtained, using an accurate and efficient Chebyshev spectral multi-domain methodology, for different Rayleigh numbers varying over the range of 103 to 106. In order to assess the impact of different geometric approximations on the flow structure, dynamics and overall heat transfer, the results for the above case, denoted WC (wide aspect ratio cell with periodic side boundaries and six internal bodies), are compared with those for the cases of UCNS (unit cell with no-slip adiabatic side boundaries with a single internal body), UCPC (unit cell with periodic side boundaries with a single internal body) and pure Rayleigh–Bénard convection. The results for the case of WC with six adiabatic bodies are also compared to those with six neutral isothermal bodies to consider the effects of the thermal boundary condition on time-dependent natural convection in the enclosure.
Integrated multiple patch-clamp array chip via lateral cell trapping junctions
Authors: Seo J. , Ionescu-Zanetti C. , Diamond J. , Lal R. , Lee L. P.
Published: March 2004

We present an integrated multiple patch-clamp array chip by utilizing lateral cell trapping junctions. The intersectional design of a microfluidic network provides multiple cell addressing and manipulation sites for efficient electrophysiological measurements at a number of patch sites. The patch pores consist of openings in the sidewall of a main fluidic channel, and a membrane patch is drawn into a smaller horizontal channel. This device geometry not only minimizes capacitive coupling between the cell reservoir and the patch channel, but also allows simultaneous optical and electrical measurements of ion channel proteins. Evidence of the hydrodynamic placement of mammalian cells at the patch sites as well as measurements of patch sealing resistance is presented. Device fabrication is based on micromolding of polydimethylsiloxane, thus allowing inexpensive mass production of disposable high-throughput biochips.
On the translation of a cylinder in a long tube
Authors: Liu Hui , Bau Haim H. , Hu Howard
Published: March 2004

We study the motion of a cylindrical particle translating slowly in a long tube as a function of the particle’s dimensions and placement both in the presence and the absence of external pressure gradients. The cylinder acts as a “leaky” piston, generating both fluid recirculation and through flow. When the particle is long, analytic expressions are obtained for both the velocity field and the force needed to sustain the particle’s motion as functions of the particle’s position and dimensions. When the particle is short, a superposition-based algorithm is outlined to facilitate economical numerical calculations. When the particle is placed off center in the tube, torque will act on the particle. When the particle is unguided, this torque will preclude coaxial motion and cause the particle to follow an oscillatory trajectory.
The evolution and subsequent dynamics of waves on a vertically falling liquid film
Authors: Nosoko T. , Miyara A.
Published: March 2004

The spatiotemporal evolution of periodic waves on a vertically falling water film was investigated at the Reynolds number Re=15–75 via physical and numerical experiments. Small periodic waves excited at a low frequency grow directly into teardrop-shaped tall pulses, small waves of an intermediate frequency first grow into close-packed humps and then into pulses sandwiching single capillary ripples, and small waves of a high frequency grow into nearly sinusoidal waves. The initial wave evolution causes the waves to accelerate at low frequencies or to decelerate at intermediate and high frequencies. The maximum deceleration occurs with the growth into the close-packed humps which then undergo the transition to the pulses without a change of the wave frequency, associated with rapid acceleration. Subsequently, the quasisteady, nearly sinusoidal waves of small amplitudes and short-separation pulses further develop into nearly solitary tall pulses through transitions of wave coalescence, associated with growth of transverse wavefront variation. The nearly sinusoidal waves of large amplitudes rapidly increase the transverse variations of wavefronts and then fade away. These spatiotemporal evolution scenarios are consistent with the predictions from the stability analysis of steady-state traveling waves and the numerical simulations of the temporal evolution of periodic waves reported in the literature. A comparison with the evolution of noise-driven natural waves suggests that the natural waves greatly compress the scenario of waves at an intermediate frequency and rapidly grow into nearly solitary pulses following the scenario. Furthermore, it was revealed that these evolution scenarios resemble the ones observed on falling films inclined slightly from the horizontal.
Mechanism of p-type-to-n-type conductivity conversion in boron-doped diamond
Authors: Dai Ying , Dai Dadi , Liu Donghong , Han Shenghao , Huang Baibiao
Published: March 2004

We report on the role of two boron–hydrogen complexes in the conductivity conversion from p-type to n-type in boron-doped diamond samples. The calculated electronic structures of the simulated clusters show that the boron–hydrogen complex of hydrogen–boron pairs (BH1) is characteristic of p-type conductivity, and that of two hydrogen–boron centers (BH2) or three-hydrogen-boron centers (BH3) have a character of n-type conductivity. The phenomenon of conversion from p-type-to-n-type conductivity through deuteration may be explained by the transformation from the BH1 into BH2 and/or BH3 complexes.
Non-Fickian diffusion and tau approximation from numerical turbulence
Authors: Brandenburg Axel , Käpylä Petri J. , Mohammed Amjed
Published: March 2004

Evidence for non-Fickian diffusion of a passive scalar is presented using direct simulations of homogeneous isotropic turbulence. The results compare favorably with an explicitly time-dependent closure model based on the tau approximation. In the numerical experiments three different cases are considered: (i) zero mean concentration with finite initial concentration flux, (ii) an initial top hat profile for the concentration, and (iii) an imposed background concentration gradient. All cases agree in the resulting relaxation time in the tau approximation relating the triple correlation to the concentration flux. The first-order smoothing approximation is shown to be inapplicable.
Two-fluid jets and wakes
Authors: Herczynski Andrzej , Weidman Patrick D. , Burde Georgy I.
Published: March 2004

Similarity solutions for laminar two-fluid jets and wakes are derived in the boundary-layer approximation. Planar and axisymmetric fan jets as well as classical and momentumless planar wakes are considered. The interface between the immiscible fluids is stabilized by the action of gravity, with the heavier fluid, taken to be a liquid, placed beneath the lighter fluid. Velocity profiles for the jets and the classical wake depend intimately, but differently, on the parameter χ=ρ1μ1/ρ2μ2, where ρi and μi are, respectively, the density and absolute viscosity of the fluid in the upper (i=1) and lower (i=2) fluid domains, while the momentumless wake profile depends on the parameter Ω=ρ1μ23/ρ2μ13. Generally, all interfaces deflect from horizontal except the fan jet. However, while the interface for the classical planar two-fluid wake is never flat, the interfaces for the planar jet and the momentumless wake become flat in the particular case μ1=μ2. Velocity profiles illustrating the strongly asymmetrical jet and wake profiles that arise in air-over-water, oil-over-water, and air-over-oil flows are presented.
Dynamics of transient eddy above rolling-grain ripples
Authors: Rousseaux Germain , Yoshikawa Harunori , Stegner Alexandre , Wesfreid José Eduardo
Published: March 2004

This study deals with the flow motion over the so-called rolling-grain ripples which are generated by water oscillations above a sand bed. We focus our efforts on quantifying by means of laboratory experiments and numerical calculations the morphology and the dynamics of transient flow patterns. We report, for the first time, on the formation of an unsteady pattern with closed streamlines (we call it “eddy”) above rolling-grain ripples using flow visualizations and particle image velocimetry measurements. This structure appears in the ripple trough during flow reversal and scales with the ripple wavelength. The experimental results are in qualitative agreement with the perturbative flow solution calculated by Vittori in 1989. Even if the relative ripple amplitude is not small in the experiment the perturbative expansion at the first order gives an accurate description of the flow dynamics.
Flow pattern and heat transfer rate in Rayleigh–Bénard convection
Authors: Watanabe Tadashi
Published: March 2004

The three-dimensional Rayleigh–Bénard convection is simulated numerically using the lattice Boltzmann method. Flow patterns are observed and the heat transfer rate is estimated in terms of the Nusselt number. The dependence of the Nusselt number on the Rayleigh number is shown to agree well with that obtained by the two-dimensional calculations of the Navier–Stokes equations. It is shown that several roll patterns with different wave numbers and heat transfer rates are established even though the ratio of the horizontal size to the vertical size is a multiple of 2. Two types of oscillatory roll patterns are shown: one is with oscillatory heat transfer rate and the other is with the constant heat transfer rate. It is found that the square pattern is possible under the same condition for the stable or oscillatory roll pattern. The heat transfer rate decreases with decreasing wave number.
Long wavelength emission in InxGa1−xAs quantum dot structures grown in a GaAs barrier by metalorganic chemical vapor deposition
Authors: Passaseo A. , Tasco V. , De Giorgi M. , Todaro M. T. , De Vittorio M. , Cingolani R.
Published: March 2004

We demonstrate a method to obtain room temperature long wavelength emission from InGaAs quantum dots (QDs) growth directly into a binary GaAs matrix. The wavelength is tuned from 1.26 up to 1.33 μm by varying the V/III ratio during growth of the GaAs cap layer, without using a seeding layer or InGaAs wells. Strong improvement in terms of line-shape narrowing and efficiency is obtained. In addition to the shift in wavelength we observe an impressive reduction of temperature dependent quenching of the emission efficiency, which decreases only by a factor of 3 between cryogenic temperatures and room temperature, very good for QD structures emitting at 1.3 μm. Photoluminescence spectroscopy and theoretical modeling were combined for interpretation of the results.
Axisymmetric inviscid interaction of a bubble and a vortex ring
Authors: Higuera F. J.
Published: March 2004

Bubble breakup induced by the interaction of a bubble and a coaxial vortex ring is described numerically using a boundary element method. Breakup is found to occur when a Weber number based on the radius of the bubble and the speed of the incoming vortex is larger than a certain critical value, which is computed as a function of the initial vortex-to-bubble radius ratio. Different modes of breakup are observed depending on the values of these two parameters. The critical Weber number has a minimum for vortices slightly smaller than the bubble, for which the breakup is mediated by capillary waves excited on the bubble surface by the passage of the vortex.
Influence of thermocapillary effect and interfacial heat release on convective oscillations in a two-layer system
Authors: Nepomnyashchy Alexander A. , Simanovskii Ilya B.
Published: March 2004

The oscillatory convection for a real system of fluids under the joint action of buoyancy and thermocapillary effect or in the presence of the interfacial heat release is investigated. The nonlinear development of the oscillatory instability is studied. In the case of periodic boundary conditions, regimes of traveling waves and pulsating traveling waves are predicted. For different types of boundary conditions, the period doubling bifurcation is obtained. It is shown that the region of the Grashof number values, where nonlinear oscillations take place, is bounded both from below (by the mechanical equilibrium state) and from above (by the steady state).
Interaction of wavy cylindrical Couette flow with endwalls
Authors: Czarny Olivier , Serre Eric , Bontoux Patrick , Lueptow Richard M.
Published: March 2004

The finite length of a Taylor–Couette cell introduces endwall effects that interact with the centrifugal instability and the subsequent wavy vortex flow. We investigate the interaction between the endwall Ekman boundary layers and the wavy vortices in a finite-length cavity via direct numerical simulation using a three-dimensional spectral method. To analyze the nature of the interaction between the vortices and the endwall layers, we consider three endwall boundary conditions: fixed endwalls, endwalls rotating with the inner cylinder, and stress-free endwalls. Near the endwalls, the waviness is diminished, primarily due to the effect of the flatness of the endwall rather than as a result of the no-slip boundary condition at the endwall. However, the waviness is present just one or two vortices away from the endwalls, indicating that the effect of the endwall on waviness does not penetrate far from the endwall. The interaction of the waviness with the endwall Ekman layer does not appear to result in disorder in the flow, either in the endwall vortex or farther from the endwall. Likewise, the waviness does not significantly alter Ekman layer thickness from that predicted based on theory.
Nonlinear model equation for three-dimensional Bunsen flames
Authors: Denet Bruno
Published: March 2004

The nonlinear description of laminar premixed flames has been very successful because of the existence of model equations describing the dynamics of these flames. The Michelson–Sivashinsky equation is the most well known of these equations and has been used in different geometries, including three-dimensional quasi-planar and spherical flames. Another interesting model, usually known as the Frankel equation, which could in principle take into account large deviations of the flame front, has been used for the moment only for two-dimensional expanding and Bunsen flames. We report here for the first time numerical solutions of this equation for three-dimensional flames.
Spin-filter devices based on resonant tunneling antisymmetrical magnetic/semiconductor hybrid structures
Authors: Xu H. Z. , Zhang Yafei
Published: March 2004

Spin filter devices based on resonant tunneling antisymmetrical magnetic-barrier structures are proposed and the spin-polarization properties are described. These devices can be realized by depositing one or two ferromagnetic stripes with perpendicular magnetization on top of two-dimensional electron gas. It is remarkable that the spin-filter efficiency of the transmission probability and the conductance through these devices could be reached completely (∼100%) in resonance, and that constant voltage applied to the metallic stripe of the systems could tune the spin polarization. These interesting features may lead to a practical voltage controlled spin filter.
Instability versus collapse in a flow driven by the radial electric current
Authors: Shtern Vladimir
Published: March 2004

A flow of an electrically conducting fluid induced by a spherically symmetric current (discharging from a point source located on the fluid free surface) has a striking feature. As increasing total current J reaches a threshold, a jet propagating from the source along the axis of symmetry collapses, i.e., velocity becomes unbounded on the axis. It is shown here that the flow becomes unstable at J smaller than the threshold. For a flow in a conical region, θ<θcone (θ is the polar angle from the axis of symmetry), the instability leads to either a time-oscillating or steady-swirling flow depending on θcone. In a half-space, θcone=90°, the instability is oscillatory. The steady swirl first develops as J increases in a cone with θcone<85°.
Site-controlled and size-homogeneous Ge islands on prepatterned Si (001) substrates
Authors: Zhong Zhenyang , Bauer G.
Published: March 2004

We report on a combination of lithography and self-assembly techniques which results in long-range two-dimensionally ordered Ge islands. Island lattices with perpendicular but also with obliquely oriented unit vectors were realized. Quantitative analysis of the island topographies demonstrates that the size dispersion of these islands is smaller than that found on flat substrates. Furthermore, island formation on the patterned substrates is observed for a smaller amount of Ge deposition. However, with further Ge deposition an increasing amount is incorporated into the sidewalls.
Theoretical study of Auger recombination in a GaInNAs 1.3 μm quantum well laser structure
Authors: Andreev A. D. , O’Reilly E. P.
Published: March 2004

We present a theoretical study of Auger recombination processes in a GaInNAs/GaAs quantum well structure designed for 1.3 μm laser emission. The calculations are based on a 10×10 k⋅p model, incorporating valence, conduction, and nitrogen-induced bands. The Auger transition matrix elements are calculated explicitly, without introducing any further approximations into the Hamiltonian used. We consider two main Auger recombination channels: the process when the energy released from the electron-hole recombination causes electron excitation (CHCC process) and the process with hole excitation to the split-off valence band (CHHS process). The CHHS process is shown to be dominant. Good agreement is found when comparing the calculated Auger rates with experimental values of the Auger contribution to the threshold current of GaInNAs quantum well lasers.
Electric field-controlled water permeation coupled to ion transport through a nanopore
Authors: Dzubiella J. , Allen R. J. , Hansen J.-P.
Published: March 2004

We report molecular dynamics simulations of a generic hydrophobic nanopore connecting two reservoirs which are initially at different Na+ concentrations, as in a biological cell. The nanopore is impermeable to water under equilibrium conditions, but the strong electric field caused by the ionic concentration gradient drives water molecules in. The density and structure of water in the pore are highly field dependent. In a typical simulation run, we observe a succession of cation passages through the pore, characterized by approximately bulk mobility. These ion passages reduce the electric field, until the pore empties of water and closes to further ion transport, thus providing a possible mechanism for biological ion channel gating.
Recombination of drifting holes with trapped electrons in stabilized a-Se photoconductors: Langevin recombination
Authors: Kasap S. O. , Fogal Bud , Zahangir Kabir M. , Johanson Robert E. , O’Leary Stephen K.
Published: March 2004

Stabilized amorphous selenium (a-Se) is one of the x-ray photoconductors that is currently used in recently developed direct conversion flat panel x-ray image detectors. We have studied the recombination of free holes with trapped electrons in stabilized a-Se. Electrons were deeply trapped in a-Se by carrying out repetitive electron time-of-flight (TOF) transient photoconductivity experiments. By using conventional and interrupted field hole time-of-flight (IFTOF) transient photoconductivity techniques in a TOF, IFTOF, TOF sequence, we were able to develop a technique that allows the measurement of the capture coefficient between free holes and trapped electrons. We find that the capture process of holes by trapped electrons closely follows the Langevin recombination mechanism.
Comparison of structure and properties of femtosecond and nanosecond laser-structured silicon
Authors: Crouch C. H. , Carey J. E. , Warrender J. M. , Aziz M. J. , Mazur E. , Génin F. Y.
Published: March 2004

We compare the optical properties, chemical composition, and crystallinity of silicon microstructures formed in the presence of SF6 by femtosecond laser irradiation and by nanosecond laser irradiation. In spite of very different morphology and crystallinity, the optical properties and chemical composition of the two types of microstructures are very similar. The structures formed with femtosecond (fs) pulses are covered with a disordered nanocrystalline surface layer less than 1 μm thick, while those formed with nanosecond (ns) pulses have very little disorder. Both ns-laser-formed and fs-laser-formed structures absorb near-infrared (1.1–2.5 μm) radiation strongly and have roughly 0.5% sulfur impurities.
Single virus particle mass detection using microresonators with nanoscale thickness
Authors: Gupta A. , Akin D. , Bashir R.
Published: March 2004

In this letter, we present the microfabrication and application of arrays of silicon cantilever beams as microresonator sensors with nanoscale thickness to detect the mass of individual virus particles. The dimensions of the fabricated cantilever beams were in the range of 4–5 μm in length, 1–2 μm in width and 20–30 nm in thickness. The virus particles we used in the study were vaccinia virus, which is a member of the Poxviridae family and forms the basis of the smallpox vaccine. The frequency spectra of the cantilever beams, due to thermal and ambient noise, were measured using a laser Doppler vibrometer under ambient conditions. The change in resonant frequency as a function of the virus particle mass binding on the cantilever beam surface forms the basis of the detection scheme. We have demonstrated the detection of a single vaccinia virus particle with an average mass of 9.5 fg. These devices can be very useful as components of biosensors for the detection of airborne virus particles.
Doubling of sensitivity and bandwidth in phonon cooled hot electron bolometer mixers
Authors: Baselmans J. J. A. , Hajenius M. , Gao J. R. , Klapwijk T. M. , de Korte P. A. J. , Voronov B. , Gol’tsman G.
Published: March 2004

We demonstrate that the performance of NbN lattice cooled hot electron bolometer mixers depends strongly on the interface quality between the bolometer and the contact structure. We show experimentally that both the receiver noise temperature and the gain bandwidth can be improved by more than a factor of 2 by cleaning the interface and adding an additional superconducting interlayer to the contact pad. Using this we obtain a double sideband receiver noise temperature TN,DSB=950 K at 2.5 THz and 4.3 K, uncorrected for losses in the optics. At the same bias point, we obtain an IF gain bandwidth of 6 GHz.
Comment on “Three-dimensional photonic-crystal emitter for thermal photovoltaic power generation” [Appl. Phys. Lett. 83, 380 (2003)]
Authors: Trupke Thorsten , Würfel Peter , Green Martin A.
Published: March 2004

In a recent article, Lin et al. [Appl. Phys. Lett. 83, 380 (2003)] reported on the light-emitting properties of three-dimensional tungsten photonic crystals and their potential applications as improved thermal emitters in thermophotovoltaic (TPV) systems. Their findings have attracted considerable interest throughout the media and the application of this type of materials has been praised as a potential superior future energy source; e.g., in waste heat-driven electrical generators (http://www.photonics.com). The results of the theoretical modeling in the work of Lin et al. suggest that a TPV system can achieve higher heat to electric energy conversion efficiencies in combination with a three-dimensional tungsten photonic crystal than with any conventional selective thermal emitter. These theoretical results are based on the experimental observation that the photonic crystal, when heated to a given temperature, emits more radiation in certain spectral regimes than a black body of the same temperature. This experimental observation shall briefly be discussed here.
Coherent nonlinear optical spectroscopy of single quantum dot excited states
Authors: Batteh E. T. , Cheng Jun , Chen Gang , Steel D. G. , Gammon D. , Katzer D. S. , Park D.
Published: March 2004

We report a coherent nonlinear optical signature of the excited states of two quantum dots. By comparing the nonlinear spectra with the linear photoluminescence excitation spectrum, a clear identification of excited states is possible.