bluuebunny/crossref_metadata_2025 huggingface dataset:
Interaction of two-dimensional separated flows with a free surface at low Froude numbers
Authors: Triantafyllou George S. , Dimas Athanassios A.
Published: July 2002

It is shown that the low Froude number wake of floating two-dimensional objects is convectively unstable. This is shown to be true for bluff objects, like a circular cylinder, and for streamlined objects, like a thin airfoil. As a result, the wake behind a floating object remains steady, even at high Reynolds numbers, characterized by a long region of recirculating flow. It is concluded that the presence of the free surface has a stabilizing effect at low Froude numbers, suppressing the unsteadiness of the wake. The structure of the dispersion relation, shows that, at low Froude numbers, a short wavelength interaction between the wake instability and ambient waves is possible in the form of a spatially growing response.
Satellite formation in capillary jet breakup
Authors: Mansour Nagi N. , Lundgren Thomas S.
Published: July 2002

Computations of finite-amplitude, spatially periodic wave growth on a cylindrical jet have been carried out using a boundary integral method. The initial wave growth is in agreement with Rayleigh’s linear theory. When followed to completion these waves pinch off large drops separated by smaller satellite drops (spherules) that decrease in size with decreasing wavelength. The computed sizes of both drops and satellites agree with experiment. It is found that satellites will form for all unstable wave numbers. The small satellites that are computed at wave numbers near the critical wave number were not predicted by near-linear analysis but are observed in experimental photographs of jet breakup. Computation of the collapse of elongated satellites shows short waves propagating on their surfaces.
Stability of core-annular flow with a small viscosity ratio
Authors: Hu Howard H. , Lundgren Thomas S. , Joseph Daniel D.
Published: July 2002

It is known that the stability problem for core-annular flow of very viscous crude oil and water is singular, the water annulus appears to be inviscid with boundary layers at the pipe wall and at the interface. In the present paper, this singular problem is treated by the method of matched asymptotic expansions using ε=m/Rα as a small parameter. There are two cases of instability corresponding to different positions of the critical point in the annulus. One case is when the critical point is far away from the interface, the other is when the critical point is close to the interface within a distance of order ε1/3. In both cases, we derive the equations for the eigenvalues, and give the explicit forms for the neutral curves. The stability problem is also treated by the modified finite element code used by Hu and Joseph [J. Fluid Mech. 205, 359 (1989); Phys. Fluids A 1, 1659 (1989)], taking into account the boundary layers at the pipe wall and at the interface. The results of the two methods agree where they overlap, but the finite element technique goes further.
Dynamic behavior of liquid sheets
Authors: Mansour Adel , Chigier Norman
Published: July 2002

An experiment was conducted to study the aerodynamic instability of liquid sheets issuing from a two-dimensional air-assisted nozzle. Detailed measurements of the frequency of oscillation of the liquid sheet have been made. The measured vibrational frequencies were then correlated with the resulting spray angle. It was shown that the liquid sheet oscillations are dynamically similar to that of hard spring systems. For each air pressure, three distinct modes of breakup are distinguished. At low liquid flow rates, the sinusoidal mode of breakup is dominant. At intermediate liquid flow rates, both the sinusoidal and the dilational modes are superimposed on the liquid sheet. With a further increase in liquid flow rate, the liquid sheet oscillations mainly become of the dilational type. It was also shown that the effect of introducing air in the nozzle is similar to the effect of inducing forced vibrations on the nozzle jaws. Thus, for each air flow rate, there is a specific vibration frequency for the nozzle. The frequency of these vibrations is proportional to the air velocity. As the liquid sheet natural frequency approaches that of the nozzle, resonance is established. At resonance, the maximum spray angle is achieved.
Nonexistence of Lyapunov functions and the instability of the von Kármán vortex streets
Authors: Lim Chjan
Published: July 2002

The instability of the von Kármán vortex streets and the existence of a global Lyapunov function at the special aspect ratio h/l=(1/π)sinh−1(1), are some of the difficulties with the well-known von Kármán model. By consistently applying the principle of genericity, its shown that a new family of near-equilibrium periodic solutions of the von Kármán model for aspect ratios near 0.281... supplies numerous theoretical candidates for observed vortex trails. This set of solutions implies that there is no global Lyapunov functions when h/l≠(1/π)sinh−1(1) which in turn leads to a rich variety of near-equilibrium solutions for the model.
Numerical analysis of a uniform flow of a rarefied gas past a sphere on the basis of the Boltzmann equation for hard-sphere molecules
Authors: Takata Shigeru , Sone Yoshio , Aoki Kazuo
Published: July 2002

A slow uniform flow of a rarefied gas past a sphere is investigated on the basis of the linearized Boltzmann equation for hard-sphere molecules and the diffuse reflection condition. With the aid of a similarity solution, the Boltzmann equation is reduced to two simultaneous integrodifferential equations with three independent variables, which are solved numerically. The collision integral is computed efficiently by the use of a numerical collision kernel [Phys. Fluids A 1, 363 (1989)]. The velocity distribution function of the gas molecules, which has discontinuity in the gas, the density, flow velocity, and temperature fields of the gas, and the drag on the sphere are obtained accurately for the whole range of the Knudsen number. In spite of slow flow, the temperature is nonuniform (thermal polarization). From the behavior of the velocity distribution function, the kinetic transition region is clearly seen to separate into the Knudsen layer and the S layer for small Knudsen numbers.
Langmuir turbulence equations with the self-generated magnetic field
Authors: Li L. H.
Published: July 2002

Starting from the dynamical plasma equations with the help of relaxed Zakharov simplification assumptions, and through taking use of the time-averaged two-time-scale two-fluid plasma description, the Zakharov equations (ZE) are generalized to contain the self-generated magnetic field. The generalized Zakharov equations (GZE) are a set of three coupled equations and of seventh-order nonlinearity with an amplification factor in almost all nonlinear terms of GZE. The conclusion that electron nonlinearities other than those appearing in ZE are not important and that the ion nonlinearities may be important when Langmuir turbulence level is fairly high, which was obtained through using both particle-in-cell (PIC) simulations and numerically integrating ZE by Newman et al. [Phys. Fluids B 2, 2600 (1990)] and Clark et al. [Phys. Fluids B 4, 708 (1992)], is demonstrated by the theoretical analysis given in this paper.
Lower-hybrid collapse, caviton turbulence, and charged particle energization in the topside auroral ionosphere and magnetosphere*
Authors: Chang Tom
Published: July 2002

In 1981, Chang and Coppi [Geophys. Res. Lett. 8, 1253 (1981)] suggested that lower-hybrid turbulence could be the prime candidate for the acceleration of ions and generation of ‘‘ion conics’’ in the high-latitude ionosphere and magnetosphere. Subsequently, Retterer, Chang, and Jasperse [J. Geophys. Res. 91, 1609 (1986)] demonstrated that nonlinear wave interactions near the lower-hybrid frequency through modulational instability, such as the collapse of waves into soliton (caviton) turbulence could play a key role in the energization of both the ambient ions and electrons. Recent sounding rocket observations in the source region of the topside auroral ionosphere seem to confirm the details of such predictions [Kintner et al., Phys. Rev. Lett. 68, 2448 (1992); Arnoldy et al., Geophys. Res. Lett. 19, 413 (1992)]. In this paper, the scenario of this interesting micro/meso scale, nonlinear wave–wave and wave–particle interaction plasma process in the auroral ionosphere/magnetosphere is briefly reviewed.
Neoclassical quasilinear transport theory of fluctuations in toroidal plasmas
Authors: Shaing K. C.
Published: July 2002

A transport theory of fluctuations with frequencies less than the gyrofrequency in toroidal plasmas is developed to calculate particle and heat fluxes, bootstrap current, Ware pinch flux, and modification to plasma conductivity. It is found that the particle and heat fluxes are proportional to ∑m,n,ωm2(eΦ̃mnω/Te)2, the bootstrap current and Ware pinch flux are proportional to ∑m,n,ωm[(m−nq)/‖m−nq‖] (eΦ̃mnω/Te)2, and the modification

to plasma conductivity is proportional to ∑m,n,ω‖m−nq‖(eΦ̃mnω /Te)2, where m (n) is the poloidal (toridal) mode number for the (m,n) mode with frequency ω, Φ̃mnω is its mode amplitude, e is the electric charge, Te is the electron temperature, and q is the safety factor. Alternatively, in terms of the poloidal wave vector kθ and the parallel wave vector k∥, the particle and heat fluxes are proportional to 〈k2θ〉〈(eΦ̃/Te)2〉, the bootstrap current and Ware pinch flux are proportional to 〈kθk∥/‖k∥‖〉〈(eΦ̃/ Te)2〉, and the modification to plasma conductivity is proportional to 〈‖k∥‖〉〈(eΦ̃/Te)2〉, where 〈Φ̃2〉 is the averaged fluctuation level. Thus the sensitivities of various transport fluxes to plasma fluctuations are different: the most sensitive ones are particle and heat fluxes, the less sensitive ones are bootstrap current and Ware pinch flux, and the least sensitive one is the modification to plasma conductivity. The effects of fluctuations on bootstrap current and Ware pinch flux and on modification to plasma conductivity are smaller than those on particle and heat fluxes by factors of 〈kθk∥/‖k∥‖〉/〈k2

θ〉 and 〈k∥‖〉/〈k2θ〉, respectively. It is demonstrated explicitly that the matrix consisting of these transport coefficients satisfies Onsager symmetry. Furthermore, it is shown that the electron and ion transport are coupled through plasma flows. The theory is employed to discuss some qualitative transport properties observed in toroidal plasma experiments.
On the elimination of wall-topography parameters from second-moment closure
Authors: Launder B. E. , Li S.-P.
Published: July 2002

Current generation second-moment closures employ wall proximity/orientation parameters to ensure that the velocity fluctuations normal to the wall die out as the wall is approached faster than fluctuations parallel to the wall. The use of such devices renders the models unsuitable for application to surfaces of even moderately complex shape. It is argued that, for the case of flow parallel to the wall, no such parameters are needed if one adopts a model of the pressure–strain process that rigorously satisfies the two-component limit. The assertion is supported by computations of flow in square and rectangular ducts as well as for the low-Reynolds-number plane channel flow.
Dissociation modeling in low density hypersonic flows of air
Authors: Boyd Iain D. , Candler Graham V. , Levin Deborah A.
Published: July 2002

Vibration–dissociation coupling in low-density, hypersonic flows of air is investigated. Radiative emission data for nitric oxide and for atomic oxygen measured by a reentry flight experiment are used to assess the modeling of this phenomenon. Flow field computations are performed using the direct simulation Monte Carlo method. Due to the relatively small number of collisions under high-altitude, low-density flow conditions, an overlay approach is used to simulate changes in chemical composition of trace species, including both nitric oxide and atomic oxygen. Radiative emission is calculated using a nonequilibrium radiation method. It is found that the strong degree of thermal nonequilibrium that occurs in high-altitude, hypersonic flows makes the chemistry very sensitive to the vibration–dissociation coupling model. A number of such models based on continuum and particle representations of the flow are assessed. A variation in dissociation rate of up to nine orders of magnitude among these models is found for the lowest-density flight conditions. By using a sophisticated dissociation model, the emission calculated at the highest altitude for which measurements are available is improved from a factor of 220 too low to within a factor of 4 too low. With the same model, improvement by a factor of 50 is also obtained for the computation of emission from atomic oxygen. This is the first time that the observed dependence of the flight data on the free-stream density has been predicted correctly.
A note on the leapfrogging between two coaxial vortex rings at low Reynolds numbers
Authors: Lim T. T.
Published: July 2002

In the early 1970s, Maxworthy [J. Fluid. Mech. 51, 15 (1972)] and Oshima et al. [J. Phys. Soc. Jpn. 38, 1159 (1975)] attempted to produce leapfrogging between two coaxial vortex rings at low Reynolds numbers in their laboratories. However, the rings failed to undergo the classical leapfrogging behavior and merged to form a single ring. In a recent numerical study, Shariff et al. (NASA Tech. Memo. 102257, 1989) attributed the failure partly to the effect of core distortion. They pointed out that vortex cores at low Reynolds numbers are thicker and therefore more susceptible to distortion during the leapfrogging. In this Brief Communication, it is shown through a systematic investigation that the initial generating condition also plays an important role in determining the success or failure of leapfrogging. The common belief that leapfrogging can be achieved simply by generating two vortex rings in quick succession may not be true for all flow conditions, especially when the Reynolds number is low. This finding may help to explain the results of Maxworthy and Oshima et al.
First transitions in circular Couette flow with axial stratification
Authors: Hua Bach Lien , Le Gentil Sylvie , Orlandi Paolo
Published: July 2002

The first flow regimes which have been observed experimentally for a circular Couette flow with a stable, axial stratification in density are investigated through direct numerical simulations of the three-dimensional Navier-Stokes equations for a Boussinesq fluid. The setup of two concentric cylinders has a nondimensional gap width of ε=(b−a)/a=0.289; the outer cylinder is fixed and the stratification in density in the axial direction is linear. The main effect of an axial density stratification is to reduce the height of the Taylor vortices and to cause the formation of density layers of small aspect ratio. For large enough Prandtl number, the primary bifurcation from circular Couette flow is found to be axisymmetric and of Hopf-type in the direct numerical simulations. An analytical solution for onset of instability and slightly different boundary conditions from the experimental ones agrees within 0.6% with numerical simulations at a Prandtl number of 700. The experimental flow regimes with well-defined density layers are well reproduced by the numerical simulations in the appropriate range of relative Reynolds number Re/Rec1, where Rec1 denotes the critical Reynolds number for the primary bifurcation from circular Couette flow. However, the increase of axial scale with Re/Rec1 is found to be continuous, whereas it is quantized in the laboratory experiments. Numerical results reveal that the first two transitions between the flow regimes are primarily due to the temporal behavior of the axially symmetric part of the flow. Onset of nonaxisymmetric motions appears at the same Re/Rec1≈1.18 as in the homogeneous fluid case at the same η=a/b. Stratification precludes large axial displacements and the azimuthal modes patterns have a quite distinct appearance from the homogeneous wavy modes. At large enough Re, a destabilization of the jet-like outflow between pairs of vortices causes the suppression of the density front which is located at the same axial height. This nonaxisymmetric flow regime presents common features with the wavy outflow boundary (WOB) pattern, which is commonly observed in the homogeneous Couette-Taylor case.
Bifurcation of the equilibrium states of a weightless liquid bridge
Authors: Slobozhanin Lev A. , Alexander J. Iwan D. , Resnick Andrew H.
Published: July 2002

The bifurcation of the solutions of the nonlinear equilibrium problem of a weightless liquid bridge with a free surface pinned to the edges of two coaxial equidimensional circular disks is examined. The bifurcation is studied in the neighborhood of the stability boundary for axisymmetric equilibrium states with emphasis on the boundary segment corresponding to nonaxisymmetric critical perturbations. The first approximations for the shapes of the bifurcated equilibrium surfaces are obtained. The stability of the bifurcated states is then determined from the bifurcation structure. Along the maximum volume stability limit, depending on values of the system parameters, loss of stability with respect to nonaxisymmetric perturbations results in either a jump or a continuous transition to stable nonaxisymmetric shapes. The value of the slenderness at which a change in the type of transition occurs is found to be Λs=0.4946. Experimental investigation based on a neutral buoyancy technique agrees with this prediction. It shows that, for Λ<Λs, the jump is finite and that a critical bridge undergoes a finite deformation to a stable nonaxisymmetric state.
Drop formation from a vibrating orifice generator driven by modulated electrical signals
Authors: Brenn G. , Lackermeier U.
Published: July 2002

The recently introduced technique of amplitude-modulated (AM) excitation of laminar liquid jets to produce drops by controlled Rayleigh-type jet disintegration is extended to frequency modulation (FM) and investigated in detail to quantify the available ranges of excitation wavenumbers and modulation depths for various jet velocities and liquids. The AM and FM techniques are compared regarding the achievable ratios of carrier and modulation frequencies. The modulated excitation is applied to a vibrating orifice drop generator (TSI Inc.). Four different drop formation processes are observed for different wavenumber domains. Experiments with three different Newtonian liquids and a dimensional analysis yield a generalized representation of the parameter windows of the technique in terms of excitation wavenumber and jet velocity for a wide range of liquid viscosity and surface tension.
Intermittency and relative scaling of subgrid-scale energy dissipation in isotropic turbulence
Authors: Cerutti Stefano , Meneveau Charles
Published: July 2002

The rate at which large-scale kinetic energy in turbulent flows is transferred to, or from, unresolved scales (smaller than a filter scale Δ) is given by Π(x,t)=−τijS̃ij, where τij is the subgrid stress, and S̃ij is the resolved strain-rate tensor. The spatial distribution of Π(x,t) is computed from DNS of isotropic turbulence, and is found to be highly intermittent with increasing levels of intermittency as the filter size decreases. Relative scaling exponents of high-order moments of Π are measured using extended self-similarity, and are compared to those of longitudinal velocity structure functions. Reasonably good agreement is found, both sets of exponents clearly departing from the Kolmogorov (1941) theory. Relative scaling exponents of the SGS dissipation as predicted by several models are measured a priori from the DNS, and are compared to those of the true dissipation. We find the constant and spectral eddy viscosity models to be significantly less intermittent, and the local dynamic model to be much more intermittent than the true SGS dissipation field. The traditional and volume-averaged dynamic Smagorinsky models, together with the similarity model, yield more realistic levels of intermittency.
Correction to the four-fifths law due to variations of the dissipation
Authors: Lindborg Erik
Published: July 2002

The full Kolmogorov equation is analyzed with respect to Galilean invariance. It is shown that the time-dependent term in typical applications as jets and wakes will cause rather large deviations from the four-fifths law, even at fairly large Reynolds numbers. With the use of the K−ε-model an inertial range model expression is derived for the third-order structure function of freely decaying turbulence at finite Reynolds number. The expression is compared with results from previously reported experiments. It is argued that experiments to test intermittency corrections to the Kolmogorov 1941 theory should be made in flows which are both stationary and homogeneous in the mean flow direction, such as pipe and channel flows.
Development and calibration of algebraic nonlinear models for terms in the Reynolds stress transport equations
Authors: Sjögren Torbjörn , Johansson Arne V.
Published: July 2002

A simple and straightforward method is presented for the derivation and calibration of algebraic nonlinear models for terms in Reynolds stress turbulence closures. The method extensively utilizes data from direct numerical simulations to allow an investigation of the model performance over the entire Reynolds stress anisotropy-invariant map. The model constants are determined from the condition of minimizing the mean square error over the invariant map, in order to give good model behavior for as wide a class as possible of flow situations. A low Reynolds number closure is proposed based on the most general form for closing the Reynolds stress transport equations in terms of Reynolds stresses and total dissipation rate. It is shown that forcing the closure to satisfy realizability in a strict sense leads to a good model behavior even for the complicated flow situation near a wall, without any use of ad-hoc wall damping functions in the closure. The model behavior in homogeneous turbulent flow is analyzed by formulating equations for invariant measures, yielding several quite general results for the behavior of the present and other existing models. A new approach to the modeling effects of rotation in the context of Reynolds stress closures is presented and tested for some different homogeneous flows subjected to rotation.
Magnetic topology and the problem of its invariant definition
Authors: Hornig G. , Schindler K.
Published: July 2002

The evolution of an ideal plasma conserves magnetic lines of force and hence magnetic topology. However, magnetic topology, i.e. the structure and linkage of magnetic flux, is a property of the magnetic field alone. Therefore, the conservation of topology can also be a property of non-ideal plasmas for which the plasma flow is not line conserving. A general definition of magnetic topology is given and it is shown that it yields a large set of non-ideal topology-conserving systems. In the application of the notion of magnetic topology to real plasmas problems arise concerning the stability of topology. Instability may inhibit one from defining the topology of a given real, i.e. not exactly prescribed, magnetic field configuration and makes it difficult to detect changes of magnetic topology, such as reconnection processes. This problem of structural instability of magnetic topology also appears in connection with changes of the frame of reference. A change of the frame of reference may lead to a transition in topology especially for topological unstable, non-ideal systems.
Ionized physical vapor deposition of integrated circuit interconnects
Authors: Hopwood J.
Published: July 2002

Interconnects, once the technological backwater of integrated circuit technology, now dominate integrated circuit cost and performance. As much as 90 percent of the signal delay time in future integrated circuit designs will be due to the interconnection of semiconductor devices while the remaining 10 percent is due to transistor-related delay. This shifts the thrust of critical research toward an improved understanding of interconnect science and technology. Shrinking circuit geometries will require high aspect ratio (AR) vias to interconnect adjacent metal layers. By the year 2007 it is predicted that logic circuits will use 6 to 7 interconnected metal layers with via ARs of 5.2:1. Memory will need fewer layers, but ARs as high as 9:1. In this paper, the demands of interconnect technology will be reviewed and the opportunities for plasma-based deposition of vias will be discussed. One promising new method of fabricating high-aspect ratio vias is ionized physical vapor deposition (I-PVD). The technique economically creates a unidirectional flux of metal which is uniform over 200–300 mm diameter wafers. Since metal ejected by conventional sputtering is primarily neutral and exhibits a cosine angular velocity distribution, sputtered metal atoms do not reach the bottom of high AR vias. By sputtering these atoms into a moderate pressure (4 Pa), high-density Ar plasma, however, the metal atoms are first thermalized and then ionized. The ions are then readily collimated by the plasma sheath and directionally deposited into narrow, deep via structures. Experiments have consistently shown that over 80% of the metal species are ionized using I-PVD. The physical mechanisms responsible for ionization will be discussed from both an experimental and modeling perspective and the spatial variation of metal ionization is experimentally determined.
Analytic treatment of electromagnetic emission near the plasma frequency via Langmuir wave decay
Authors: Edney S. D. , Robinson P. A.
Published: July 2002

The three-wave decay of a Langmuir wave into an ion-sound wave and an electromagnetic wave close to the plasma frequency is analyzed. Realistic approximations to the spectra for the Langmuir and ion-sound waves are used to enable analytic calculation of the resulting spectrum of transverse electromagnetic waves in terms of a single well-behaved integral. This integral is approximated analytically and evaluated numerically and the results are compared with each other and with previous estimates. Applications of the results to type III solar radio bursts and laboratory plasmas are also discussed and it is found that the present analytic work is relevant to emission by fast beams in both contexts, provided the plasma temperature is not too high.
E×B
shear flows and electromagnetic gyrofluid turbulence
Authors: Scott B.
Published: July 2002

Low frequency tokamak edge turbulence is modelled numerically using gyrofluid equations for electrons and ions on an equal footing. The electrons are electromagnetic, and arbitrarily strong finite gyroradius effects are included for the ions. Computations are in a globally consistent truncation of flux surface geometry arising from ideal tokamak equilibria. The turbulence is similar to that in the fluid model in steep gradient regimes, for which the electron transit frequency is comparable to that of the turbulence. The nonlinear drift wave instability is shown to be caused by E×B self-advection, and is similar for both two-and three-dimensional models. The turbulence always has drift wave mode character for the parameter regime of interest, except when the ideal ballooning threshold is reached. Turbulence interacts strongly with E×B shear flows, but does not build the flow shear to significant levels by itself. On the other hand, an imposed shear layer arising from the neoclassical equilibrium of the edge region does have the necessary properties and scaling to eventually result in a credible edge transport transition scenario.
Optical properties of thin nematic liquid crystal cells
Authors: Wu Shin-Tson , Efron Uzi
Published: July 2002

Optical properties of thin liquid crystal cells are found to deviate significantly from those of thick cells. Results on the liquid crystal thickness dependent phase retardation, effective birefringence, and decay time of thin E-7 cells are presented. A simple model which is based on the partially disordered surface layers is developed to explain the observed phenomena. The concept of virtually inactive surface layers is introduced.
Frequency locking, quasiperiodicity, and chaos in modulated self-pulsing semiconductor lasers
Authors: Winful H. G. , Chen Y. C. , Liu J. M.
Published: July 2002

A transition to chaos via quasiperiodicity is observed in the output of a directly modulated self-pulsing semiconductor laser. By sweeping the frequency and amplitude of the current modulation, several frequency-locked states (Arnol’d tongues) are mapped out directly. Good agreement with the predictions of a rate equation model is obtained.
Kinetics of implantation enhanced interdiffusion of Ga and Al at GaAs-GaxAl1−xAs interfaces
Authors: Cibert J. , Petroff P. M. , Werder D. J. , Pearton S. J. , Gossard A. C. , English J. H.
Published: July 2002

The kinetics of implantation enhanced interdiffusion at GaAs-GaxAl1−xAs interfaces is investigated by cathodoluminescence and transmission electron microscopy. Localized Ga+ implantation leads to enhancement of the interdiffusion by about two orders of magnitude at 950 °C. A complete recovery of the optical quality of the material and local alteration of the band gap is observed after rapid thermal annealing. The role of intrinsic interdiffusion is identified. Control of the interdiffusion kinetics has allowed the fabrication of ultrasmall structures with good optical properties.
Growth of ultrapure InP by atmospheric pressure organometallic vapor phase epitaxy
Authors: Chen C. H. , Kitamura M. , Cohen R. M. , Stringfellow G. B.
Published: July 2002

We report the growth and characterization of ultrapure InP using trimethylindium and phosphine by atmospheric pressure organometallic vapor phase epitaxy (APOMVPE). The 77 K mobility of 131 000 cm2 /V s is the highest ever obtained by APOMVPE and among the highest ever measured for InP using any growth technique. The low-temperature photoluminescence measurements reveal that impurity reduction occurs at higher growth temperatures.
Roughening of Si (111) surface under high-temperature thermal cycling
Authors: Zuo J.-K. , Harper R. A. , Wang G.-C.
Published: July 2002

High-resolution low-energy electron diffraction has been used to study the generation of defects on the surface of commercial Si (111) wafers under high-temperature thermal cycling in ultrahigh vacuum. We observed a gradual increase of single-atomic step density from 0.15 to 0.30% after several thermal annealings at ∼1200 °C. However, a reduction of step density, accompanied by a change of step height from single-atomic to double-atomic step height, was observed after the sample was annealed to near melting temperature (∼1400 °C). At the same time, the surface was broken into micrograins having a small mosaic orientation. Due to the limited instrumental resolution, these low-density step structures and the small-angle mosaic structures cannot be resolved by using the conventional low-energy electron diffraction technique.
Thermal properties of AlAs/GaAs superlattices
Authors: Yao Takafumi
Published: July 2002

Thermal properties of semiconductor superlattices have been investigated for the first time. The thermal properties of AlAs/GaAs superlattices were measured with the ac calorimetric method. It is found that the thermal diffusivity and thermal conductivity of the AlAs/GaAs superlattices are larger than those of the AlGaAs alloy due to the suppression of alloy scattering in the superlattice. However, the thermal diffusivity and thermal conductivity decrease with a decrease in the superlattice period and seem to approach those of the Al0.5Ga0.5As alloy in the limit of short-period superlattices.
Transient photocapacitance and photocurrent studies of undoped hydrogenated amorphous silicon
Authors: Gelatos A. V. , Mahavadi K. K. , Cohen J. D. , Harbison J. P.
Published: July 2002

We have applied transient photocapacitance and transient junction photocurrent measurements to the study of undoped hydrogenated amorphous silicon (a-Si:H) films, and find that the electronic optical transition from the dominant deep defect is very similar in energy to the D−→D0+e optical transition identified in n-type doped a:Si:H films. In addition, we have observed a competing hole thermal transition, and we have obtained estimates of its thermal emission rate and of the thermal gap. We have used the difference between photocapacitance and photocurrent in the valence-band tail region to determine the quantity (μτ)hNT at different temperatures. Finally, we have observed that light-induced metastable defects have a hole capture cross section significantly larger than that of the intrinsic defects.
Antivirally active MxA protein sequesters La Crosse virus nucleocapsid protein into perinuclear complexes
Authors: Kochs Georg , Janzen Christian , Hohenberg Heinz , Haller Otto
Published: July 2002

Bunyaviruses replicate in the cytoplasm of infected cells. New viral particles are formed by budding of nucleocapsids into the Golgi apparatus. We have previously shown that the IFN-induced human MxA protein inhibits bunyavirus replication by an unknown mechanism. Here we demonstrate that MxA binds to the nucleocapsid protein of La Crosse virus (LACV) and colocalizes with the viral protein in cytoplasmic complexes. Electron microscopy revealed that these complexes accumulated in the perinuclear area and consisted of highly ordered fibrillary structures. A similar MxA-mediated redistribution of viral nucleocapsid proteins was detected with other bunyaviruses, such as Bunyamwera virus and Rift Valley fever virus. MxA(E645R), a carboxy-terminal mutant of MxA without antiviral activity against LACV, did not lead to complex formation. Wild-type MxA, but not MxA(E645R), was able to bind to LACV nucleocapsid protein in coimmunoprecipitation assays, demonstrating the importance of the carboxy-terminal effector domain of MxA. These results illustrate an efficient mechanism of IFN action whereby an essential virus component is trapped in cytoplasmic inclusions and becomes unavailable for the generation of new virus particles.
Identification by redox proteomics of glutathionylated proteins in oxidatively stressed human T lymphocytes
Authors: Fratelli Maddalena , Demol Hans , Puype Magda , Casagrande Simona , Eberini Ivano , Salmona Mario , Bonetto Valentina , Mengozzi Manuela , Duffieux Francis , Miclet Emeric , Bachi Angela , Vandekerckhove Joel , Gianazza Elisabetta , Ghezzi Pietro
Published: July 2002

Formation of mixed disulfides between glutathione and the cysteines of some proteins (glutathionylation) has been suggested as a mechanism through which protein functions can be regulated by the redox status. The aim of this study was to identify the proteins of T cell blasts that undergo glutathionylation under oxidative stress. To this purpose, we radiolabeled cellular glutathione with35S, exposed T cells to oxidants (diamide or hydrogen peroxide), and performed nonreducing, two-dimensional electrophoresis followed by detection of labeled proteins by phosphorimaging and their identification by mass spectrometry techniques. We detected several proteins previously not recognized to be glutathionylated, including cytoskeletal proteins (vimentin, myosin, tropomyosin, cofilin, profilin, and the already known actin), enzymes (enolase, aldolase, 6-phosphogluconolactonase, adenylate kinase, ubiquitin-conjugating enzyme, phosphoglycerate kinase, triosephosphate isomerase, and pyrophosphatase), redox enzymes (peroxiredoxin 1, protein disulfide isomerase, and cytochromecoxidase), cyclophilin, stress proteins (HSP70 and HSP60), nucleophosmin, transgelin, galectin, and fatty acid binding protein. Based on the presence of several protein isoforms in control cells, we suggest that enolase and cyclophilin are heavily glutathionylated under basal conditions. We studied the effect of glutathionylation on some of the enzymes identified in the present study and found that some of them (enolase and 6-phosphogluconolactonase) are inhibited by glutathionylation, whereas the enzymatic activity of cyclophilin (peptidylprolyl isomerase) is not. These findings suggest that protein glutathionylation might be a common mechanism for the global regulation of protein functions.
Dynamic interplay between antagonistic pathways controlling the σ
32
level in
Escherichia coli
Authors: Morita Miyo T. , Kanemori Masaaki , Yanagi Hideki , Yura Takashi
Published: July 2002

The heat-shock response in
Escherichia coli
depends
primarily on the transient increase in the cellular level of heat-shock
sigma factor σ
32
encoded by the
rpoH
gene,
which results from both enhanced synthesis and transient stabilization
of normally unstable σ
32
. Heat-induced synthesis of
σ
32
was previously shown to occur at the translation
level by melting the mRNA secondary structure formed within the 5′
coding sequence of
rpoH
including the translation
initiation region. The subsequent decrease in the σ
32
level during the adaptation phase has been thought to involve both
shutoff of synthesis (translation) and destabilization of
σ
32
-mediated by the DnaK–DnaJ chaperones, although
direct evidence for translational repression was lacking. We now show
that the heat-induced synthesis of σ
32
does not shut off
at the translation level by using a reporter system involving
translational coupling. Furthermore, the apparent shutoff was not
observed when the synthesis rate was determined by a very short pulse
labeling (15 s). Examination of σ
32
stability at 10 min
after shift from 30 to 42°C revealed more extreme instability
(
t
1/2
=20 s) than had previously been
thought. Thus, the dynamic change in σ
32
stability during
the heat-shock response largely accounts for the apparent shutoff of
σ
32
synthesis observed with a longer pulse. These results
suggest a mechanism for maintaining the intricate balance between the
antagonistic pathways: the
rpoH
translation as
determined primarily by ambient temperature and the turnover of
σ
32
as modulated by the chaperone (and presumably
protease)-mediated autogenous control.
Activation-dependent changes in receptor distribution and dendritic morphology in hippocampal neurons expressing P2X
2
-green fluorescent protein receptors
Authors: Khakh Baljit S. , Smith W. Bryan , Chiu Chi-Sung , Ju Donghong , Davidson Norman , Lester Henry A.
Published: July 2002

ATP-gated P2X
2
receptors are widely expressed in
neurons, but the cellular effects of receptor activation are unclear.
We engineered functional green fluorescent protein (GFP)-tagged
P2X
2
receptors and expressed them in embryonic hippocampal
neurons, and report an approach to determining functional and total
receptor pool sizes in living cells. ATP application to dendrites
caused receptor redistribution and the formation of varicose hot spots
of higher P2X
2
-GFP receptor density. Redistribution in
dendrites was accompanied by an activation-dependent enhancement of the
ATP-evoked current. Substate-specific mutant T18A P2X
2
-GFP
receptors showed no redistribution or activation-dependent enhancement
of the ATP-evoked current. Thus fluorescent P2X
2
-GFP
receptors function normally, can be quantified, and reveal the dynamics
of P2X
2
receptor distribution on the seconds time scale.
A new strategy to decrease
N
-methyl-
d
-aspartate (NMDA) receptor coactivation: Inhibition of
d
-serine synthesis by converting serine racemase into an eliminase
Authors: Panizzutti Rogério , De Miranda Joari , Ribeiro Cátia S. , Engelender Simone , Wolosker Herman
Published: July 2002

Serine racemase is a brain-enriched enzyme that synthesizes
d
-serine, an endogenous modulator of the
glycine site of
N
-methyl-
d
-aspartate (NMDA)
receptors. We now report that serine racemase catalyzes an elimination
reaction toward a nonphysiological substrate that provides a powerful
tool to study its neurobiological role and will be useful to develop
selective enzyme inhibitors. Serine racemase catalyzes robust
elimination of
l
-serine
O
-sulfate that is
500 times faster than the physiological racemization reaction,
generating sulfate, ammonia, and pyruvate. This reaction provides the
most simple and sensitive assay to detect the enzyme activity so far.
We establish stable cell lines expressing serine racemase and show that
serine racemase can also be converted into a powerful eliminase in
cultured cells, while the racemization of
l
-serine is
inhibited. Likewise,
l
-serine
O
-sulfate
inhibits the synthesis of
d
-serine in primary astrocyte
cultures. We conclude that the synthetic compound
l
-serine
O
-sulfate is a better substrate than
l
-serine as well as an inhibitor of
d
-serine
synthesis. Inhibition of serine racemase provides a new strategy to
selectively decrease NMDA receptor coactivation and may be useful in
conditions in which overstimulation of NMDA receptors plays a
pathological role.
Carbocyclic fatty acids in plants: Biochemical and molecular genetic characterization of cyclopropane fatty acid synthesis of
Sterculia
foetida
Authors: Bao Xiaoming , Katz Sue , Pollard Mike , Ohlrogge John
Published: July 2002

Fatty acids containing three-member carbocyclic rings are found in bacteria and plants. Bacteria synthesize cyclopropane fatty acids (CPA-FAs) only by the addition of a methylene group from
S
-adenosylmethionine to the
cis
-double bond of monoenoic phospholipid-bound fatty acids. In plants CPA-FAs are usually minor components with cyclopropene fatty acids (CPE-FAs) more abundant.
Sterculia foetida
seed oil contains 65–78% CPE-FAs, principally sterculic acid. To address carbocyclic fatty acid synthesis in plants, a cDNA library was constructed from developing seeds during the period of maximum oil deposition. About 0.4% of 5,300 expressed sequence tags were derived from one gene, which shared similarities to the bacterial CPA-FA synthase. However, the predicted protein is twice as large as the bacterial homolog and represents a fusion of an FAD-containing oxidase at the N terminus and a methyltransferase at the C terminus. Functional analysis of the isolated full-length cDNA was conducted in tobacco suspension cells where its expression resulted in the accumulation of up to 6.2% dihydrosterculate of total fatty acids. In addition, the dihydrosterculate was specifically labeled by [
methyl
-
14
C]methionine and by [
14
C]oleic acid in the transgenic tobacco cells. In
in vitro
assay of
S. foetida
seed extracts,
S
-adenosylmethionine served as a methylene donor for the synthesis of dihydrosterculate from oleate. Dihydrosterculate accumulated largely in phosphatidylcholine in both systems. Together, a CPA-FA synthase was identified from
S. foetida
, and the pathway in higher plants that produce carbocyclic fatty acids was defined as by transfer of C
1
units, most likely from
S
-adenosylmethionine to oleate.
Loss of the membrane anchor of the target receptor is a mechanism of bioinsecticide resistance
Authors: Darboux Isabelle , Pauchet Yannick , Castella Claude , Silva-Filha Maria Helena , Nielsen-LeRoux Christina , Charles Jean-François , Pauron David
Published: July 2002

The mosquitocidal activity of
Bacillus sphaericus
is because of a binary toxin (Bin), which binds to
Culex pipiens
maltase 1 (Cpm1), an α-glucosidase present in the midgut of
Culex pipiens
larvae. In this work, we studied the molecular basis of the resistance to Bin developed by a strain (GEO) of
C. pipiens
. Immunohistochemical and
in situ
hybridization experiments showed that Cpm1 was undetectable in the midgut of GEO larvae, although the gene was correctly transcribed. The sequence of the
cpm1

GEO

cDNA differs from the sequence we previously reported for a susceptible strain (
cpm1

IP

) by seven mutations: six missense mutations and a mutation leading to the premature termination of translation. When produced in insect cells, Cpm1
IP
was attached to the membrane by a glycosylphosphatidylinositol (GPI). In contrast, the premature termination of translation of Cpm1
GEO
resulted in the targeting of the protein to the extracellular compartment because of truncation of the GPI-anchoring site. The interaction between Bin and Cpm1
GEO
and the enzyme activity of the receptor were not affected. Thus, Bin is not toxic to GEO larvae because it cannot interact with the midgut cell membrane, even though its receptor site is unaffected. This mechanism contrasts with other known resistance mechanisms in which point mutations decrease the affinity of binding between the receptor and the toxin.
FGF-2 regulation of neurogenesis in adult hippocampus after brain injury
Authors: Yoshimura Shinichi , Takagi Yasushi , Harada Jun , Teramoto Tetsuyuki , Thomas Sunu S. , Waeber Christian , Bakowska Joanna C. , Breakefield Xandra O. , Moskowitz Michael A.
Published: July 2002

Fibroblast growth factor-2 (FGF-2) promotes proliferation of
neuroprogenitor cells in culture and is up-regulated within brain after
injury. Using mice genetically deficient in FGF-2
(FGF-2
−/−
mice), we addressed the importance of
endogenously generated FGF-2 on neurogenesis within the
hippocampus, a structure involved in spatial, declarative, and
contextual memory, after seizures or ischemic injury. BrdUrd
incorporation was used to mark dividing neuroprogenitor cells and NeuN
expression to monitor their differentiation into neurons. In the
wild-type strain, hippocampal FGF-2 increased after either kainic acid
injection or middle cerebral artery occlusion, and the numbers of
BrdUrd/NeuN-positive cells significantly increased on days 9 and 16
as compared with the controls. In FGF-2
−/−
mice, BrdUrd
labeling was attenuated after kainic acid or middle cerebral artery
occlusion, as was the number of neural cells colabeled with both BrdUrd
and NeuN. After FGF-2
−/−
mice were injected
intraventricularly with a herpes simplex virus-1 amplicon vector
carrying FGF-2 gene, the number of BrdUrd-labeled cells increased
significantly to values equivalent to wild-type littermates after
kainate seizures. These results indicate that endogenously
synthesized FGF-2 is necessary and sufficient to stimulate
proliferation and differentiation of neuroprogenitor cells in the adult
hippocampus after brain insult.
MRP9, an unusual truncated member of the ABC transporter superfamily, is highly expressed in breast cancer
Authors: Bera Tapan K. , Iavarone Carlo , Kumar Vasantha , Lee Sanghyuk , Lee Byungkook , Pastan Ira
Published: July 2002

Combining a computer-based screening strategy and functional genomics, we previously identified MRP9 (ABCC12), a member of the ATP-binding cassette (ABC) superfamily. We now show that the gene has two major transcripts of 4.5 and 1.3 kb. In breast cancer, normal breast, and testis, the
MRP9
gene transcript is 4.5 kb in size and encodes a 100-kDa protein. The protein is predicted to have 8 instead of 12 membrane-spanning regions. When compared with closely related ABC family members, it lacks transmembrane domains 3, 4, 11, and 12 and the second nucleotide-binding domain. In other tissues including brain, skeletal muscle, and ovary, the transcript size is 1.3 kb. This smaller transcript encodes a nucleotide-binding protein of ≈25 kDa in size. An
in situ
hybridization study shows that the 4.5-kb transcript is expressed in the epithelial cells of breast cancer. An antipeptide antibody designed to react with the amino terminus of the protein detects a 100-kDa protein in testis and the membrane fraction of a breast cancer cell line. Because the 4.5-kb RNA is highly expressed in breast cancer and not expressed at detectable levels in essential normal tissues, MRP9 could be a useful target for the immunotherapy of breast cancer. Because of the unusual topology of the two variants of
MRP9
, we speculate that they may have a different function from other family members.