bluuebunny/crossref_metadata_2025 huggingface dataset:
Film flow down an inclined plane over a three-dimensional obstacle
Authors: Blyth M. G. , Pozrikidis C.
Published: August 2006

The low-Reynolds-number flow of a liquid film down an inclined plane wall over a particle attached to the wall is considered. The effect of the particle is described on the assumption that the free surface suffers only a minor disturbance from its basic flat state, and the disturbance velocity over the free surface is small, even though the particle size may not be small compared to the unperturbed film thickness. The problem is formulated using the boundary integral method for Stokes flow to describe the surface velocity, the deformation of the film surface, and the distribution of the traction over the particle surface. Results are presented for small particles following the earlier asymptotic analysis of Pozrikidis and Thoroddsen, and for moderate-sized particles with semispherical, spherical, and semispheroidal shapes. The simulations reveal that, in all cases, the free surface causes an upstream hump and a horseshoe type of deformation downstream whose intensity depends on the Bond number and is largely insensitive to the specific particle shape.
Plane Poiseuille flow of a sedimenting suspension of Brownian hard-sphere particles: Hydrodynamic stability and direct numerical simulations
Authors: Yiantsios Stergios G.
Published: August 2006

The pressure driven flow of a suspension of sedimenting Brownian hard-sphere particles in a plane channel is considered. The balance of gravity and Brownian forces leads to a stationary state where a concentration profile of the particles is established in the channel, with a transition from a viscous sediment to clear fluid. The hydrodynamic stability of the flow and the nonlinear evolution of unstable disturbances are studied numerically by spectral element/Fourier expansion techniques. Two modes of instability with different characteristics are identified. The first is of the Tollmien-Schlichting type, similar to the one present in single-fluid parallel shear flows. This instability appears at much lower Reynolds numbers than for a single fluid when the transition in viscosity is gradual and the sediment is receptive to the fluid motion in the bulk, that is, for relatively small colloidal particles well into the submicron range. An interesting feature, observed through three-dimensional numerical simulations, is the formation of longitudinal striation patterns in the sediment, reminiscent of drag reducing surfaces with organized roughness known as riblets. The second type of instability is similar to the interfacial instability in stratified shear flows with a jump in viscosity. This type of instability appears also at low Reynolds numbers, when the transition from sediment to clear fluid is sharper, that is, for relatively larger particles. This instability results in slow waves traveling with velocities characteristic of those in the sediment, and gives rise to a significant resuspension and formation of low concentration regions in the vicinity of the sediment. An interesting feature is the formation of patterns in the sediment reminiscent of sand ripples.
Evaporation induced self assembly and rheology change during sol-gel coating
Authors: Lee Chang H. , Lu Yunfeng , Shen Amy Q.
Published: August 2006

Thin films with self-assembled nanostructures are important in applications such as catalysis and biosensor technology. A major technique used to prepare such films is sol-gel processing. This technique involves depositing a complex fluid on a substrate by dip, spin, or spray coating, followed by allowing the film to evaporate and form self-assembled nanostructures. Since the composition of the film during coating is central to understanding how changing chemical and physical conditions affect the properties and microstructures of the films, we investigate the rheological properties of the entrained fluid film and its subsequent impact on the steady state film thickness during the evaporation-induced self-assembly process. We perform systematic experiments to measure the meniscus shape and film thickness during sol-gel dip coating. We observe that the experimental data of film thickness lie way below the Landau-Levich-Derjaguin prediction from the classic film-coating results. To explain this discrepancy, we present a thin film model based on the lubrication approximation with an evaporation effect. Our results show that evaporation-induced self-assembly leads to rheological variations in the entrained film and, consequently, alters the film thickness. The predicted film thickness based on the evaporation-induced rheology variation model compares well with the experiments.
Electrospinning of viscoelastic Boger fluids: Modeling and experiments
Authors: Carroll Colman P. , Joo Yong Lak
Published: August 2006

An experimental and theoretical investigation of electrospun Newtonian and viscoelastic jets is presented. In particular, the effect of electrical conductivity and viscoelasticity on the jet profile during the initial stage of electrospinning is examined. In the theoretical study, the fluid is described as a leaky dielectric with charges only on the jet surface and viscoelastic models for polymer solutions such as Oldroyd-B and FENE-P are fully coupled with the fluid momentum equations and Gauss’ law. A theoretical model for the jet is derived using a thin filament approximation, and the resulting differential equations governing electrically charged, stable polymeric jets are solved numerically. Two different experimental systems are considered: Newtonian solutions of glycerol containing trace amounts of lithium chloride salt, and viscoelastic PIB/PB Boger fluid solutions. The experimental jet profiles from electrospinning experiments are compared with the model predictions. Our results reveal that increasing the electrical conductivity of the fluid by adding salt tends to delay the jet thinning. Increasing the fluid viscoelasticity causes a more rapid initial jet thinning, however further away from the spinneret viscoelastic jets are thicker than their Newtonian counterparts due to the higher elongational viscosity. We also investigate whether the general trends observed for these fluids can be applied to predict the qualitative behavior for the spinning of other fluid systems such as PEO/water solutions that have high conductivity and viscoelasticity.
Short-wave centrifugal instability in the vicinity of vanishing total vorticity streamlines
Authors: Gallaire F. , Rousset F.
Published: August 2006

The purpose of the present Brief Communication is to show under which circumstances the criterion conjectured by Sipp and Jacquin [Phys. Fluids 12, 1740 (2000)] relative to the stability of incompressible inviscid planar rotating basic flows with closed streamlines is valid.
The physics of vortex merger: Further insight
Authors: Brandt Laura K. , Nomura Keiko K.
Published: August 2006

Results of numerical simulations provide further insight on the physical mechanisms associated with symmetric vortex merger. The relative contributions of filament and exchange band fluid to the reduction in the vortex separation are determined and the latter is found to be dominant. A key underlying process is the interaction of strain rate and vorticity gradient near the center of rotation, through which a tilt in the vorticity contours is established. This leads to the entrainment of core fluid into the exchange band, which is transformed into a single vortex.
Relationship between stirring rate and Reynolds number in the chaotically advected steady flow in a container with exactly counter-rotating lids
Authors: Lackey Tahirih C. , Sotiropoulos Fotis
Published: August 2006

We solve numerically the three-dimensional incompressible Navier-Stokes equations to simulate the flow in a cylindrical container of aspect ratio one with exactly counter-rotating lids for a range of Reynolds numbers for which the flow is steady and three dimensional (300⩽Re⩽850). In agreement with linear stability results [C. Nore et al., J. Fluid Mech. 511, 45 (2004)] we find steady, axisymmetric solutions for Re<300. For Re>300 the equatorial shear layer becomes unstable to steady azimuthal modes and a complex vortical flow emerges, which consists of cat’s eye radial vortices at the shear layer and azimuthally inclined axial vortices. Upon the onset of the three-dimensional instability the Lagrangian dynamics of the flow become chaotic. A striking finding of our work is that there is an optimal Reynolds number at which the stirring rate in the chaotically advected flow is maximized. Above this Reynolds number, the integrable (unmixed) part of the flow begins to grow and the stirring rate is shown conclusively to decline. This finding is explained in terms of and appears to support a recently proposed theory of chaotic advection [I. Mezić, J. Fluid Mech. 431, 347 (2001)]. Furthermore, the calculated rate of decay of the stirring rate with Reynolds numbers is consistent with the Re−1∕2 upper bound predicted by the theory.
Experimental verification of Type-II-eigenmode destabilization in the boundary layer over a compliant rotating disk
Authors: Colley Andrew J. , Carpenter Peter W. , Thomas Peter J. , Ali Reza , Zoueshtiagh Farzam
Published: August 2006

Destabilization of the Type-II eigenmode in boundary layers over compliant rotating disks was predicted theoretically by Cooper and Carpenter [J. Fluid Mech. 350, 231 (1997)]. Their results showed that for relatively low levels of compliance the Type-II eigenmode was destabilized, to be stabilized and ultimately eliminated for higher levels of compliance. The goal of the present study was to obtain the first experimental verification of the prediction that the Type-II mode can be destabilized at low levels of compliance. To this end a new type of rotating-disk apparatus was designed and a new type of material was used to produce suitable compliant walls for the experiments. Background noise in the new facility is substantially reduced in comparison with that in facilities used in related previous studies. This enabled the detection of substantially cleaner hot-film signals. Although the mean base flow remained unchanged, noise characteristics have been improved and turbulence intensities are significantly reduced. The measurements reveal not only the comparatively strong signals from the Type-I (cross-flow vortices) instability mode but also clear evidence of the Type-II eigenmode. In agreement with the theory of Cooper and Carpenter the data analysis shows that relatively low levels of wall compliance destabilize the Type-II mode.
Identification and analysis of coherent structures in the near field of a turbulent unconfined annular swirling jet using large eddy simulation
Authors: García-Villalba M. , Fröhlich J. , Rodi W.
Published: August 2006

Large eddy simulations of incompressible turbulent flow in an unconfined annular swirling jet at Reynolds number 81500 are reported, based on the outer radius of the jet. The results are in excellent agreement with experimental data for mean flow, turbulent statistics, and power spectral densities of velocity fluctuations. Two dominant families of large-scale coherent structures are identified in the flow. Both are orthogonal to the mean three-dimensional streamlines, which suggests that they are formed as the result of a Kelvin-Helmholtz instability. Instantaneous vortex structures as well as different types of spectra and two-point correlations are presented to further elucidate the properties of the flow.
The destabilizing effects of wall permeability in channel flows: A linear stability analysis
Authors: Tilton Nils , Cortelezzi Luca
Published: August 2006

We perform a three-dimensional linear stability analysis of a laminar flow in a channel delimited by rigid, homogeneous, isotropic, porous blocks sealed by impermeable walls. We consider porous materials with small permeability and neglect inertial effects in the porous flow. We solve the coupled linear stability problem, arising from the adjacent channel and porous flows, using a spectral collocation technique. We find that very small amounts of wall permeability can dramatically decrease the stability of the channel flow with respect to Poiseuille flow. We also observe two new wall modes and a new class of modes in the porous regions.
Capillary pressure spectrometry: Toward a new method for the measurement of the fractional wettability of porous media
Authors: Sygouni Varvara , Tsakiroglou Christos D. , Payatakes Alkiviades C.
Published: August 2006

A transparent porous medium of controlled fractional wettability is fabricated by mixing intermediate-wet glass microspheres with strongly oil-wet polytetrafluouroethylene microspheres, and packing them between two transparent glass plates. Silicon oil is displaced by water, the growth pattern is video-recorded, and the transient response of the pressure drop across the pore network is measured for various fractions of oil-wet particles. The measured global capillary pressure fluctuates as the result of the variation of the equilibrium curvature of menisci between local maxima and local minima. With the aid of wavelets, the transient response of the capillary pressure is transformed to a capillary pressure spectrum (CPS). The peaks of the CPS are used to identify the most significant flow events and correlate their amplitude with the spatial distribution of fractional wettability. The flow events are closely related with the fluctuations of the capillary pressure and are classified into three main categories: motion in pore clusters, generation/expansion of capillary fingers, coalescence of interfaces. The amplitude of the peaks of CPS is related quasilinearly with a local coefficient of fractional wettability presuming that the same class of flow events is concerned. Approximate calculations of the maximum meniscus curvature in pores of converging-diverging geometry and uniform wettability in combination with simple mixing laws predict satisfactorily the experimentally measured average prebreakthrough capillary pressure as a function of the fraction of the oil-wet particles.
Turbulence characteristics of the Bödewadt layer in a large enclosed rotor-stator system
Authors: Randriamampianina Anthony , Poncet Sébastien
Published: August 2006

A three-dimensional (3D) direct numerical simulation is combined with a laboratory study to describe the turbulent flow in an enclosed annular rotor-stator cavity characterized by a large aspect ratio G=(b−a)∕h=18.32 and a small radius ratio a∕b=0.152, where a and b are the inner and outer radii of the rotating disk and h is the interdisk spacing. The rotation rate Ω under consideration is equivalent to the rotational Reynolds number Re=Ωb2∕ν=9.5×104, where ν is the kinematic viscosity of the fluid. This corresponds to a value at which an experiment carried out at the laboratory has shown that the stator boundary layer is turbulent, whereas the rotor boundary layer is still laminar. Comparisons of the 3D computed solution with velocity measurements have given good agreement for the mean and turbulent fields. The results enhance evidence of weak turbulence at this Reynolds number, by comparing the turbulence properties with available data in the literature [M. Lygren and H. I. Andersson, J. Fluid Mech. 426, 297 (2001)]. An approximately self-similar boundary layer behavior is observed along the stator side. The reduction of the structural parameter a1 under the typical value 0.15 and the variation in the wall-normal direction of the different characteristic angles show that this boundary layer is three-dimensional. A quadrant analysis [H. S. Kang, H. Choi, and J. Y. Yoo, Phys. Fluids 10, 2315 (1998)] of conditionally averaged velocities is performed to identify the contributions of different events (ejections and sweeps) on the Reynolds shear stress producing vortical structures. The asymmetries observed in the conditionally averaged quadrant analysis are dominated by Reynolds stress-producing events in this Bödewadt layer. Moreover, case 1 vortices (with a positive wall induced velocity) are found to be the major source of generation of special strong events, in agreement with the conclusions of Lygren and Andersson [J. Fluid Mech. 426, 297 (2001)].
Averaged dynamics of two-phase media in a vibration field
Authors: Straube Arthur V. , Lyubimov Dmitry V. , Shklyaev Sergey V.
Published: August 2006

The averaged dynamics of various two-phase systems in a high-frequency vibration field is studied theoretically. The continuum approach is applied to describe such systems as solid particle suspensions, emulsions, bubbly fluids, when the volume concentration of the disperse phase is small and gravity is insignificant. The dynamics of the disperse system is considered by means of the method of averaging, when the fast pulsation and slow averaged motion can be treated separately. Two averaged models for both nondeformable and deformable particles, when the compressibility of the disperse phase becomes important, are obtained. A criterion when the compressibility of bubbles cannot be neglected is figured out. For both cases the developed models are applied to study the averaged dynamics of the disperse media in an infinite plane layer under the action of transversal vibration.
On the asymptotic solution of the Maxey-Riley equation
Authors: Mograbi E. , Bar-Ziv E.
Published: August 2006

The Maxey-Riley [Phys. Fluids 26, 883 (1983)] particle equation of motion is considered without the history term and for an asymptotically small Stokes number. The equation admits a globally attractive invariant manifold identified as the Eulerian particle velocity field asymptotically close to the unperturbed fluid velocity field, thus suppressing the inconsequential initial transients. A recursive asymptotic scheme is obtained for the calculation of the invariant manifold in any order of accuracy. The dimension of the particle equation on the invariant manifold is reduced by half, which considerably facilitates the analysis of its motion in physical space. Structural stability theory provides comprehensive qualitative description of the particle motion.
Analysis of the wall pressure trace downstream of a separated region using extended proper orthogonal decomposition
Authors: Hoarau C. , Borée J. , Laumonier J. , Gervais Y.
Published: August 2006

Multipoint wall pressure measurements and Laser Doppler Velocimetry (LDV) measurements of the velocity field generated over a 30° forward facing ramp are discussed in this paper. Mean and fluctuating signals are analyzed both in the mean separated region and in the recovering boundary layer flow. We present an analysis based on proper orthogonal decomposition (POD). The choice of a “POD region” as a subset of the sensor array is discussed. In a “POD region” restricted to the mean separated region, we show that the spatio-temporal coherence of the signal is efficiently captured by the POD decomposition. Extended POD is then introduced to extract in an objective way the part of the surface pressure signal, downstream of the mean reattachment point, which is correlated with the pressure signal under the separated region. The properties of this decomposition are demonstrated. We show that the power spectral density of the correlated part always displays a peak in the frequency domain associated with vortex shedding while the high frequency region is dominated by the uncorrelated part. A physical model is proposed to link large scale vortex structures and the correlated surface pressure fluctuations. We prove that the correlated pressure signal downstream of the mean separation length carries the properties of the distorted mixing layer eddies being advected and dissipating into the outer boundary layer flow. Finally, the spectral content of the correlated pressure signal is studied and we show that the first extended mode is able to capture the normalized power spectral density of the correlated pressure accurately. The method proposed here is therefore able to extract the contribution of separated shear layer eddies from the complex pressure trace in the recovering boundary layer near the mean reattachment point. The fast convergence of the POD decomposition in the separated region and the properties of the first extended mode downstream of the reattachment may be used to obtain an efficient reduced description for control or analysis purposes.
Interaction of dipolar vortices with a step-like topography
Authors: Tenreiro M. , Zavala Sansón L. , van Heijst G. J. F.
Published: August 2006

The interaction of a barotropic, dipolar vortex with a step-like topography is studied by means of laboratory experiments in a rotating tank and by numerical simulations based on a quasi-two-dimensional model. Two main configurations are analyzed: when the dipole approaches a “low” or a “high” step with respect to the maximum water depth. For relatively low steps, the vortex crosses the topography with a deflected trajectory, while maintaining its dipolar structure. The sense of this deflection depends on whether the dipole reaches a step-up or a step-down. For high steps, in contrast, the dipole is not able to cross the topography, and the reflection of one of the dipole structures is observed. In both cases, one observes a weak flow along the topography with shallow water on its right. The essential features of the flow evolution for low and high steps can be explained by using arguments of potential vorticity conservation (due to the weakness of viscous effects). In order to determine whether a barotropic dipole is able to cross the step or is reflected, qualitative criteria based on the step height and dipole strength are derived using inviscid arguments.
Passive flow control in microdevices using thermally responsive polymer solutions
Authors: Stoeber Boris , Hu Che-Ming Jack , Liepmann Dorian , Muller Susan J.
Published: August 2006

Thermally responsive polymer solutions have previously been used in microchannels with integrated heaters for active valving. Here we present a detailed study of the rheological behavior of these complex fluids, suggesting the broad range of conditions under which active valving could be used, along with an investigation of their use for passive microflow control in which viscous heating leads to channel blockage. We discuss the scaling of viscous heating in the channels under constant flow rate and constant pressure drop conditions. In the case of pressure-driven flow through a microchannel, we demonstrate that viscous heating can cause periodic gel formation leading to flow instabilities. Velocity fields above and below the gel temperature were mapped using digital particle image velocimetry; above the gel temperature, flow fluctuations may be caused by the competing mechanisms of thermothickening and shear-thinning.
Euler closure of the Boltzmann equations for resonant bimolecular reactions
Authors: Groppi M. , Spiga G. , Zus F.
Published: August 2006

The problem of a rigorous hydrodynamic closure at the Euler level of the complicated set of integro-differential nonlinear Boltzmann-type equations, describing the evolution of a chemically reactive gas mixture at the kinetic level, is addressed. The case in which the process is driven by both elastic scattering and resonant transitions is worked out and discussed by a formal mathematical procedure based on typical asymptotic methods of kinetic theory.
A simple model for liquid metal electric current limiters
Authors: Boeck Thomas , Thess André , Terhoeven Peter
Published: August 2006

We formulate an analytical model for the dynamics of an infinitely thin horizontal liquid metal sheet carrying a dc electric current and connected to a vertical slot. This problem is relevant to electrical engineering and represents the greatest possible simplification of a model considered recently by Thess et al. [J. Fluid Mech527, 67 (2005)]. It is demonstrated that the behavior of the fluid under the action of the electromagnetic forces is completely described by a nonlinear ordinary differential equation for the fluid height which can be solved analytically. We show that the system has a single stable solution as long as the electrical current I is below a critical value Ic, whereas the system exhibits pinch-off for I>Ic. Moreover, the pinch-off time is calculated, and the system is shown to have a peculiar behavior close to the singularity time.
A theoretical prediction of friction drag reduction in turbulent flow by superhydrophobic surfaces
Authors: Fukagata Koji , Kasagi Nobuhide , Koumoutsakos Petros
Published: August 2006

We present a theoretical prediction for the drag reduction rate achieved by superhydrophobic surfaces in a turbulent channel flow. The predicted drag reduction rate is in good agreement with results obtained from direct numerical simulations at Reτ≃180 and 400. The present theory suggests that large drag reduction is possible also at Reynolds numbers of practical interest (Reτ∼105–106) by employing a hydrophobic surface, which induces a slip length on the order of ten wall units or more.
Solitary water wave interactions
Authors: Craig W. , Guyenne P. , Hammack J. , Henderson D. , Sulem C.
Published: August 2006

This article concerns the pairwise nonlinear interaction of solitary waves in the free surface of a body of water lying over a horizontal bottom. Unlike solitary waves in many completely integrable model systems, solitary waves for the full Euler equations do not collide elastically; after interactions, there is a nonzero residual wave that trails the post-collision solitary waves. In this report on new numerical and experimental studies of such solitary wave interactions, we verify that this is the case, both in head-on collisions (the counterpropagating case) and overtaking collisions (the copropagating case), quantifying the degree to which interactions are inelastic. In the situation in which two identical solitary waves undergo a head-on collision, we compare the asymptotic predictions of Su and Mirie [J. Fluid Mech. 98, 509 (1980)] and Byatt-Smith [J. Fluid Mech. 49, 625 (1971)], the wavetank experiments of Maxworthy [J. Fluid Mech. 76, 177 (1976)], and the numerical results of Cooker, Weidman, and Bale [J. Fluid Mech. 342, 141 (1997)] with independent numerical simulations, in which we quantify the phase change, the run-up, and the form of the residual wave and its Fourier signature in both small- and large-amplitude interactions. This updates the prior numerical observations of inelastic interactions in Fenton and Rienecker [J. Fluid Mech. 118, 411 (1982)]. In the case of two nonidentical solitary waves, our precision wavetank experiments are compared with numerical simulations, again observing the run-up, phase lag, and generation of a residual from the interaction. Considering overtaking solitary wave interactions, we compare our experimental observations, numerical simulations, and the asymptotic predictions of Zou and Su [Phys. Fluids 29, 2113 (1986)], and again we quantify the inelastic residual after collisions in the simulations. Geometrically, our numerical simulations of overtaking interactions fit into the three categories of Korteweg-deVries two-soliton solutions defined in Lax [Commun. Pure Appl. Math. 21, 467 (1968)], with, however, a modification in the parameter regime. In all cases we have considered, collisions are seen to be inelastic, although the degree to which interactions depart from elastic is very small. Finally, we give several theoretical results: (i) a relationship between the change in amplitude of solitary waves due to a pairwise collision and the energy carried away from the interaction by the residual component, and (ii) a rigorous estimate of the size of the residual component of pairwise solitary wave collisions. This estimate is consistent with the analytic results of Schneider and Wayne [Commun. Pure Appl. Math. 53, 1475 (2000)], Wright [SIAM J. Math. Anal. 37, 1161 (2005)], and Bona, Colin, and Lannes [Arch. Rat. Mech. Anal. 178, 373 (2005)]. However, in light of our numerical data, both (i) and (ii) indicate a need to reevaluate the asymptotic results in Su and Mirie [J. Fluid Mech. 98, 509 (1980)] and Zou and Su [Phys. Fluids 29, 2113 (1986)].
Intermittency in passive scalar turbulence under the uniform mean scalar gradient
Authors: Watanabe Takeshi , Gotoh Toshiyuki
Published: August 2006

Small scale statistics of a passive scalar convected by turbulence under the uniform mean scalar gradient is studied by high-resolution direct numerical simulation. It is found that the scaling exponents of the structure functions of scalar increments in parallel and perpendicular directions to the mean gradient are the same and saturate approximately 1.3 at large order, and that they are dependent on scalar injection scheme at large scales within the Reynolds numbers studied. Tails of the probability density functions for the scalar increment in the inertial convective range are well fitted by a scaling form inferred from the saturation and the tail of the one point scalar probability density function.
Controlling electrohydrodynamic pumping in microchannels through defined temperature fields
Authors: Felten Maika , Geggier Peter , Jäger Magnus , Duschl Claus
Published: August 2006

An electrohydrodynamic micropump driven by a high-frequency traveling electric wave (1–10MHz) at low voltage (4–10V) is presented. The pumping rate is dependent on the profile (inhomogeneities) of the dielectric properties of the medium. Defined temperature fields serve as a versatile mean for the control of these profiles. We produce temperature fields in two ways: by the electric wave itself and by external Peltier elements. We investigated experimentally the influence of two-dimensional and three-dimensional electrode arrays on the pumping behavior and compared it to theoretical predictions of temperature field calculations using finite element analysis. The two results are in good agreement. Furthermore, we demonstrate the effect of an additional external temperature field that results in an advanced performance of the pump.
Quasilinear calculation of Langmuir wave generation and beam propagation in the presence of density fluctuations
Authors: Li B. , Robinson P. A. , Cairns I. H.
Published: August 2006

The generation of beam-driven Langmuir waves and the propagation of an electron beam in the presence of ambient density fluctuations are numerically studied using quasilinear calculations in one spatial dimension. The random spatiotemporal density fluctuations are driven externally as ion-sound-like turbulence. The effects of Langmuir wave scattering off density inhomogeneities in three spatial dimensions are represented through effective damping of the Langmuir waves, and are included in the quasilinear model. The numerical results are explored for illustrative parameters, and Langmuir wave field statistics are compared with stochastic growth theory (SGT) predictions. Due to the combined effects of quasilinear interaction with the beam and scattering off density fluctuations, the Langmuir waves show burstiness and the levels are generally lower than when the density is homogeneous, qualitatively consistent with previous predictions. Apart from early evolution, the average beam speed is approximately the same as in the homogeneous case, but relaxation of the beam is significantly retarded. Both features are in qualitative agreement with earlier predictions. Moreover, the beam distribution function displays relatively smooth variations, which implies that the burstiness in the wave levels originates predominantly from the randomness in the damping rate due to density perturbations, rather than from the stochasticity in the beam growth rate. The statistics of the Langmuir wave field show good agreement with SGT predictions, thus indicating the beam-Langmuir wave system is in a SGT state. Furthermore, variations of the density fluctuation parameters are found to affect the evolution of both beam and Langmuir waves.
Integration of polymer light-emitting diode and polymer waveguide on Si substrate
Authors: Lin Yuan-Yu , Cheng Chung , Liao Hua-Hsien , Horng Sheng-Fu , Meng Hsin-Fei , Hsu Chain-Shu
Published: August 2006

We integrate a polymer light-emitting diode (PLED) and a polymer waveguide on a Si substrate. The light emitted from the PLED is coupled to the waveguide by a diffuser and a reflection layer with coupling efficiency about 1%. There is no delay nor distortion between PLED emission and the light propagation in the waveguide. Good direct modulation characteristics of the waveguide output are demonstrated up to 200kHz. The device structure and processes are based on easy spin coating and are compatible to Si technology.
Instrumentation and analytical methods of an x-ray photoelectron spectroscopy–scanning tunneling microscopy surface analysis system for studying nanostructured materials
Authors: Lahtonen K. , Lampimäki M. , Jussila P. , Hirsimäki M. , Valden M.
Published: August 2006

The design and performance of an x-ray photoelectron spectroscopy (XPS)–scanning tunneling microscopy (STM) surface analysis system for studying nanostructured materials are described. The analysis system features electron spectroscopy methods (XPS and Auger electron spectroscopy) in addition to a variable temperature STM. With the analytical methods of the system, surface chemical analysis as well as surface morphology down to atomic resolution can be obtained. The system also provides facilities for sample cleaning, annealing, gas dosing, depth profiling, and surface modifications by sputtering and evaporation. Controlled gas exposures from ultrahigh vacuum to atmospheric pressures in the adjustable temperature range of 120–1100K can be carried out in different chambers. A fast entry air lock allows the transfer of samples and STM tips into the system without air exposures. The surface analysis system uses a common sample holder in all five chambers which are independently pumped and separated from each other by gate valves. Thus, it is possible to make all sample preparations and experiments in situ under well-defined conditions as illustrated by the formation and characterization of strained, self-assembled nano-oxides on Cu(100).
Thermal stability of amorphous LaScO3 films on silicon
Authors: Edge L. F. , Schlom D. G. , Rivillon S. , Chabal Y. J. , Agustin M. P. , Stemmer S. , Lee T. , Kim M. J. , Craft H. S. , Maria J.-P. , Hawley M. E. , Holländer B. , Schubert J. , Eisenbeiser K.
Published: August 2006

The thermal stability of amorphous LaScO3 thin films deposited by molecular-beam deposition directly on (001) Si was investigated by high-resolution transmission electron microscopy (HRTEM), transmission infrared absorption spectroscopy (IRAS), and x-ray diffraction (XRD). IRAS indicated that the as-deposited films contained <0.1Å of SiO2 at the interface between LaScO3 and silicon. XRD studies showed that the films remained amorphous after annealing in N2 at 700°C, although HRTEM showed structural order on an ∼1nm length scale even in the as-deposited films. By 800°C, the LaScO3 had started to crystallize and formed a ∼5nm thick Sc-deficient interlayer between it and silicon.
Potential distribution in an ion sheath of non-Maxwellian plasma
Authors: Gurovich V. Tz. , Gleizer J. Z. , Bliokh Yu. , Krasik Ya. E.
Published: August 2006

A model is suggested that shows that in a plasma with a group of fast (non-Maxwellian) electrons, a nonmonotonic distribution of the potential can be formed inside the ion sheath with the potential maximum larger than the biased electrode potential. It was shown that the excess of the maximum potential above the biased electrode potential value depends on the ratio between the density of non-Maxwellian electrons and the density of the plasma bulk electrons. The results obtained in experiments with a hollow anode plasma source, which is characterized by the existence of a fast electron group, qualitatively agree with the model predictions.
Strong magnetocaloric effects in La1−zCez(Fex−yMnySi1−x)13 at low temperatures
Authors: Fujieda S. , Fujita A. , Kawamoto N. , Fukamichi K.
Published: August 2006

The Curie temperature TC of La1−zCez(Fex−yMnySi1−x)13 is decreased by adjusting the composition, and hence the itinerant-electron metamagnetic transition is observed at 5K in La0.75Ce0.25(Fe0.850Mn0.035Si0.110)13 after zero-field cooling. As a result, the La1−zCez(FexMnySi1−x)13 compounds exhibit strong magnetocaloric effects in relatively low magnetic fields in a wide range of temperatures between about 19 and 180K. Consequently, the La1−zCez(Fex−yMnySi1−x)13 compounds are useful for the magnetic refrigerants working at low temperatures.
Annealing effects on single Shockley faults in 4H-SiC
Authors: Miyanagi Toshiyuki , Tsuchida Hidekazu , Kamata Isaho , Nakamura Tomonori , Nakayama Koji , Ishii Ryousuke , Sugawara Yoshitaka
Published: August 2006

We investigated the annealing effect on single Shockley faults (SSFs) in the SiC epitaxial layers by photoluminescence mapping in combination with high-power laser illumination. Comparing before and after annealing at 350–550°C, it became obvious that annealing results in the shrinking of the faulted area of SSFs. When high-power laser illumination is performed again on the same area annealed at 550°C, the right-angled triangular SSFs reformed into exactly the same features as those before annealing, but the isosceles triangular SSFs did not reform. The annealing temperature to start shrinking the faulted area differs according to the type of SSF.
Magnetomechanical hydraulic-servo apparatus for investigation of magnetomechanical coupling properties of magnetic materials
Authors: Pei Yongmao , Fang Daining
Published: August 2006

An apparatus for carrying out magnetomechanical experiments has been constructed, which allows investigations of the physical and mechanical properties of magnetic materials under the application of coupled uniaxial stress and magnetic fields. The apparatus combines a hydraulically driven mechanical loading system with a dipole electromagnet, which is capable of producing mechanical forces up to 2000kg and magnetic fields up to 2.4T with a pole gap of 100mm. And the mechanical force can be applied in any angle with the magnetic field because the mechanical loading device can be rotated in the pole gap. The apparatus allows the automatic measurement of magnetomechanical parameters under on-line computer control of both the magnetic field and the mechanical force. As an example, the simultaneous measurement of magnetostriction and magnetization at various fixed compressive pre stresses applied in the rod axial axis for Tb0.3Dy0.7Fe1.95 samples are presented.
Evidence for spin injection in a single metallic nanoparticle: A step towards nanospintronics
Authors: Bernand-Mantel A. , Seneor P. , Lidgi N. , Muñoz M. , Cros V. , Fusil S. , Bouzehouane K. , Deranlot C. , Vaures A. , Petroff F. , Fert A.
Published: August 2006

We have fabricated nanometer-sized magnetic tunnel junctions using a conductive tip nanoindentation technique in order to study the transport properties of a single metallic nanoparticle. Coulomb blockade effects show clear evidence for single-electron tunneling through a single 2.5nm Au cluster. The observed magnetoresistance is the signature of spin conservation during the transport process through a nonmagnetic cluster.
Efficient fabrication of field electron emitters from the multiwalled carbon nanotube yarns
Authors: Wei Yang , Weng Ding , Yang Yuanchao , Zhang Xiaobo , Jiang Kaili , Liu Liang , Fan Shoushan
Published: August 2006

An efficient method has been developed to fabricate field electron emitters by cutting a continuous multiwalled carbon nanotube (MWNT) yarn into segments. The cross section of each segment was composed of open-ended MWNTs which serve as field emitters. The emission current can reach several milliamperes, and the emitters can work stably for a long time. The excellent field emission properties are attributed to the large enhancement factor over 100 000, dense emitting centers, and the robust structure. The emitters might be easily manipulated and assembled into devices, such as electron guns, x-ray tubes, pixel tubes, etc.
Performance of an ultrahigh resolution cylindrical grating monochromator undulator beamline
Authors: Song Y. F. , Yuh J.-Y. , Lee Y.-Y. , Chung S. C. , Huang L. R. , Tsuei K.-D. , Perng S. Y. , Lin T. F. , Fung H. S. , Ma C.-I. , Chen C. T. , Tsang K. L.
Published: August 2006

An ultrahigh resolution and high-flux undulator beamline U9-CGM, with a cylindrical grating monochromator and covering an energy range from 5to100eV, has been constructed. Two separate entrance slits are included to provide different incident angles, hence to optimize the overall performance. This beamline delivers an average photon flux of 6×1010photons∕s with the entrance and exit slit openings at 10μm. An energy resolving power of 110 000 with a photon flux of 1×1010photons∕s was obtained at 16eV with the 1800line∕mm grating and at 64eV with the 1600line∕mm grating, with the entrance and exit slit openings at 3 and 5μm, respectively. A differentially pumped gas cell is used to attenuate high-order light by up to four orders of magnitude. This beamline will be a powerful tool for research in ultrahigh resolution spectroscopy of atoms, molecules, and condensed matter.
Application of frequency combs in the measurement of the refractive index of air
Authors: Zhang J. , Lu Z. H. , Menegozzi B. , Wang L. J.
Published: August 2006

We report a new method in the precision measurement of the refractive index of air using a highly unbalanced Michelson interferometer with a femtosecond optical frequency comb as the light source. Standard dry air is filled into a 30m multipass cell, serving as the long arm of the interferometer, while a short arm acts as the reference path. Both time and frequency domain interferograms are recorded to measure the refractive index of air. The deviation of our experimental results with Edlén’s formula is 1.4×10−9 at 800nm. Our experiment has a standard error of 5.2×10−9 at fixed parameters (pressure and temperature). This is achieved by putting the multipass cell into a temperature-stabilized box, and also by locking the interferometer path length with a He–Ne laser. We achieved a temperature stabilization of 0.8mK for 25h. This corresponds to 0.4μm multipass cell length change. The locking of the He–Ne interferometer enables us to achieve 7nm path-length change outside the multipass cell. Combined with accurate measurement of temperature and pressure, we were able to achieve an accuracy of 7.7×10−9.
Advanced feedback control methods in EXTRAP T2R reversed field pinch
Authors: Yadikin D. , Brunsell P. R. , Paccagnella R.
Published: August 2006

Previous experiments in the EXTRAP T2R reversed field pinch device have shown the possibility of suppression of multiple resistive wall modes (RWM). A feedback system has been installed in EXTRAP T2R having 100% coverage of the toroidal surface by the active coil array. Predictions based on theory and the previous experimental results show that the number of active coils should be sufficient for independent stabilization of all unstable RWMs in the EXTRAP T2R. Experiments using different feedback schemes are performed, comparing the intelligent shell, the fake rotating shell, and the mode control with complex feedback gains. Stabilization of all unstable RWMs throughout the discharge duration of td≈10τw is seen using the intelligent shell feedback scheme. Mode rotation and the control of selected Fourier harmonics is obtained simultaneously using the mode control scheme with complex gains. Different sensor signals are studied. A feedback system with toroidal magnetic field sensors could have an advantage of lower feedback gain needed for the RWM suppression compared to the system with radial magnetic field sensors. In this study, RWM suppression is demonstrated, using also the toroidal field component as a sensor signal in the feedback system.
Stability of the resistive wall mode in HBT-EP plasmas
Authors: Fitzpatrick Richard , Bialek James
Published: August 2006

A relatively simple model of the resistive wall mode (RWM) is derived for a large aspect ratio, low β, circular cross section, tokamak plasma, surrounded by a concentric, thin, uniform resistive wall. The model employs uniform toroidal plasma rotation, and includes the following realistic edge dissipation mechanisms: dissipation due to charge-exchange with cold neutrals, and dissipation due to neoclassical flow damping. The model is applied to the HBT-EP tokamak [T. Ivers, E. Eisner, A. Garofalo et al., Phys. Plasmas 3, 1926 (1996)], with the wall parameters determined by fitting to output from the VALEN code [J. Bialek, A. H. Boozer, M. E. Mauel, and G. A. Navratil, Phys. Plasmas 8, 2170 (2001)]. Dissipation due to charge-exchange with cold neutrals is found to be not quite large enough to account for the observed rotational stabilization of the RWM in HBT-EP plasmas. On the other hand, dissipation due to neoclassical flow damping is sufficiently large to explain the observations.
Experimental evidence of domain wall tilting in periodically poled lithium niobate crystals grown by the Czochralski off-center technique
Authors: Bazzan M. , Argiolas N. , Sada C. , Cattaruzza E.
Published: August 2006

Periodically poled lithium niobate crystals are expected to have ferroelectric domain boundaries parallel to the direction of the spontaneous polarization. The authors report, however, that this is not the case for periodic structures grown by the off-center Czochralski technique. By exploiting the high resolution x-ray diffraction technique in reciprocal space mapping mode, the authors demonstrate that the angle between the domain border and the spontaneous polarization directions is different from zero, reaching a value as high as 5°.
Local resistance measurement across grain boundaries in low-temperature polycrystalline silicon layer of thin film transistor using scanning spreading resistance microscopy
Authors: Yamagiwa Hiroto , Abo Satoshi , Wakaya Fujio , Takai Mikio , Sakamoto Tadao , Tokioka Hidetada , Nakagawa Naoki
Published: August 2006

Local resistance distribution in a low-temperature polycrystalline Si layer of a thin film transistor with a lightly doped drain (LDD) structure was investigated using scanning spreading resistance microscopy. The local resistance around the grain boundaries was found to be lower than that at grain insides in the drain, LDD, and channel regions. At the center of the grain boundaries, however, slightly higher resistance part was sandwiched between low resistance regions. Identifying the drain, the LDD, and the channel regions is succeeded by an analysis in which only the lower local resistances were used.
Three-dimensional recording by tightly focused femtosecond pulses in LiNbO3
Authors: Juodkazis Saulius , Sudzius Markas , Mizeikis Vygantas , Misawa Hiroaki , Gamaly Eugene G. , Liu Youwen , Louchev Oleg A. , Kitamura Kenji
Published: August 2006

The authors report on a three-dimensional single-shot optical recording by 150fs pulses at 800nm wavelength in Fe doped LiNbO3. The rewritable bits (2x×2y×8zμm3) are demonstrated. The highest refractive index modulation of ∼10−3 per single pulse has been formed by preferential photovoltaic effect at close to the dielectric breakdown irradiance of ∼TW∕cm2 and was independent of polarization (in respect to the c axis). The achievable refractive index modulation is evaluated and the recording mechanisms are discussed.
Absolute photoluminescence quantum yield enhancement of poly(2-methoxy 5-[2′-ethylhexyloxy]-p-phenylenevinylene)
Authors: Marchioni F. , Chiechi R. , Patil S. , Wudl F. , Chen Y. , Shinar J.
Published: August 2006

A twofold absolute photoluminescence quantum yield (PLQY) enhancement of poly(2-methoxy 5-[2′-ethylhexyloxy]-p-phenylenevinylene) (MEH-PPV) is demonstrated by simple preparation of films where the polymer is blended with the small organic molecule 7,8,10-triphenylfluoranthene (3PF). The photophysical investigation of this particular energy transfer process was carried out using steady state absorption/luminescence spectroscopy and optically detected magnetic resonance techniques. The enhanced PLQY is attributed to direct sensitization of the intrachain emitting state of MEH-PPV by energy transfer from 3PF.