Particle-free Region Research Articles

On Lieb–Robinson Bounds for the Bose–Hubbard Model

We consider the dynamics of the Bose-Hubbard model on general lattices and prove a Lieb-Robinson bound for observables whose supports are separated by an initially almost particle-free region. We further obtain a maximal velocity bound for particle transport through an initially empty region which also applies to long-range hopping. Our techniques originate in the proofs of maximal velocity bounds for Schr\"odinger operators and scattering theory in non-relativistic QED.

Structuring of colloidal silica nanoparticle suspensions near water-silica interfaces probed by specular neutron reflectivity.

Structuring of aqueous suspensions of colloidal silica nanoparticles near an isolated planar silica-water interface is studied by specular neutron reflectivity. The reflectivity data clearly show that the suspensions develop a damped, oscillatory concentration profile in the normal direction to the interface. The wavelengths of these oscillations agree well with those independently determined by direct force measurements in the slit-geometry. The reflectivity data further demonstrate that the oscillatory structure persists over several layers and that the first particle layer is separated from the interface by a particle-free region.

Production of AA6061-T6/Al2O3 reinforced nanocomposite using friction stir welding

Heat-treatable AA6061-T6 weldability leads to a decline in the physio-mechanical and tribological properties due to strengthening precipitates dissolution in the nugget zone of FS-welded joints. Improvement in these properties is governed by reinforcement particles addition in the weld joint line for non-heat treatable Al-alloys. Although, its application to AA6061-T6 is scant. In the present work, FS-welded joints were produced by reinforcing Al2O3 nanoparticles into the faying surface of the base matrix. The purpose of Al2O3 nanoparticles addition is to refine the microstructure of the nugget zone and to obstruct the granular growing in the heat-affected zone. FSW was conducted at constant rotating 2000 rpm, transverse 70 mm min−1 speed to illustrate the influence of Al2O3 addition on mechanical, microstructure, micro-hardness, and wear properties of welds compared to parent alloy and unreinforced joints. Findings revealed that Al2O3 nanoparticles addition along the joint line leads to noteworthy grains refinement structure of weld zone (6μm) compared with the base material (38μm) and unreinforced nugget (18μm) joints. In addition, reinforced FS-W nugget zone have banded structure of alternate regions namely, nanoparticles-rich and free those have a difference in grain size, such that, the nanoparticle-rich region has fine grain size (6μm) than nano particle-free (14μm) region. Due to Zener-influence occurred via Al2O3 nanoparticles helps to avoid the grain growth accompanied by dynamic recrystallization throughout FSW process which results in granular size reduction. Tensile strength (246 MPa/73%), microhardness (88HV/52%), and wear properties was notably increased for reinforced FSW joint compared to unreinforced joint (tensile strength (180MPa), micro-hardness (46 HV), and lower wear resistance, under similar parameters which attribute to nanoparticles presence in processed area. Moreover, single FSW pass leads to non-uniform distribution of Al2O3 nanoparticles and nucleation of voids at Al/Al2O3 boundary in the heat-affected zone which leads to early fracture of welded joint during tensile loading.

Aging behavior of the extruded SiCp-reinforced AZ91 Mg alloy composite

Abstract

Dissipative solitary wave at the interface of a binary complex plasma

The propagation of a dissipative solitary wave across an interface is studied in a binary complex plasma. The experiments were performed under microgravity conditions in the PK-3 Plus Laboratory on board the International Space Station using microparticles with diameters of 1.55 μm and 2.55 μm immersed in a low-temperature plasma. The solitary wave was excited at the edge of a particle-free region and propagated from the sub-cloud of small particles into that of big particles. The interfacial effect was observed by measuring the deceleration of particles in the wave crest. The results are compared with a Langevin dynamics simulation, where the waves were excited by a gentle push on the edge of the sub-cloud of small particles. Reflection of the wave at the interface is induced by increasing the strength of the push. By tuning the ion drag force exerted on big particles in the simulation, the effective width of the interface is adjusted. We show that the strength of reflection increases with narrower interfaces.

Armoring confined bubbles in the flow of colloidal suspensions.

We study the process of coating the interface of a long gas bubble, which is translating in a horizontal circular capillary tube filled with a colloidal suspension. A typical elongated confined bubble is comprised of three distinct regions: a spherical front cap, a central body that is separated from the tube wall by a thin liquid film, and a spherical cap at the back. These three regions are connected by transitional sections. Particles gradually coat the bubble from the back to the front. We investigate the mechanisms that govern the initial accumulation of the particles and the growth of the particle-coated area on the interface of the bubble. We show that the initial accumulation of particles starts at the stable stagnation ring on the rear cap of the bubble, and the particles will completely coat the spherical cap at the back of the bubble before accumulating on the central body. Armoring the central interface of the bubble with particles thickens the liquid film around the bubble relative to that around the particle-free interface. This effect creates a rather sharp step on the interface of the bubble in the central region, which separates the armored region from the particle-free region. After the bubble is completely coated, the liquid film around the body of the bubble will adjust again to an intermediate thickness. We show that the three distinct thicknesses that the liquid film acquires during the armoring process can be well described analytically.

Ageing behavior of as-cast SiCp/AZ91 Mg matrix composites

SiC particles exhibited necklace-type distribution in SiCp/AZ91 Mg matrix composites fabricated by stir casting, and thus, the SiC particles could be divided into segregated particles region and particle free region. The effects of this particle distribution on the discontinuous precipitation of Mg17Al12 phase during ageing were investigated. The high density dislocations were induced due to the sharp mismatch between the thermal expansion coefficients of SiC and AZ91 alloy. The overlapped mismatch among the segregated particles causes higher density dislocations. And as a result, the discontinuous precipitations as the ageing products started born mainly near particles, especially near segregated particles. However, few Mg17A112 precipitated in the matrix within the particle segregation. The high-density dislocations and the numerous SiC/Mg interfaces made Mg17A112 phases much easier and earlier to precipitate in the composite. Thus, the SiCp addition reduced the peak ageing time from 38h (for AZ91 alloy) to 28h (for the composite). The ultimate tensile strength of the composite was enhanced by 46% after peak-ageing. SiC particles could also improve the age hardening efficiency of the composite because the high-density dislocations, the fine grain sizes and the numerous SiC/Mg interfaces cause the finer and more uniform precipitation of Mg17A112 Above all, the addition of SiC particles is effectively to improve the mechanical properties of magnesium matrix composite materials.

A fully Eulerian approach to particle inertial deposition in a physiologically realistic bifurcation

The investigation of aerosol flows in bifurcations is significant in biomedical applications, because this geometry resembles the branching airways of the lung. Most of the existing computational studies regarding aerosol flow in branching airways use Lagrangian description for the particulate phase, when particle inertial effects are taken into consideration. In the present study a fully Eulerian model is used for studying the transport and deposition of inertial particles under steady state inspiratory flow in the single physiologically realistic bifurcation created by generations G3-G4 of the human lung. In-house codes are used to create the geometry and the computational grid, obtain the fluid flow field and calculate particle concentration. Deposition fractions are calculated, concentration profiles are shown and deposition sites are indicated under different flow conditions (Reynolds number and flow asymmetry in the branches) and particles of various sizes (1–10μm). It is found that total particle deposition fraction is loosely dependent on fluid flow Reynolds number and our results are in agreement with experimental findings. Moreover, total deposition fraction does not change significantly between symmetric Q1/Q2=1 and asymmetric Q1/Q2=2 flow conditions, but is considerably lower for the totally obstructed case, Q2=0. On the contrary, deposition sites and particle concentration profiles depend on the various flow conditions. Apart from deposition at the bifurcation, which is present at all cases, deposition sites downstream the bifurcation move towards the outer bifurcation wall and daughter tube exit as Reynolds number increases. For the Q2=0, it is also shown that particles are trapped and deposit even in the obstructed daughter tube. In all studied cases, it is found that there is a particle-free region at the outer wall, at the beginning of the daughter tubes opposite the bifurcation. The straightforward formulation of deposition modelling and derivation of detailed concentration profiles along the bifurcation are major advantages of the fully Eulerian methodology used in the study.

Quantitative characterization of the focusing process and dynamic behavior of differently sized microparticles in a spiral microchannel

In this paper, a spiral microchannel was fabricated to systematically investigate particle dynamics. The focusing process or migration behavior of different-sized particles in the outlet region was presented. Specifically, for focused microparticles, quantitative characterization and analysis of how particles migrate towards the equilibrium positions with the increase in flow rate (De = 0.31–3.36) were performed. For unfocused microparticles, the particle migration behavior and the particle-free region’s formation process were characterized over a wide range of flow rates (De = 0.31–4.58), and the emergence of double particle-free regions was observed at De ≥ 3.36. These results provide insights into the design and operation of high-throughput particle/cell filtration and separation. Furthermore, using the location markers pre-fabricated along with the microchannel structures, the focusing or migration dynamics of different-sized particles along the spiral microchannel was systematically explored. The particle migration length effects on focusing degree and particle-free region width were analyzed. These analyses may be valuable for the optimization of microchannel structures. In addition, this device was successfully used to efficiently filter rare particles from a large-volume sample and separate particles of two different sizes according to their focusing states.

Inkjet Printing of Electrically Conducting Micron-Wide Lines and Transparent Conducting Films by Edge-Enhanced Twin-Deposition

ABSTRACTRoll-to-roll manufacturing holds the potential to rapidly and cheaply produce electronic devices in a flexible format as well as to effectively scale up production of emerging nanotechnologies. Developing scalable techniques for the efficient and effective use of solution-processed functional material is a significant factor in realizing the potential of roll-to-roll manufacturing. We present a novel inkjet deposition process developed to rapidly deposit arrays of micron-wide lines of silver nanoparticles for use as an optically transparent and electrically conducting film. The technique involves jetting a controlled number of space-overlapped drops of a dilute nanoparticle silver ink onto a substrate to form a long stable ink rivulet with two parallel and pinned edges. Subsequently, nanoparticles deposit preferentially at the two parallel rivulet edges due to edge-enhanced evaporation of the solvent. The final result is a twin-deposit of parallel continuous nanoparticle lines, each with a characteristic width less than 5μm and height less than 300 nm. The twin lines are separated by a predominantly particle-free region with the spacing between the lines ranging from 100 μm to 600 μm, where the spacing is a function of ink, substrate, and printing conditions. The effect of substrate surface and jetting parameters on nanoparticle line morphology is presented. Arrays of such lines have been printed and evaluated as potential transparent conducting films, showing an effective sheet resistance of ∼5 Ω/□. This edge-enhanced twin-deposition technique has the potential for rapid, material-efficient, and lithography-free patterned deposition of functional material for use in roll-to-roll manufacturing.

Particles Inside the Void of a Complex Plasma

Experiments with complex (“dusty”) plasmas in microgravity often show a particle-free region in the center of the discharge, which is the so-called “void.” The cause of this unexpected feature is assumed to be the ions streaming outward from the discharge center toward the radio-frequency electrodes and walls, dragging the microparticles with them out of the center. In several experiments onboard the International Space Station, single particles moving out of the void could be traced. The obtained trajectories reveal the distribution of the forces inside the void.

The effect of SiC particles on the microstructure and mechanical properties of friction stir welded pure copper joints

The SiC particles with mean size of 5 μm were introduced into the pure Cu joints during the friction stir welding process. After one pass processing, the distribution of SiC particle is not uniform and some pores formed around the aggregation of SiC particles. However, the particle distribution became rather uniform after two passes processing and the previous pores can be refilled. In addition, banded structure consisting of particle-rich and particle-free region can be observed in the stir zone. The particle-rich region has an average grain size of less than 2 μm, much smaller than that of about 8 μm in the particle-free region. Microstructural observation confirmed that the SiC particles can act as the heterogeneous nucleation site in the dynamic recrystallization of Cu grains. The SiC dispersed Cu joints exhibit a Vickers hardness of 110 HV, much higher than 70 HV in the stir zone without SiC particles.

Synthesis and characterization of in-situ reinforced Fe-TiC steel FGMs

This paper presents a novel technique to synthesize TiC-reinforced iron base (Fe-TiC) functionally graded materials (FGM) using SHS reaction followed by centrifugal casting. The in-situ reinforced Fe-TiC steel composites were prepared by aluminothermic SHS reactions using metal oxides and elemental powders. Ferritic as well as austenitic matrix Fe-TiC composites were obtained by judicious thermite charge composition. FGM microstructure was characterized in terms of TiC area fraction, particle size, and chemical analysis using optical and analytical electron microscopy. The results reveal a gradient of TiC particles across the thickness in the inner face and a particle-free region in the outer surface. Hardness profiles have shown an increasing trend from outer surface to TiC-rich inner surface. Wear studies indicated better wear performance of TiC-rich inner face as compared to particle-free outer surface of ferritic or austenitic steel matrices.

Gravity currents from a line source in an ambient flow

We present a mainly theoretical study of high-Reynolds-number planar gravity currents in a uniformly flowing deep ambient. The gravity currents are generated by a constant line source of fluid, and may also be supplied with a source of horizontal momentum and a source of particles. We model the motion using a shallow-water approximation and represent the effects of the ambient flow by imposing a Froude-number condition in a moving frame. We present analytic and numerical expressions for the threshold ambient flow speed above which no upstream propagation can occur at long times. For homogeneous gravity currents in an ambient flow below threshold, we find similarity solutions in which the up- and downstream fronts spread at a constant rate and the current propagates indefinitely in both directions. For gravity currents consisting of both interstitial fluid of a different density to the ambient and a sedimenting particle load, we find long-time asymptotic solutions for ambient flow strengths below threshold. These consist of a steady particle-rich near-source region, in which settling and advection of particles balance, and an effectively particle-free frontal region. The homogeneous behaviour of the fronts ensures that they also spread at a constant rate and therefore can propagate upstream indefinitely. For gravity currents driven solely by a sedimenting particle load, we find numerically that a single regime exists for ambient flow strengths below threshold. In these solutions, settling balances advection near the source leading to a steady region, which joins on to a complex frontal boundary layer. The upstream front progressively decelerates. Our solutions for homogeneous and particle-driven gravity currents compare well with published experimental results.

Rotation of a nanoparticle cloud in an inductively coupled plasma induced by weak static magnetic fields

Hydrocarbon nanoparticles with diameters between 10 and 30 nanometres are created in a low pressure plasma combining capacitive and inductive power coupling. The particles are generated in the capacitive phase of the experiment and stay confined in the plasma in the inductive phase. The presence of these embedded particles induces a rotation of a particle-free region (void) around the symmetry axis of the reactor. The phenomenon is analysed using optical emission spectroscopy both line integrated and spatially resolved via an intensified charge coupled device camera. From these data, electron temperatures and densities are deduced. We find that the rotation of the void is driven by a tangential component of the ion drag force induced by an external static magnetic field. Two modes are observed: a fast rotation of the void in the direction opposite to that of the tangential component and a slow rotation in the same direction. The rotation speed decreases linearly with the size of the particles. In the fast mode the dependence on the applied magnetic field is weak and consequently the rotation speed can serve as a monitor to detect particle sizes in low temperature plasmas.

Circulation' dynamo in complex plasma

In ground based experiments (performed in Ar using a radio frequency (rf)-discharge), we observe the interaction of individual agglomerated particles with a monodisperse (bulk) complex plasma cloud containing (melamine-formaldehyde (MF)) microparticles of 7.17 μm± 3% diameter. The particles are levitated by thermophoresis. For this purpose, a gas temperature gradient of 2000 K m−1 is applied. The particle cloud has a complicated ‘sandwich-like’ vertical structure of two dense slabs (filled by particles), separated by a void, a central particle-free region. The void is impenetrable for small particles, but not for heavier and/or accelerated agglomerates, which may slide through the entire void and therefore can be used as natural test particles for determination of the force acting inside the void. The bulk particles remain in quasi-equilibrium for a long time and are dynamically active, e.g., intense edge rotations (vortices) and nonlinear vertical waves. We traced particle motions in detail and studied the correlation of particle vibrations inside the clouds and the motion of agglomerates and/or accelerated particles penetrating through the void. A possible physical explanation of the cloud's activity is based on the assumption that the phenomenon can be considered as a consequence of the non-Hamiltonian character of complex plasmas.

Where surface physics and fluid dynamics meet: Rupture of an amphiphile layer by fluid flow

We investigate the fluctuating pattern created by a jet of fluid impingent upon an amphiphile-covered surface. This microscopically thin layer is initially covered with 50 microm floating particles so that the layer can be visualized. A vertical jet of water located below the surface and directed upward drives a hole in this layer. The hole is particle-free and is surrounded by the particle-laden amphiphile region. The jet ruptures the amphiphile layer creating a particle-free region that is surrounded by the particle-covered surface. The aim of the experiment is to understand the (fluctuating) shape of the ramified interface between the particle-laden and particle-free regions.

A comparative study on the phase behaviour of highly charged colloidal spheres in aconfining wedge geometry

We studied the structures formed in aqueous dispersions ofcharged colloidal spheres under a constant low salt concentration ofc = 6 × 10−6 mol l−1. Particlesof diameter 2a = 1000 nm were confined to a low angle wedge geometry with plate separation0<S<50 µm and observed with video microscopy. Irrespective of the initial particle densityn we reproducibly observe the particles to migrate to the narrow wedge side on thetimescale of a few days. Thereby an interface between a crystalline structure anda near particle free region is formed, which propagates slowly until the diluteregion is exhausted of particles. While the origin of this separation is still unclear,the final extension of the crystalline region is stable on the timescale of months.Within the crystal phase we observe a characteristic sequence of structures withincreasing plate separation similar to that seen in previous experiments on hardsphere-like systems but here with non-touching particles. Moreover, we find thatmechanical equilibrium is a necessary prerequisite for observing the full richness ofdifferent phases. A detailed comparison to recent theoretical calculations for bilayerswas performed and semi-quantitative agreement with the predictions observed.

Transport phenomena and structuring in shear flow of suspensions near solidwalls

In this paper we apply the lattice Boltzmann method and an extension to particlesuspensions as introduced by Ladd et al to study transport phenomena and structuringeffects of particles suspended in a fluid near sheared solid walls. We find that a particle freeregion arises near walls, which has a width depending on the shear rate and the particleconcentration. The wall causes the formation of parallel particle layers at lowconcentrations, where the number of particles per layer decreases with increasing distancefrom the wall.

Effect of MA on microstructure and synthesis path of in-situ TiC reinforced Fe–28at.% Al intermetallic composites

An in-situ composite consisting of titanium carbide and iron aluminide was synthesized by mechanical alloying of elemental powders and pulse discharge sintering. The microstructure and synthesis path of in-situ TiC reinforced Fe–28at.% Al composite were examined as a function of milling time and heat treatment temperature. Mechanical alloying of the elemental powders promoted bcc solid solution formation, and the solid solution completed at milling of 400 h. During sintering Fe 3Al and TiC precipitated from the supersaturated Fe solid solution containing Al, Ti and C. Microstructure of the sintered body made of powder milled for 200 h consists of three regions, that is, large TiC particles of about 5 μm in size, Fe 3Al matrix with TiC particles of submicron size and particle-free Fe 3Al region. For the sintered body after milling of 400 h, however, the large TiC particles disappeared due to the complete solid solution during mechanical alloying. Sintering of the powders milled for the period shorter than 50 h formed intermediate phases, such as Al 5Fe 2, Al 3Ti and Fe 3AlC x , by heat treatment up to 1273 K, while that of the powder milled for 400 h showed direct precipitation of Fe 3Al and TiC from the Fe solid solution.