Layer Of Large Particles Research Articles

Origin of granular axial segregation bands in a rotating tumbler: An interface-mixing driven Rayleigh-Taylor instability

The origin of large and small particle axial bands in long rotating tumblers is a long-standing question. Using DEM simulations, we show that this axial segregation is due to a Rayleigh-Taylor instability which is characterized by the fact that the density of a granular medium increases with mixing and decreases with segregation. For initially mixed particles, segregation and collisional diffusion in the flowing layer balance and lead to a three-layer system, with a layer of large particles over a layer of small particles, and, interposed between these layers, a layer of more densely packed mixed particles. The higher density mixed particle layer over the lower density small particle layer induces a Rayleigh-Taylor instability, evident as waviness in the interface between the layers. The waviness destabilizes into ascending plumes of small particles and descending plumes of mixed particles with large particles enriched near the surface, which become evident as small and large particle bands visible at the free surface. Rolls driven by segregation at the tilted interface between plumes maintain the pattern of frozen plumes. Published by the American Physical Society 2024

Rotating disk electrode measurements on low and high loading catalyst layers: Diffusion limitations and application to Pt catalysts supported on porous micrometric carbon xerogel particles designed for proton exchange membrane fuel cells

Innovative fuel cell catalysts are now often being supported on nanostructured materials in order to improve mass transport, contact with the ionomer and active phase distribution. However, to maintain their inner structure, these supports are used as µm-sized particles, which hampers to perform accurate kinetic experiments on rotating disk electrode since a thin layer of catalyst is in principle required. The present study thus aims at checking if accurate measurements remain possible for thick layers of large particles. To do so, oxygen reduction was first performed on a commercial Pt catalyst supported on carbon black; both a typical thin and a thick layer (37 and 370 µgPt cm−2) were used. The impact of the diffusion limitations on the kinetic current was determined. The same measurement protocol was then performed with a catalyst supported on 5 µm particles of a nanostructured carbon (∼15 µm layer thickness, 370 µgPt cm−2). Results and calculation show that, for both thick layers, the kinetic current should be measured at higher potential (1.00 or 1.05 V/ESH) than usual to avoid diffusion limitations. Also, the voltage scanning rate should be lowered to 0.5 or 1 mV s−1 to decrease the impact of the double layer capacity on the measurements.

Formation and Stability of Smooth Thin Films with Soft Microgels Made of Poly(N-Isopropylacrylamide) and Poly(Acrylic Acid).

In this work, soft microgels of Poly(N-Isopropylacrylamide) (PNIPAm) at two different sizes and of interpenetrated polymer network (IPN) composed of PNIPAm and Poly(Acrylic Acid) (PAAc) were synthesized. Then, solutions of these different types of microgels have been spin-coated on glass substrates with different degrees of hydrophobicity. PNIPAm particles with a larger diameter form either patches or a continuous layer, where individual particles are still distinct, depending on the dispersion concentration and spin speed. On the other, PNIPAm particles with a smaller diameter and IPN particles form a continuous and smooth film, with a thickness depending on the dispersion concentration and spin-speed. The difference in morphology observed can be explained if one considers that the microgels may behave as colloidal particles or macromolecules, depending on their size and composition. Additionally, the microgel size and composition can also affect the stability of the depositions when rinsed in water. In particular, we find that the smooth and continuous films show a stimuli-dependent stability on parameters such as temperature and pH, while large particle layers are stable under any condition except on hydrophilic glass by washing at 50 °C.

Model of the windward slope formation of the eolian relief form

The model for establishing the windward slope upon exposure to the air flow is based on the analysis of the state of the layer composed of sandy incoherent particles. It is assumed that the wind removal of individual particles of this layer leads to a change in its length due to rolling and immobility of larger particles. Accounting for precipitating saltating particles gives an idea of the reasons for the alternation of layers of large particles with layers of smaller ones involved in wind-sand transport. Assuming a linear dependence of the dynamic velocity on the height of the location on the windward slope, the forms of the windward slope, which are close to the observed ones, are obtained. An algorithm for determining the linear coefficient of change of dynamic velocity is proposed. The calculation of the relative change at different levels of the windward slope of the values of the roughness parameter and the scale of the laminar layer made it possible to estimate the coefficient of surface resistance.

Windward Aeolian Slope Formation Model

The model of the formation of windward slope by the air-flow is based on an analysis of the state of a layer composed of noncohesive sand particles. It is assumed that the wind removal of individual particles of this layer leads to a change in its length due to rolling and the immobility of larger particles. The reasons for the alternation of layers of large particles with layers of smaller ones involved in wind-sand transport can be revealed by taking into account deposited saltating particles. The windward slope forms close to the observed ones are obtained under the assumption of a linear dependence of the dynamic velocity on the height of the location on the windward slope. An algorithm for determining the linear coefficient of variation of dynamic velocity is proposed. The calculation of the relative change of the values of the roughness parameter and the scale of the laminar layer at different levels of the windward slope made it possible to estimate the coefficient of surface resistance.

Modeling Segregation in Modulated Granular Flow

We consider unsteady segregating granular flows, focusing on stratification in bidisperse bounded heap flow. Experiments indicate that periodically changing the feed rate of particles falling onto the upstream portion of the heap results in a stratified segregation much like that which occurs at low feed rates, but with more regular stratified layers of large and small particles and a higher overall feed flow rate. Experiments clarify how a front of large particle at a high feed rate deposits a layer of large particles that is subsequently covered over by a layer of small particles during a low feed rate, thus generating layers of large and small particles. The stratification can be modelled using a version of an advection-segregation-diffusion equation with a segregation velocity. Simply repeatedly switching the model from a low volume flow rate to a high volume flow rate instantaneously along the entire length of the flowing layer results in stratification patterns that are similar to those observed in experiments. Using a modulated feed rate offers the potential to intentionally create extended layers of bidisperse granular materials to enhance the effective degree of mixing of the deposited materials at heap length scales.

Depinning and heterogeneous dynamics of colloidal crystal layers under shear flow.

Using Brownian dynamics (BD) simulations and an analytical approach we investigate the shear-induced, nonequilibrium dynamics of dense colloidal suspensions confined to a narrow slit-pore. Focusing on situations where the colloids arrange in well-defined layers with solidlike in-plane structure, the confined films display complex, nonlinear behavior such as collective depinning and local transport via density excitations. These phenomena are reminiscent of colloidal monolayers driven over a periodic substrate potential. In order to deepen this connection, we present an effective model that maps the dynamics of the shear-driven colloidal layers to the motion of a single particle driven over an effective substrate potential. This model allows us to estimate the critical shear rate of the depinning transition based on the equilibrium configuration, revealing the impact of important parameters, such as the slit-pore width and the interaction strength. We then turn to heterogeneous systems where a layer of small colloids is sheared with respect to bottom layers of large particles. For these incommensurate systems we find that the particle transport is dominated by density excitations resembling the so-called "kink" solutions of the Frenkel-Kontorova (FK) model. In contrast to the FK model, however, the corresponding "antikinks" do not move.

Mechanistic study of the hydrothermal reduction of palladium on the Tobacco mosaic virus

The fundamental mechanisms governing reduction and growth of palladium on the genetically engineered Tobacco mosaic virus in the absence of an external reducer have been elucidated via in situ X-ray absorption spectroscopy. In recent years, many virus-inorganic materials have been synthesized as a means to produce high quality nanomaterials. However, the underlying mechanisms involved in virus coating have not been sufficiently studied to allow for directed synthesis. We combined XAS, via XANES and EXAFS analysis, with TEM to confirm an autocatalytic reduction mechanism mediated by the TMV1Cys surface. This reduction interestingly proceeds via two first order regimes which result in two linear growth regimes as spherical palladium nanoparticles are formed. By combining this result with particle growth data, it was discovered that the first regime describes growth of palladium nanoparticles on the virion while the second regime describes a second layer of larger particles which grew sporadically on the first palladium nanoparticle layer. Subsequent aggregation of free solution based spherical particles and metallized nanorods characterize a third and final regime. At the end of the second reduction regime, the average particle diameter of particles tethered to the TMV1Cys surface are approximately 4.5nm. The use of XAS to simultaneously monitor the kinetics of biotemplated reactions along with growth of metal nanoparticles will provide insight into the pertinent reduction and growth mechanisms so that nanorod properties can be controlled through their populating nanoparticles.

In situ 3D characterization of bidisperse cakes using confocal laser scanning microscopy

In situ 3D characterization of a filtration cake obtained with a model bidisperse suspension containing two sizes of fluorescent particles (1µm and 4.8µm) has been performed using Confocal Laser Scanning Microscopy (CLSM). The suspensions were dead-end filtered through 0.8µm pore diameters silicon nitride microsieves, under a constant flow rate. The membrane was put in a specific filtration chamber previously designed [1], allowing direct on-line microscopic observations of particles deposition. The final position of the first particles arriving on the membrane was investigated. Then, the online deposition of the following particles was recorded and, thus, the cake build-up could be analyzed layer by layer. In addition, to analyze the protective effect of a layer of large particles, 1µm particles were filtrated over a previously deposited cake formed with the 4.8µm microspheres.

713 光閉じ込め効果を用いた色素増感型太陽電池の作製(OS7-(4),OS7 オーガナイズドセッション《精密/微細加工と評価》)

Dye-sensitized solar cell (DSC) is expected as a portable power of flexible printed electronics device. However Problem of DSC is low efficiency. We have optimized titania layer for high efficiency by utilizing inkjet technology. We have achieved efficiency of 8.3%. Therefore, we focused on light trapping effect for increasing efficiency. Light trapping effect is a phenomenon to increase the absorption of light by reflecting light. This effect is obtained by titania layer of large particles on titania layer of small particles. In this study, we fabricated DSC of two titania layers structure. Efficiency of DSC is the highest 8.7% in this laboratory.

Effect of the “inversion” of a scattering medium in layers of close-packed titanium dioxide nanoparticles

The features of the behavior of the diffuse transmission of layers of close-packed titanium dioxide nanoparticles in the visible and near infrared spectral ranges with an increase in the volume fraction f of the particles in a layer have been analyzed. It has been found that an increase in f for layers of small particles (about 25 nm) with a relatively low volume fraction (0.20–0.25) is accompanied by the expected decrease in diffuse transmission. At the same time, an increase in f for layers of large particles (about 100 nm) with a volume fraction of 0.45–0.50 results in a strong increase in transmission. The described phenomenon has been interpreted in terms of the concepts of inverse scattering systems, where the main scattering centers are air nanocavities in a TiO2 matrix rather than TiO2 particles in an air matrix.

Experimental investigation into segregating granular flows down chutes

We experimentally investigated how a binary granular mixture made up of spherical glass beads (size ratio of 2) behaved when flowing down a chute. Initially, the mixture was normally graded, with all the small particles on top of the coarse grains. Segregation led to a grading inversion, in which the smallest particles percolated to the bottom of the flow, while the largest rose toward the top. Because of diffusive remixing, there was no sharp separation between the small-particle and large-particle layers, but a continuous transition. Processing images taken at the sidewall, we were able to measure the evolution of the concentration and velocity profiles. These experimental profiles were used to test a recent theory developed by Gray and Chugunov [J. Fluid Mech. 569, 365 (2006)], who derived a nonlinear advection diffusion equation that describes segregation and remixing in dense granular flows of binary mixtures. We found that this theory was able to provide a consistent description of the segregation/remixing process under steady uniform flow conditions. To obtain the correct depth-averaged concentration far downstream, it was very important to use an accurate approximation to the downstream velocity profile through the avalanche depth. The S-shaped concentration profile in the far field provided a useful way of determining what we refer to as the Péclet number for segregation, a dimensionless number that quantifies how large the segregation is compared to diffusive remixing. While the theory was able to closely match the final fully developed concentration profile far downstream, there were some discrepancies in the inversion region (i.e., the region in which the mixing occurs). The reasons for this are not clear. The difficulty to set up the experiment with both well controlled initial conditions and a steady uniform bulk flow field is one of the most plausible explanations. Another interesting lead is that the flux of segregating particles, which was assumed to be a quadratic function of the concentration in small beads, takes a more complicated form.

Shear-driven size segregation of granular materials: Modeling and experiment

Granular materials segregate by size under shear, and the ability to quantitatively predict the time required to achieve complete segregation is a key test of our understanding of the segregation process. In this paper, we apply the Gray-Thornton model of segregation (developed for linear shear profiles) to a granular flow with an exponential shear profile, and evaluate its ability to describe the observed segregation dynamics. Our experiment is conducted in an annular Couette cell with a moving lower boundary. The granular material is initially prepared in an unstable configuration with a layer of small particles above a layer of large particles. Under shear, the sample mixes and then resegregates so that the large particles are located in the top half of the system in the final state. During this segregation process, we measure the velocity profile and use the resulting exponential fit as input parameters to the model. To make a direct comparison between the continuum model and the observed segregation dynamics, we map the local concentration (from the model) to changes in packing fraction; this provides a way to make a semiquantitative comparison with the measured global dilation. We observe that the resulting model successfully captures the presence of a fast mixing process and relatively slower resegregation process, but the model predicts a finite resegregation time, while in the experiment resegregation occurs only exponentially in time.

Scalar Conservation Laws with Nonconstant Coefficients with Application to Particle Size Segregation in Granular Flow

Granular materials will segregate by particle size when subjected to shear, as occurs, for example, in avalanches. The evolution of a bidisperse mixture of particles can be modeled by a nonlinear first order partial differential equation, provided the shear (or velocity) is a known function of position. While avalanche-driven shear is approximately uniform in depth, boundary-driven shear typically creates a shear band with a nonlinear velocity profile. In this paper, we measure a velocity profile from experimental data and solve initial value problems that mimic the segregation observed in the experiment, thereby verifying the value of the continuum model. To simplify the analysis, we consider only one-dimensional configurations, in which a layer of small particles is placed above a layer of large particles within an annular shear cell and is sheared for arbitrarily long times. We fit the measured velocity profile to both an exponential function of depth and a piecewise linear function which separates the shear band from the rest of the material. Each solution of the initial value problem is nonstandard, involving curved characteristics in the exponential case, and a material interface with a jump in characteristic speed in the piecewise linear case.

Luminescence properties of ZnS:Mn nanocrystals embedded in SiO 2 by ion implantation

Sequential multi-energy implantations of zinc and sulphur ions have been performed in a 250-nm thick SiO 2 layer thermally grown on 〈1 1 1〉 silicon. Energies and doses have been chosen to produce 10 at.% constant concentration profiles overlapping over about 100 nm. Manganese is subsequently introduced at various levels by the same way. Thermal treatments (from 700 to 1100 °C) lead to the formation of nanometric precipitates of the luminescent compound ZnS:Mn. A bimodal size distribution is observed, with a quasi-single layer of large particles (40 nm) in the end-of-range region and much smaller precipitates between this layer and the surface. The orange emission is maximal when the Mn concentration is close to 3%. Several hours at 900 °C is the best thermal budget for maximal luminescence intensity at room temperature. A shift of the excitation spectrum related to size variations, shows that the particles of smaller size are mainly responsible for the observed luminescence. In agreement with other authors, the luminescence lifetime is found in the ms range and increases with the nanocrystal diameter, tending to the lifetime of bulk ZnS. The luminescence of ZnS:Mn nanoparticles embedded in SiO 2 by ion implantation is also shown to be very stable during long UV light irradiation.

Stable solutions of a scalar conservation law for particle-size segregation in dense granular avalanches

Dense, dry granular avalanches are very efficient at sorting the larger particles towards the free surface of the flow, and finer grains towards the base, through the combined processes of kinetic sieving and squeeze expulsion. This generates an inversely graded particle-size distribution, which is fundamental to a variety of pattern formation mechanisms, as well as subtle size-mobility feedback effects, leading to the formation of coarse-grained lateral levees that create channels in geophysical flows, enhancing their run-out. In this paper we investigate some of the properties of a recent model [Gray, J. M. N. T. & Thornton, A. R. (2005) A theory for particle size segregation in shallow granular free-surface flows. Proc. R. Soc. 461, 1447–1473]; [Thornton, A. R., Gray, J. M. N. T. & Hogg, A. J. (2006) A three-phase mixture theory for particle size segregation in shallow granular free-surface flows. J. Fluid. Mech. 550, 1–25] for the segregation of particles of two sizes but the same density in a shear flow typical of shallow avalanches. The model is a scalar conservation law in space and time, for the volume fraction of smaller particles, with non-constant coefficients depending on depth within the avalanche. It is proved that for steady flow from an inlet, complete segregation occurs beyond a certain finite distance down the slope, no matter what the mixture at the inlet. In time-dependent flow, dynamic shock waves can develop; they are interfaces separating different mixes of particles. Shock waves are shown to be stable if and only if there is a greater concentration of large particles above the interface than below. Constructions with shocks and rarefaction waves are demonstrated on a pair of physically relevant initial boundary value problems, in which a region of all small particles is penetrated from the inlet by either a uniform mixture of particles or by a layer of small particles over a layer of large particles. In both cases, and under a linear shear flow, solutions are constructed for all time and shown to have similar structure for all choices of parameters.

Formation of Hierarchical Nanoparticle Pattern Arrays Using Colloidal Lithography and Two-Step Self-Assembly: Microspheres atop Nanospheres

We report a simple approach to the fabrication of hierarchical nanoparticle arrays and film patterns using a novel combination of colloidal lithography (CL), two-step self-assembly, and reactive-ion etching (RIE). In this approach, a uniform nanoparticle film (∼15−50 nm particle diameter) is first deposited on a substrate. Then, larger (several hundred to thousands of nanometers diameter) microparticles with a different composition are self-assembled into well-ordered patterns atop the nanoparticle film. Next, reactive-ion etching is used to remove parts of the initial nanoparticle film using the upper layer of large particles as a mask. After selective removal of the remaining upper layer of large particles, hierarchical nanoparticle patterns are obtained on flat surfaces. Hexagonal patterns of small nanoparticle film arrays were easily fabricated with a monolayer of large spheres using this approach. Moreover, the shape and diameter of nanoparticle film disks depend on the etching duration while the per...

Microscale variability in the distributions of large fluorescent particles observed in situ with a planar laser imaging fluorometer

It has been known for decades that particle-size and biomass spectra show regular patterns in the ocean, and that these patterns often show systematic variations with other properties such as total biomass, nutrient concentration, season, and distance (both vertical and horizontal). The recent finding of the ubiquitous nature of layers of phytoplankton < 1 m thick prompted us to explore the fine- and microscale vertical variations of size- and fluorescence-abundance spectra in the ocean. Using a two-dimensional planar laser imaging system mounted on a free-falling platform, we quantified the properties of large fluorescent particles (∼ 20 μm–2 cm) through the water column, obtaining images every 10–30 cm. These images showed systematic relationships of the spectral properties to total chlorophyll: increased proportions of the smallest particles at high chlorophyll concentrations, and a lengthening of the spectral size range at high total chlorophyll concentrations (more large particles at high chlorophyll concentrations). Further, we observed significant variations of the spectral properties over scales of 1 m and less, and recorded the frequent occurrence of unusual layers of large particles. Our new instrument, which is sensitive to thin layers of enhanced phytoplankton biomass, shows the planktonic community to be highly structured vertically on scales of 1–2 m, particularly within the DCM.

Novel photonic crystal thin films using the Langmuir–Blodgett approach

Photonic crystals prepared via the Langmuir–Blodgett (LB) method are shown to be quite different to those structures prepared by other self-assembly approaches. In LB prepared films the layer spacing is expanded relative to that expected for a true FCC structure while the correlation between layers is reduced. The resulting films are thus best described as (2+1) dimensional in nature, since they do not possess the full three-dimensional (3-D) ordering present in opaline structures. A simple analysis of Bragg reflection peaks for a sample prepared from silica particles of diameter ca. 250nm reveals layer spacing for the LB-grown films of 0.941D, where D is the particle diameter. We account for these phenomena in terms of a model in which each layer of spheres forms within the confines of the LB trough before being transferred as a complete layer to the substrate. Under such conditions the likelihood of the spheres finding the optimum sites for the formation of a true FCC structure is low and the resulting structures, although still consisting of ordered layers, do not adopt the FCC structure and hence they may not be described as ‘opals’. Results are also presented for bilayer (size matched) heterostructure photonic crystals. For the size-matched bilayer systems where the diameter of one set of particles is twice that of the other, it is shown that when sampled in a particular geometry, cooperative effects can occur involving the first-order Bragg reflections from the layer of smaller particles combining with the second-order Bragg reflections from the layer of larger particles. As a consequence an enhancement of the intensity of the first-order Bragg peak from the layer comprised of the smaller particles is observed in the binary heterostructure.

A void-based description of compaction and segregation in flowing granular materials

Guided by the kinematical treatment of vacancies in theories for solid-state diffusion, we develop a theory for compaction and segregation in flowing granular materials. This theory leads to a partial differential equation for the macroscopic motion of the material coupled to a system of partial differential equations for the volume fractions of the individual particle types. When segregation is ignored, so that the focus is compaction, the latter system is replaced by a scalar partial differential equation that closely resembles equations arising in theories of traffic flow. To illustrate the manner in which the theory describes compaction and segregation, we present three explicit solutions. In particular, for an arbitrary loosely packed mixture of small and large particles in a fixed container under the influence of gravity, we show that a layer of large particles forms at the free surface and grows with time, while a closely packed mixture of large and small particles forms and grows from the base of the container; the final solution, attained in a finite time, consists of a layer of closely packed large particles above a closely packed mixed state. At the level of everyday experience, this solution at least qualitatively explains why in a container of mixed nuts, Brazil nuts are generally found at the top.