Spontaneous air-driven separation in vertically vibrated fine granular mixtures

Structured bubbling in vibrated gas-fluidized beds of binary granular particles: experiments and simulations

Mixing and segregation of granular particles on the basis of size and density from vertical vibration or upward gas flow is critical to a wide range of industrial, agricultural and natural processes. Recently, combined vibration and gas flow under certain conditions has been shown to create periodically repeating structured bubbling patterns within a fluidized bed of spherical, monodisperse particles. Here, we demonstrate with experiments and simulations that structured bubbling can form in binary mixtures of particles with different size and density, but with similar minimum fluidization velocities. Structured bubbling leads to particles mixing regardless of initial particle configuration, while exciting particles with only gas flow produces smaller unstructured bubbles which act to segregate particles. Discrete particle simulations match the experimental results qualitatively and, in some regards quantitatively, while continuum particle simulations do not predict mixing in the case of structured bubbling, highlighting areas for future model improvement.

Mechanical vibration patterns elicit behavioral transitions and habituation in crawling Drosophila larvae

How animals respond to repeatedly applied stimuli, and how animals respond to mechanical stimuli in particular, are important questions in behavioral neuroscience. We study adaptation to repeated mechanical agitation using the Drosophila larva. Vertical vibration stimuli elicit a discrete set of responses in crawling larvae: continuation, pause, turn, and reversal. Through high-throughput larva tracking, we characterize how the likelihood of each response depends on vibration intensity and on the timing of repeated vibration pulses. By examining transitions between behavioral states at the population and individual levels, we investigate how the animals habituate to the stimulus patterns. We identify time constants associated with desensitization to prolonged vibration, with re-sensitization during removal of a stimulus, and additional layers of habituation that operate in the overall response. Known memory-deficient mutants exhibit distinct behavior profiles and habituation time constants. An analogous simple electrical circuit suggests possible neural and molecular processes behind adaptive behavior.

Lift force acting on an intruder in dense, granular shear flows

We report a lift force model for intruders in dense, granular shear flows. Our derivation is based on the thermal buoyancy model of Trujillo and Hermann [Physica A 330, 519 (2003)10.1016/S0378-4371(03)00621-6], but it takes into account both granular temperature and pressure differences in the derivation of the net buoyancy force acting on the intruder. In a second step, the model is extended to take into account also density differences between the intruder and the bed particles. The model predicts very well the rising and sinking of intruders, the lift force acting on intruders as determined by discrete element model simulations, and the neutral-buoyancy limit of intruders in shear flows. Phenomenologically, we observe a cooling upon the introduction of an intruder into the system. This cooling effect increases with intruder size and explains the sinking of large intruders. On the other hand, the introduction of small to midsized intruders, i.e., up to four times the bed particle size, leads to a reduction in the granular pressure compared to the hydrostatic pressure, which in turn causes the rising of small to midsized intruders.

Microscopic structure and dynamics study of granular segregation mechanism by cyclic shear

Granular mixtures with size difference can segregate upon shaking or shear. However, the quantitative study of this process remains difficult because it can be influenced by many mechanisms. Conflicting results on similar experimental systems are frequently obtained when the experimental conditions are not well controlled, which is mainly due to the fact that many mechanisms can be at work simultaneously. Moreover, it is often that macroscopic or empirical measures, which lack microscopic physical bases, are used to explain the experimental findings and therefore cannot provide an accurate and complete depiction of the overall process. Here, we carry out a detailed and systematic microscopic structure and dynamics study of a cyclically sheared granular system with rigorously controlled experimental conditions. We find that both convection and arching effect play important roles in the segregation process in our system, and we can quantitatively identify their respective contributions.

Gravitational instabilities in binary granular materials

The motion and mixing of granular media are observed in several contexts in nature, often displaying striking similarities to liquids. Granular dynamics occur in geological phenomena and also enable technologies ranging from pharmaceuticals production to carbon capture. Here, we report the discovery of a family of gravitational instabilities in granular particle mixtures subject to vertical vibration and upward gas flow, including a Rayleigh-Taylor (RT)-like instability in which lighter grains rise through heavier grains in the form of "fingers" and "granular bubbles." We demonstrate that this RT-like instability arises due to a competition between upward drag force increased locally by gas channeling and downward contact forces, and thus the physical mechanism is entirely different from that found in liquids. This gas channeling mechanism also generates other gravitational instabilities: the rise of a granular bubble which leaves a trail of particles behind it and the cascading branching of a descending granular droplet. These instabilities suggest opportunities for patterning within granular mixtures.

Phase-coexisting patterns, horizontal segregation, and controlled convection in vertically vibrated binary granular mixtures

We report patterns consisting of coexistence of synchronous and asynchronous states [for example, a granular gas co-existing with (i) bouncing bed, (ii) undulatory subharmonic waves, and (iii) Leidenfrost-like states] in experiments on vertically vibrated binary granular mixtures in a Hele-Shaw cell. Most experiments have been carried out with equimolar binary mixtures of glass and steel balls of same diameter by varying the total layer height (F) for a range of shaking acceleration (Γ). All patterns as well as the related phase diagram in the (Γ,F) plane have been reproduced via molecular dynamics simulations of the same system. The segregation of heavier and lighter particles along the horizontal direction is shown to be the progenitor of such phase-coexisting patterns as confirmed in both experiment and simulation. At strong shaking we uncover a partial convection state in which a pair of convection rolls is found to coexist with a Leidenfrost-like state. The crucial role of the relative number density of two species on controlling the buoyancy-driven granular convection is demonstrated. The onset of horizontal segregation can be explained in terms of an anisotropic diffusion tensor.

Diversity and controllability of particle distribution under coupling vibration and airflow

The vertical, horizontal, and three-dimensional (3D) distribution of binary particles with different diameters are investigated simultaneously in a cylindrical container under coupling vibration and airflow. Airflow is blown into the vertically vibrating granular bed from its bottom with vibration frequency of f = 15, 30, and 45 Hz; vibration acceleration Γ = 2-7; and air velocity u = 0-1.82 m s^-1 (fluidization number u/u_mf = 0-1.49). In our experiments, several distribution states have been observed, such as the mixing state, where particles are uniformly mixed; Brazil nut separation (BN), where large particles rise to the top; reverse Brazil nut separation (RBN), where large particles sink to the bottom; horizontal separation (HS), where binary particles separate horizontally; and combined distribution of BN/RBN and HS, in which one type of particles is above the other and close to one side of the container wall. Briefly, the vertical distribution forms five pattern types, and the horizontal distribution has four pattern types. The resultant pattern of 3D distribution exhibits 15 pattern types. These patterns can be always obtained under the conditions of certain control parameters, but the condition differs with the diameter ratio of binary particles. Phase diagrams of vertical, horizontal, and 3D distributions are drawn to provide the excitation condition of various patterns. These experimental results contribute to the further understanding of the entire distribution of particles and the control of particle distribution for different industry requirements.

Granular core phenomenon induced by convection in a vertically vibrated cylindrical container

A mixture of 13X molecular sieve (13XMS) particles and glass particles with identical diameters is placed in a cylindrical container. Under vertical vibration, heavier glass particles tend to cluster and are wrapped inside the convection of 13XMS particles, resulting in the granular core phenomenon. The vibration frequency f strongly influences particle convection and particle cluster modes. By contrast, the effect of the dimensionless acceleration amplitude Γ can be neglected. For different f ranges, the granular core is classified as center-type and ring-type cores. For the center-type core, heavy particles are distributed as an approximate zeroth-order Bessel function of the first kind in the radial direction and an exponential function in the height direction. For the ring-type core, the concentration of heavy particles follows the power-series function in the radial direction. A granular transport model is then established based on heavy-particle movements under steady state to analyze the effect of vibration parameters and granular convection on density segregation.

Modelling density segregation in flowing bidisperse granular materials

Preventing segregation in flowing granular mixtures is an ongoing challenge for industrial processes that involve the handling of bulk solids. A recent continuum-based modelling approach accurately predicts spatial concentration fields in a variety of flow geometries for mixtures varying in particle size. This approach captures the interplay between advection, diffusion and segregation using kinematic information obtained from experiments and/or discrete element method (DEM) simulations combined with an empirically determined relation for the segregation velocity. Here, we extend the model to include density-driven segregation, thereby validating the approach for the two important cases of practical interest. DEM simulations of density bidisperse flows of mono-sized particles in a quasi-two-dimensional-bounded heap were performed to determine the dependence of the density-driven segregation velocity on local shear rate and particle concentration. The model yields theoretical predictions of segregation patterns that quantitatively match the DEM simulations over a range of density ratios and flow rates. Matching experiments reproduce the segregation patterns and quantitative segregation profiles obtained in both the simulations and the model, thereby demonstrating that the modelling approach captures the essential physics of density-driven segregation in granular heap flow.

Pattern transition, microstructure, and dynamics in a two-dimensional vibrofluidized granular bed

Experiments are conducted in a two-dimensional monolayer vibrofluidized bed of glass beads, with a goal to understand the transition scenario and the underlying microstructure and dynamics in different patterned states. At small shaking accelerations (Γ=Aω^{2}/g<1, where A and ω=2πf are the amplitude and angular frequency of shaking and g is the gravitational acceleration), the particles remain attached to the base of the vibrating container; this is known as the solid bed (SB). With increasing Γ (at large enough shaking amplitude A/d) and/or with increasing A/d (at large enough Γ), the sequence of transitions/bifurcations unfolds as follows: SB ("solid bed") to BB ("bouncing bed") to LS ("Leidenfrost state") to "2-roll convection" to "1-roll convection" and finally to a gas-like state. For a given length of the container, the coarsening of multiple convection rolls leading to the genesis of a "single-roll" structure (dubbed the multiroll transition) and its subsequent transition to a granular gas are two findings of this work. We show that the critical shaking intensity (Γ_{BB}^{LS}) for the BB→LS transition has a power-law dependence on the particle loading (F=h_{0}/d, where h_{0} is the number of particle layers at rest and d is the particle diameter) and the shaking amplitude (A/d). The characteristics of BB and LS states are studied by calculating (i) the coarse-grained density and temperature profiles and (ii) the pair correlation function. It is shown that while the contact network of particles in the BB state represents a hexagonal-packed structure, the contact network within the "floating cluster" of the LS resembles a liquid-like state. An unsteadiness of the Leidenfrost state has been uncovered wherein the interface (between the floating cluster and the dilute collisional layer underneath) and the top of the bed are found to oscillate sinusoidally, with the oscillation frequency closely matching the frequency of external shaking. Therefore, the granular Leidenfrost state is a period-1 wave as is the case for the BB state.

Shear-induced segregation of particles by material density

Recently, shear rate gradients and associated gradients in velocity fluctuations (e.g., granular temperatures or kinetic stresses) have been shown to drive segregation of different-sized particles in a manner that reverses at relatively high solids fractions (〈f〉>0.50). Here we investigate these effects in mixtures of particles differing in material density through computational and theoretical studies of particles sheared in a vertical chute where we vary the solids fraction from 〈f〉=0.2 to 0.6. We find that in sparse flows, 〈f〉=0.2 to 0.4, the heavier (denser) particles segregate to lower shear rates similarly to the heavier (larger) particles in mixtures of particles differing only in size. However, there is no segregation reversal at high f in mixtures of particles differing in density. At all solids fractions, heavier (denser) particles segregate to regions of lower shear rates and lower granular temperatures, in contrast with segregation of different-sized particles at high f, where the heavier (larger) particles segregate to the region of higher shear rates. Kinetic theory predicts well the segregation for both types of systems at low f but breaks down at higher f's. Our recently proposed mixture theory for high f granular mixtures captures the segregation trends well via the independent partitioning of kinetic and contact stresses between the two species. In light of these results, we discuss possible directions forward for a model framework that encompasses segregation effects more broadly in these systems.

Competition between geometrically induced and density-driven segregation mechanisms in vibrofluidized granular systems

The behaviors of granular systems are sensitive to a wide variety of particle properties, including size, density, elasticity, and shape. Differences in any of these properties between particles in a granular mixture may lead to segregation, or "demixing," a process of great industrial relevance. Despite the known influence of particle geometry in granular systems, a considerable fraction of research into these systems concerns only uniformly spherical particles. We address, for the case of vertically vibrated granular systems, the important question of whether the introduction of differing particle geometries entirely invalidates our existing knowledge based on purely spherical granulates, or whether current models may simply be adapted to account for the effects of particle shape. We demonstrate that while shape effects can indeed influence the dynamical and segregative behaviors of a granular system, the segregative mechanisms associated with particle geometry are decidedly secondary to those related to particle density. The relevant control parameters determining the extent of geometrically induced segregation are established. Finally, a manner in which shape effects may be accounted for in simulations utilizing purely spherical particles is proposed.

Experimental study of transport of a dimer on a vertically oscillating plate

It has recently been shown that a dimer, composed of two identical spheres rigidly connected by a rod, under harmonic vertical vibration can exhibit a self-ordered transport behaviour. In this case, the mass centre of the dimer will perform a circular orbit in the horizontal plane, or a straight line if confined between parallel walls. In order to validate the numerical discoveries, we experimentally investigate the temporal evolution of the dimer's motion in both two- and three-dimensional situations. A stereoscopic vision method with a pair of high-speed cameras is adopted to perform omnidirectional measurements. All the cases studied in our experiments are also simulated using an existing numerical model. The combined investigations detail the dimer's dynamics and clearly show that its transport behaviours originate from a series of combinations of different contact states. This series is critical to our understanding of the transport properties in the dimer's motion and related self-ordered phenomena in granular systems.

Symmetrically periodic segregation in a vertically vibrated binary granular bed

Periodic segregation behaviors in fine mixtures of copper and alumina particles, including both percolation and eruption stages, are experimentally investigated by varying the ambient air pressure and vibrational acceleration. For the cases with moderate air pressure, the heaping profile of the granular bed keeps symmetrical in the whole periodic segregation. The symmetrical shape of the upper surface of the granular bed in the eruption stage, which resembles a miniature volcanic eruption, could be described by the Mogi model that illuminates the genuine volcanic eruption in the geography. When the air pressure increases, an asymmetrical heaping profile is observed in the eruption stage of periodic segregation. With using the image processing technique, we estimate a relative height difference between the copper and the alumina particles as the order parameter to quantitatively characterize the evolution of periodic segregation. Both eruption and percolation time, extracted from the order parameter, are plotted as a function of the vibration strength. Finally, we briefly discuss the air effect on the granular segregation behaviors.

Dynamics of a grain-filled ball on a vibrating plate

We study experimentally how the bouncing dynamics of a hollow ball on a vibrating plate is modified when it is partially filled with liquid or grains. Whereas empty and liquid-filled balls display a dominant chaotic dynamics, a ball with grains exhibits a rich variety of stationary states, determined by the grain size and filling volume. In the collisional regime, i.e., when the energy injected to the system is mainly dissipated by interparticle collisions, an unexpected period-1 orbit appears independently of the vibration conditions, over a wide range. This is a self-regulated state driven by the formation and collapse of a granular gas within the ball during one cycle. In the frictional regime (dissipation dominated by friction), the grains move collectively and generate different patterns and steady modes: oscillons, waves, period doubling, etc. From a phase diagram and a geometrical analysis, we deduce that these modes are the result of a coupling (synchronization) between the vibrating plate frequency and the trajectory followed by the particles inside the cavity.

Convecting particle diffusion in a binary particle system under vertical vibration

We studied the separation behaviour of binary granular particles in a vertically vibrated container. The final separation of the binary particle system exhibited the Brazil-Nut (BN) effect, though it was not complete. Particle convection occurred, and four different typical convection modes were observed when the frequency f changed from 20 Hz to 80 Hz at constant dimensionless acceleration Γ = 4πAf(2)/g. However, when Γ changed from 2 to 4 at constant f, the system's convection mode stayed almost the same. In our experiments, one type of particle generally moved much faster than the other, so the former was termed the 'convecting' particle, and the latter was termed the 'non-convecting' particle. To study the separation results qualitatively, we divided the system into vertical layers and calculated the mass distribution of the binary particles along the z axis. The results showed that when f increased at constant Γ or Γ decreased at constant f, the convecting particles, usually the smaller and lighter ones, distributed less to the top side and more to the bottom side of the container. Finally, to explain the experimental results, we derived a mass conservation equation for the convecting particles considering simultaneous convection and diffusion. The equation described the experimental results well. We also analysed the effects of f, Γ, diameter ratio, density ratio, etc., on the final separation results.

Effects of packing density on the segregative behaviors of granular systems

We present results concerning the important role of system packing in the processes of density- and inelasticity-induced segregation in vibrofluidized binary granular beds. Data are acquired through a combination of experimental results acquired from positron emission particle tracking and simulations performed using the discrete particle method. It is found that segregation due to inelasticity differences between particle species is most pronounced in moderately dense systems, yet still exerts a significant effect in all but the highest density systems. Results concerning segregation due to disparities in particles' material densities show that the maximal degree to which a system can achieve segregation is directly related to the density of the system, while the rate at which segregation occurs shows an inverse relation. Based on this observation, a method of minimizing the time and energy requirements associated with producing a fully segregated system is proposed.

Segregation in mixtures of granular chains and spherical grains under vertical vibration

We experimentally investigate segregation behaviors of binary granular mixtures consisting of granular chains and spherical grains with different interstitial media under vertical vibrations. A quantitative criterion is proposed to locate the boundaries between different vibrating phases. The water-immersed granular mixture exhibits two interesting types of segregation behaviors: chain-on-top and sandwich patterns. However, the phenomenon of sandwich segregation is absent for the air-immersed mixture. The topological differences of phase diagrams between two different environments indicate that the interstitial fluid plays an important role on the granular demixing. Additionally, the phase behaviors of mixtures for the different chain lengths show a not significant discrepancy. Finally, the vibrating thickness ratio determining the phase boundary characterizes the mixing extent of the granular bed. The estimated ratios for various chain lengths exhibit a monotonically decreasing dependence, when the vibration frequency increases.

Recovering metallic fractions from waste electrical and electronic equipment by a novel vibration system

The need to recover and recycle valuable resources from Waste Electrical and Electronic Equipment (WEEE) is of growing importance as increasing amounts are generated due to shorter product life cycles, market expansions, new product developments and, higher consumption and production rates. The European Commission (EC) directive, 2002/96/EC, on WEEE became law in UK in January 2007 setting targets to recover up to 80% of all WEEE generated. Printed Wire Board (PWB) and/or Printed Circuit Board (PCB) is an important component of WEEE with an ever increasing tonnage being generated. However, the lack of an accurate estimate for PCB production, future supply and uncertain demands of its recycled materials in international markets has provided the motivation to explore different approaches to recycle PCBs. The work contained in this paper focuses on a novel, dry separation methodology in which vertical vibration is used to separate the metallic and non-metallic fractions of PCBs. When PCBs were comminuted to less than 1mm in size, metallic grades as high as 95% (measured by heavy liquid analysis) could be achieved in the recovered products.

Dynamics of dense granular flows of small-and-large-grain mixtures in an ambient fluid

Dense grain flows in nature consist of a mixture of solid constituents that are immersed in an ambient fluid. In order to obtain a good representation of these flows, the interaction mechanisms between the different constituents of the mixture should be considered. In this article, we study the dynamics of a dense granular flow composed of a binary mixture of small and large grains immersed in an ambient fluid. In this context, we extend the two-phase approach proposed by Meruane et al. [J. Fluid Mech. 648, 381 (2010)] to the case of flowing dense binary mixtures of solid particles, by including in the momentum equations a constitutive relation that describes the interaction mechanisms between the solid constituents in a dense regime. These coupled equations are solved numerically and validated by comparing the numerical results with experimental measurements of the front speed of gravitational granular flows resulting from the collapse, in ambient air or water, of two-dimensional granular columns that consisted of mixtures of small and large spherical particles of equal mass density. Our results suggest that the model equations include the essential features that describe the dynamics of grains flows of binary mixtures in an ambient fluid. In particular, it is shown that segregation of small and large grains can increase the front speed because of the volumetric expansion of the flow. This increase in flow speed is damped by the interaction forces with the ambient fluid, and this behavior is more pronounced in water than in air.

Penetration and blown air effect in granular media

Sand is known to oppose an increasing resistance to penetration with depth. This is different from what happens in liquids since granular media, usually nonthermal systems, oppose solid friction to the motion. We report another striking and "counterintuitive" difference between the penetration dynamics observed in sand and in liquids. When pushing a top-closed shell (e.g., an upside down glass) into a liquid, the trapped air increases the buoyancy and opposes the penetration. It is more difficult to push a top capped cylinder than an opened one vertically into liquids. In contrast, the penetration is considerably easier in dense sand when cylinders are top capped. In this discrete and biphasic medium, the trapped air escapes from the shell, fluidizes the sand, and eases the motion.

Continuum simulations of shocks and patterns in vertically oscillated granular layers

We study interactions between shocks and standing-wave patterns in vertically oscillated layers of granular media using three-dimensional, time-dependent numerical solutions of continuum equations to Navier-Stokes order. We simulate a layer of grains atop a plate that oscillates sinusoidally in the direction of gravity. Standing waves form stripe patterns when the accelerational amplitude of the plate's oscillation exceeds a critical value. Shocks also form with each collision between the layer and the plate; we show that pressure gradients formed by these shocks cause the flow to reverse direction within the layer. This reversal leads to an oscillatory state of the pattern that is subharmonic with respect to the plate's oscillation. Finally, we study the relationship between shocks and patterns in layers oscillated at various frequencies and show that the pattern wavelength increases monotonically as the shock strength increases.

Effects of gas flow on granular size separation

A gas flow is introduced into a vibrating bed from its perforated bottom to clarify the effects of gas flow on granular size separation. The rising or sinking time of a single intruder does not follow a monotonic relationship, and the granular mixtures show four different types of distribution with increasing gas velocity. The different drag forces exerted on large particles and small ones are the main factors that influence the granular size separation. Under the "wall effect" the gas flow may speed up the rising of the large particle, and cause a change in size distribution.

Volume fraction variations and dilation in colloids and granulars

I discuss the importance of spatial and temporal variations in particle volume fraction to understanding the force response of concentrated colloidal suspensions and granular materials.

Three-dimensional simulations of a vertically vibrated granular bed including interstitial air

We present a numerical study of the effect of interstitial air on a vertically vibrated granular bed within one period of oscillation. We use a three-dimensional molecular-dynamics simulation including air phenomenologically. The simulations are validated with experiments made with spherical glass beads in a rectangular container. After validation, results are reported for a granular column of 9000 grains and approximately 50 layers deep (at rest), agitated with a sinusoidal excitation with maximal acceleration 4.7g at 11.7 Hz. We report the evolution of density, granular temperature, and coordination number within a vibration cycle, and the effect of interstitial air on those parameters. In three-dimensional computer simulations we found that the presence of interstitial air can promote the collective motion of the granular material as a whole.

Driven inelastic-particle systems with drag

We study steady states of the motion of a large number of particles in a closed box that are excited by a vibrating boundary and experience a linear drag force from the interstitial fluid. The dissipation in such systems arises from two main sources: Inelasticity in particle collisions and the effects of interstitial fluid on the particles. In many applications, order of magnitude estimates suggest that the dissipation due to interstitial fluid effects may greatly exceed that due to inelasticity and one is naturally led to neglect inelastic effects. In this study, we show that, if one adopts a linear drag force and inelastic effects are neglected, a steady state only exists when the vibration speed of the boundary is below a critical value. For vibration speeds above this critical value, no steady state exists since the kinetic energy of the particles grows without bound. We show that, for vibration speeds above the critical value, inelastic effects must be included to obtain a steady state even if order of magnitude estimates suggest they are negligible. Numerical simulations confirm these theoretical predictions. We also show that inclusion of apparently small nonlinear drag terms can also play a similar role in preventing the kinetic energy of the particles growing without bound.

Segregation induced by phase synchronization in a bidisperse granular layer

We propose an alternative segregation mechanism where the species-dependent interactions are dynamically induced by the phase synchronization of beads. Based on this scenario, we report an alternative segregation among beads of different restitution coefficients by molecular dynamics simulations. Since the beads are of equal size and mass, this is not related to the Brazilian-nut effect, nor can it be explained by the depletion force. Instead, this phenomenon derives from the phase synchronization, a concept which helps us determine the criteria for segregation and the phase boundaries that agree excellently with the simulation results.

Migration of an asymmetric dimer in oscillatory fluid flow

We describe the motion of an asymmetric dimer across a horizontal surface when exposed to an oscillatory fluid flow. The dimer consists of two spheres of distinct sizes, rigidly attached to each other. The dimer is found to move in a direction perpendicular to the fluid flow, with the smaller sphere foremost. We have determined how the speed depends upon the vibratory conditions, on the fluid viscosity, and on the dimer size and aspect ratio. Computer simulations are used to give an insight into the mechanism responsible for the motion. We use a scaling argument based on the asymmetry of the streaming flow to predict the approximate dependence of the migration speed on the system parameters.

Simulation of density segregation in vibrated beds

We have investigated by numerical simulation the density segregation of fine equal-sized bronze and glass particles subject to vertical vibrations. The model was found to be capable of predicting the two main segregation forms ("bronze on top" and "sandwich") in roughly the same regions of the phase diagram as was found experimentally by Burtally We investigated the effects of pressure air forcing, friction and restitution of kinetic energy in collisions, and box size on the segregation behavior. We find that next to the interstitial air friction also has a large influence on the formation of the sandwich structure.

Eliminating segregation in free-surface flows of particles

By introducing periodic flow inversions, we show both experimentally and computationally that forcing with a value above a critical frequency can effectively eliminate both density and size segregation. The critical frequency is related to the inverse of the characteristic time of segregation and is shown to scale with the shear rate of the particle flow. This observation could lead to new designs for a vast array of particle processing applications and suggests a new way for researchers to think about segregation problems.