Sensitivity of granular segregation of mixtures in quasi-two-dimensional fluidized layers

Abstract

Size segregation is studied using a quasi-two-dimensional rotating cylinder system for mixtures of different size, near-spherical particles. Flow occurs only in a thin surface layer, whereas the remaining particles rotate as a fixed bed. In most of the systems studied, the measured radial weight fraction profiles in the bed show significant double segregation ͑a core of small particles as well as a thin layer of small particles at the periphery͒. The profiles are found to be sensitively dependent on the surface roughness of the particles in the mixture, and double segregation reduces with particle roughness. Double segregation is also sensitive to cylinder diameter and no double segregation is observed for the smaller diameter cylinders used. The system, however, shows two unexpected scalings: ͑i͒ the scaled profiles are nearly the same for different cylinder diameters, when the cylinder diameter to the cylinder length ratio is the same, and ͑ii͒ the profiles obtained are found to be insensitive to the size of the large particles in the mixture but depend strongly on the size of the small particles. DOI: 10.1103/PhysRevE.69.031304 PACS number͑s͒: 45.70.Mg, 64.75.ϩg, 83.80.Fg Flowing granular mixtures segregate, and this can have a significant impact in many industrial processes ranging from pharmaceutical tablet manufacture to cement production. The process, however, is not understood to the level that quantitative predictions can be made. Even predicting the direction of segregation is not trivial. For example, Hong et al. ͓1͔ showed using molecular dynamics simulations that fluidized binary mixtures of elastic spheres under gravity segregate with the larger particles on the top or the bottom of the layer, depending on the temperature and the diameter and mass ratios. They obtained a criterion to predict the direction of segregation based on competition between condensation and percolation. Jenkins and Yoon ͓2͔ showed that kinetic theory also yields a similar criterion for the direction of segregation. Reversals in the direction of segregation have been observed in flowing fluidized layers as well, but the situation in this case is complicated by dissipation and gradients across the layer ͓3,4͔.Segregation in fluidized layers has been studied in a number of systems including heap flows ͓5-7͔, chute flows ͓3,8 -10͔, and rotating cylinder flows ͓9,11-19͔. In most experimental systems involving size segregation, the larger particles are at the top of the flowing layer and small particles are at the bottom. There are a few exceptions. Nityanand et al. ͓11͔ reported reverse segregation in a rotating cylinder system ͑small particle concentrate at the top of the flowing layer at high rotational speeds͒. Dolgunin and Ukolov ͓8͔ found small particles concentrated at the top and bottom of the layer with larger particles concentrated near the middle of the layer in a chute flow. We refer to this as double segregation since it involves regions of regular and reverse segregation. Thomas ͓9͔ also found double segregation...

Rheology and Mixing of Granular Materials

“…In mixtures of particles of different sizes but the same density, radial segregation results in the smaller particles concentrating in a radial core (r < R c ), where R c is the radius of the core. [62,[69][70][71][72][73][74][75][76] Similarly, in mixtures of particles of equal size but different density, the higher density particles concentrate in the core. [51,71] Variations on this broad theme are seen in special circumstances.…”

Particle-size segregation and diffusive remixing in shallow granular avalanches

“…Experiments suggest that the dependencies are non-trivial. For instance, Hajra & Khakhar (2004) used rotating drum experiments, such as those illustrated in figure 1, to infer that even small size differences are sufficient to cause segregation, but once the size ratio reaches a critical value the driving force for segregation saturates. This is consistent with the notion that once sufficient dilatation has occurred for the grains to percolate freely, there is no further increase in the segregation velocity.…”

Attraction-driven aggregation of dipolar particles in an external magnetic field

“…Recently, it has been shown experimentally that the dynamics of the aggregation process of dipolar particles and the resulting structure has some dependence on the shape of the magnetic field [20]. In our case similar questions could be addressed by varying the height of the magnetized cylinder h c at a fixed size of the cork disk R. This way we can tune the shape of the magnetic field of the particles from a pointlike dipole to an extended distribution of magnetic moments.…”

“…Using an external magnetic field with appropriate orientation various types of crystal structures can also be achieved in magnetic dipolar monolayers [12]. At higher length scales, centimeter sized magnetic particles provide interesting granular assemblies, where, using vibrofluidization techniques, aggregation phenomena and the phase structure of granular fluids-gases can be investigated [18][19][20][21]. Using centimeter sized steel balls of permanent magnetic moments the effect of long range anisotropic forces on the angle of repose and avalanche dynamics of granular materials have been studied [22].…”

“…The large particle size in these experimental studies offers several advantages; however, it implies some additional difficulties to overcome the frictional barrier. In these cases usually vibrating tables are used to mobilize the particles tuning the frequency and amplitude of the excitations [14,[18][19][20][21].…”