Vibration-Induced Granular Segregation: A Phenomenon Driven by Three Mechanisms

Abstract

The segregation of large spheres in a granular bed under vertical vibrations is studied. In our experiments we systematically measure rise times as a function of density, diameter and depth; for two different sinusoidal excitations. The measurements reveal that: at low frequencies, inertia and convection are the only mechanisms behind segregation. Inertia (convection) dominates when the relative density is greater (less) than one. At high frequencies, where convection is suppressed, fluidization of the granular bed causes either buoyancy or sinkage and segregation occurs.PACS numbers: 46.10.+z, 64.75.+g, 83.80.Fg Many theoretical and experimental studies have been carried out in the last five decades aimed to reveal the physics of one of the most intriguing phenomena in granular matter: vibration-induced segregation [1,2,3,4,5,6,7,8,9,10,11,12,13,14]. However, when this phenomenon intensively investigated appeared to be well understood, new scientific puzzles came into scene [15,16,17,18,19,20,21,22]. Air-driven segregation, inertia, condensation, are now new words added to the already vast list of concepts important in the subject. Thus, since this problem is an important concern to industries dealing with granulates, these recently disclosed effects should be further investigated. Granular segregation was first reported in 1939 by Brown [23] and studied ever since by the engineering community [1,2,3,4,5], until it was brought in 1987 to the physics realm with the suggestive name of "Brazil Nut Problem" (BNP) [6]. Results related to this problem established themselves as benchmarks of granular segregation. But the question: why the Brazil nuts are on top, seems to be yet an open matter of discussion. Both theoretical and experimental studies have focused on the influence of size, friction, density and excitation parameters [7,8,9,10,12,13,16,22,26,27] and the results explain, or obscure, bit by bit the underlying mechanism behind the BNP. Some of these results support the idea that it is "void-filling" beneath large ascending particles, the mechanism promoting the upward movement of an intruder in a shaken granular bed [6,14]. Other researches claim that global convection is the driven force behind the BNP [8], and others, that arching [7] or inertia [16,22] are crucial elements to explain it . The dilemma is not yet settled on the verge of even more recent findings [15]; being the most relevant the surprising result that decreasing the density of the intruder does not necessarily mean a monotonic increasing of the rise time, as might be previously suggested by studies in 3D [9] and 2D [16]. Furthermore, based on computer simulations Hong et al even dared to predict the reverse segregation effect in the BNP [17,19] (known now in the literature as the RBNP), which was immediately confronted by two groups [20,24], but nevertheless observed in the laboratory by Breu et al [25]. Finally, Yan et al [21] recently failed to confirm the experimental findings of Möbius et al [15].Based on this debate, a simple, yet...

Snooping in the sand

“…Not only in nature and our daily lives, segregation of particles is also of great importance to the processing of powders in industries [36]. In the past two decades, there has been an upsurging number of laboratory experiments to unveil the mechanism of the segregation driven by vertical vibrations or rotation [37][38][39][40][41][42][43]. Counterintuitively, it has been found that large light particles might sink under vertical vibrations while in the meanwhile equally sized heavy particle rises [44][45][46].…”

Investigation of the effect of a bumpy base on granular segregation and transport properties under vertical vibration

“…The wellknown "Brazil nut problem," in which a large particle (the so-called intruder) rises relative to the surrounding smaller-sized granular material when vertically vibrated, has been studied extensively by experiments, theoretical models, and numerical simulations. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] In a vibrating granular bed, the segregation may also be influenced by the vibration conditions, bed height, ambient pressure, humidity, size ratio, density ratio, restitution coefficient, and the friction coefficient of the particles. 8,9,12,16 Granule segregation in a vertically-vibrated granular bed is highly complicated and much remains unknown.…”

Segregation and stability of a binary granular heap

“…In fact, heterogeneous mixtures commonly undergo segregation, a phenomenon of great interest for materials processing [6]. Segregation of binary mixtures also occurs in the laboratory, in vibrating bed [7,8] and rotating drum experiments [9][10][11][12][13][14][15][16][17][18][19][20][21][22], and upon pouring into a Hele-Shaw cell [23][24][25]. It can be caused by differences in particle size, shape, smoothness, or density.…”