Size segregation of irregular granular materials captured by time-resolved 3D imaging

When opening a box of mixed nuts, a common experience is to find the largest nuts at the top. This well-known effect is the result of size-segregation where differently sized ‘particles’ sort themselves into distinct layers when shaken, vibrated or sheared. Colloquially this is known as the ‘Brazil-nut effect’. While there have been many studies into the phenomena, difficulties observing granular materials mean that we still know relatively little about the process by which irregular larger particles (the Brazil nuts) reach the top. Here, for the first time, we capture the complex dynamics of Brazil nut motion within a sheared nut mixture through time-lapse X-ray Computed Tomography (CT). We have found that the Brazil nuts do not start to rise until they have first rotated sufficiently towards the vertical axis and then ultimately return to a flat orientation when they reach the surface. We also consider why certain Brazil nuts do not rise through the pack. This study highlights the important role of particle shape and orientation in segregation. Further, this ability to track the motion in 3D will pave the way for new experimental studies of segregating mixtures and will open the door to even more realistic simulations and powerful predictive models. Understanding the effect of size and shape on segregation has implications far beyond food products including various anti-mixing behaviors critical to many industries such as pharmaceuticals and mining.

Brazil Nut Effect Drives Pattern Formation in Early Mammalian Embryos

ABSTRACTThe formation of three-dimensional ordered spatial patterns, which is essential for embryonic development, tissue regeneration, and cancer metastasis, is mainly guided by the chemical concentration gradient of morphogens. However, since no chemical concentration gradient has been observed in the early embryonic development (pre-implantation) of mammals, the pattern formation mechanism has been unsolved for a long time. During the second cell fate decision of mouse embryos, the inner cell mass (ICM) segregates into topographically regionalized epiblast (EPI) and primitive endoderm (PrE) layers. Here, we report that the segregation process of PrE/EPI precursors coincides with an emerged periodic expansion-contraction vibration of the blastocyst cavity, which induces phase transition in the ICM compartment to a higher fluidity state and generates directional tissue flows. By experiments and modeling, we demonstrate that the spatial segregation of PrE and EPI precursors is mediated by a “Brazil nut effect”-like viscous segregation mechanism in which PrE precursors with low affinity gradually migrate to the surface of ICM along with the tissue flow, while EPI precursors with high affinity remains inside ICM under cavity vibration. Artificially manipulation of the frequency and amplitude of cavity vibration could control the process of spatial separation as well as lineage specification of PrE/EPI. Furthermore, disruption of the cavity vibration in the initial stage after segregation could reverse the ICM cells back to a mixed state. Therefore, this study reveals a fundamental mechanism that guarantees the robustness of cell segregation and pattern formation without specific morphogens in early mammalian embryos. Our model also emphasizes a conserved function of cavity structure that widely exists in organisms as an energy reservoir and converter between different forms, such as chemical and mechanical energy.

Microbial Brazil nut effect

The Brazil nut effect (BNE) is a counter-intuitive process of segregation of a large object inside a vibrated granular medium (GM), which has been studied widely by subjecting GMs to...

How universal is the vibration-velocity controlled granular convection?

Recently, a number of articles have reported that granular convection induced by continuous vibration is controlled by vibration velocity, in contrast with some previous studies. We have reported such an example for the Brazil nut effect when the vibration is given discontinuously, using a one-layer granular bed in a cell with down-facing side walls. Here, we report the effect of vibration phase and wall friction using the same experimental system, to confirm rising motion of an intruder induced by granular convection is again governed by vibration velocity. We compare two different cases of vibration phase for giving intermittent vibration cycles, and found one, in which granular packing is well established before grains start to lose contacts due to vibration, provides distinctly high reproducibility. We further control the side wall friction using a microfabrication technique, and found that significantly high reproducibility is attained in a cell with vertical side walls when a millimeter texture is introduced on the side walls. Our results indicate that the granular convection is universally controlled by vibration velocity. The present study opens a way to conduct highly reproducible experiments on granular dynamics, which is indispensable for deep physical understanding of granular flow and segregation.

Vibration induced segregation of single large particles

The vibration-induced segregation (e.g., rising of one large intruder - so called Brazil Nut Effect (BNE)) is studied by discrete element method. Vibration frequency and amplitude are two dominating factors in the occurrence of BNE and a phase diagram is constructed. For fixed vibration amplitude, segregation only occurs when vibration frequency is within a certain range. Larger vibration amplitude can expand the range of vibration frequency for BNE. Size ratio and the intruder shape are studied under certain vibration conditions. Larger size ratio can enlarge the segregation intensity. The shape of the intruder influences the segregation process by the intruder′s orientation. Standing-like initial orientation can increase the time required for the intruder to reach the top while lying-like initial orientation cannot significantly affect the vertical segregation.

Granular segregation on the rubble-pile asteroid Itokawa

We investigate the dynamics of regolith on rubble-pile asteroids to explain granular processes observed in reality. In particular, we explain how the appearance of boulders on the surface of asteroid Itokawa could have resulted from a size sorting process in granular media called the Brazil Nut Effect (BNE). The Discrete Element Method (DEM) is implemented to perform numerical simulations of the BNE in a micro-gravity environment caused by inter-particle collisions during seismic vibrations. Firstly, we present the results of how the BNE depends on the magnitude of surface gravity. It is estimated that segregation processes on Itokawa occur over much longer time-scales (in the order of a few hundred years) than the same processes would require in the presence of a strong gravitational field, like on Earth. Secondly, we also find that the size sorting could also result from kinetic sieving encountered during granular avalanches. Finally, we discuss how the void-filling mechanism becomes more efficient when there is a higher relative size difference between the boulders and the surrounding grains. Our model has important implications in understanding the resurfacing of Itokawa by trying to explain one of the many complex geophysical processes that occur in such unique conditions.

A modified calculation of particle buoyant forces in vibro-fluidized beds

Segregation of granular materials under vibration or flow conditions such as the Brazil nut effect has been well known, however, there is yet no consensus mechanisms to explain this phenomenon. This study attempts to investigate particle buoyant forces in the segregation process. To explain the difference of the segregation behavior for the large particle with different size, a modified calculation method of particle buoyant force is suggested for considering the effect of particle size ratio. A simple verification illustrates its validity.