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The role of interstitial gas in the Brazil Nut effect

机译：间隙气体在巴西坚果效应中的作用

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We have studied the Brazil Nut effect - the rise of a large intruder particle within a vertically vibrated bed of smaller particles. In our study both intruder and bed particles were spherical and the vibration was such that bed convection was negligible. The rise of the intruder was found to be influenced by humidity of the air within the interstices of the particle bed and on the electrostatic charge developed on the bed particles during preparation and vibration. High relative humidity and high electrostatic charge each had the effect of slowing the rise of the intruder. Because increasing relative humidity of the interstitial air caused the electrostatic charge to diminish, the rise rate of the intruder achieved a maximum at a relative humidity of around 55%. Under controlled humidity and charge conditions, the time for the intruder to rise through the bed was found to decrease with increase in intruder diameter. As intruder density was varied under controlled humidity and charge conditions, the intruder rise time was found to exhibit a maximum when the intruder density of approximately one half of the bulk density of the bed of particles. This interesting trend was modelled by taking account of the pressure gradient that is generated across a bed of particles vertically vibrated within a gas. The tentative model suggests that the gas flows associated with this pressure gradient restrict the motion of the bed more than that of the intruder and that it is this difference that accounts for the rise of the intruder. Also incorporated in the model is the buoyancy force on the intruder that results from the pressure gradient across the bed.
机译：我们已经研究了巴西坚果效应-较大的入侵者颗粒在较小颗粒的垂直振动床中的上升。在我们的研究中，入侵者和床颗粒都是球形的，并且振动使得床对流微不足道。发现入侵者的上升受到颗粒床空隙中空气的湿度以及在制备和振动过程中床颗粒上产生的静电荷的影响。高的相对湿度和高的静电荷都具有减慢入侵者上升的作用。由于组织间空气相对湿度的增加导致静电荷减少，因此入侵者的上升速度在相对湿度约为55％时达到了最大值。在受控的湿度和充电条件下，发现入侵者通过床上升的时间会随着入侵者直径的增加而减少。由于入侵者的密度在受控的湿度和电荷条件下变化，因此当入侵者的密度约为颗粒床堆积密度的一半时，入侵者的上升时间将达到最大值。通过考虑在气体中垂直振动的颗粒床两端产生的压力梯度来模拟这种有趣的趋势。初步模型表明，与该压力梯度相关的气流比入侵者更限制床的运动，并且正是这种差异才说明了入侵者的上升。该模型中还包含了入侵者上的浮力，该浮力是由跨床的压力梯度产生的。

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