Bose M, Kumar U U, Nott P R, Kumaran V
Department of Chemical Engineering, Indian Institute of Science, Bangalore, India.
Phys Rev E Stat Nonlin Soft Matter Phys. 2005 Aug;72(2 Pt 1):021305. doi: 10.1103/PhysRevE.72.021305. Epub 2005 Aug 25.
A two dimensional bi-disperse vibrofluidized granular mixture is studied in the rapid flow regime, where particle interactions occur due to instantaneous collisions. Both experiments and simulations are carried out, and these show the existence of two phenomena which have been observed only in very dense granular flows or in equilibrium systems. The Brazil nut phenomenon, which involves the rise of larger particles in a granular mixture upon vibration, has been observed in dense systems due to the percolation of small particles though the interstitial spaces between the large particles, or due to convection rolls. In the present case, where neither effect is present, it is observed that the fluidization of the smaller particles by vibration results in an exponentially decaying density profile, at heights large compared to the particle diameter, and thereby a pressure field that decreases with height. The larger particles, suspended in this decaying pressure field, experience a larger pressure at the bottom and a smaller pressure on top, and they rise to a height where the net force caused by the decreasing pressure is balanced by the weight of the particle. An attractive force between the large particles, similar to the entropic attraction effect in mixtures of colloids and polymers, is also observed in this nonequilibrium system, because when the distance between the large particles is less than the small particle diameter, the pressure between the large particles is smaller than that on the outside. Analytical results are derived for each of these effects, and these are in agreement with the experimental and simulation results.
在快速流动状态下研究了二维双分散振动流化颗粒混合物，其中颗粒相互作用是由于瞬时碰撞而发生的。进行了实验和模拟，结果表明存在两种仅在非常致密的颗粒流或平衡系统中才观察到的现象。巴西果效应，即在振动时颗粒混合物中较大颗粒的上升现象，在致密系统中已被观察到，这是由于小颗粒通过大颗粒之间的间隙渗透，或者由于对流滚动。在当前情况下，这两种效应都不存在，观察到振动使较小颗粒流化会导致在与颗粒直径相比很大的高度处密度分布呈指数衰减，从而形成一个随高度降低的压力场。悬浮在这个衰减压力场中的较大颗粒，底部受到的压力较大，顶部受到的压力较小，它们上升到一个高度，此时压力降低所产生的净力与颗粒重量平衡。在这个非平衡系统中还观察到了大颗粒之间的吸引力，类似于胶体和聚合物混合物中的熵吸引效应，因为当大颗粒之间的距离小于小颗粒直径时，大颗粒之间的压力比外部的压力小。对每种效应都得出了分析结果，这些结果与实验和模拟结果一致。
