Simple model for reverse buoyancy in a vibrated granular system

¹ Departamento de Fisica, Universidad Simón Bolivar Apartado 89000, Caracas 1080-A, Venezuela
² Centro de Fisica, Instituto Venezolano de Investigaciones Cientificas Apartado Postal 21827, Caracas 1020-A, Venezuela
³ University of Maryland - College Park, MD 20742-2431, USA

Corresponding author: gustav@usb.ve

Received: 15 August 2003
Accepted: 1 June 2004

Abstract

We report an experimental study of the motion of a large sphere immersed in a deep dry granular bed, subjected to vertical sinusoidal vibrations. Under certain conditions large heavy objects immersed in a homogeneous vibrated granular medium rise and similar light ones sink to the bottom. This is called reverse buoyancy. We have measured the vertical velocity of immersed large spheres, as a function of their density, under conditions for which reverse buoyancy is observed. We propose a one-dimensional mechanical model, based on the assumption that a cyclic fluidization of the medium occurs. With this model, that takes into account a buoyancy force and a frictional drag, we obtain, under the conditions of our measurements, the rising velocity for heavy objects and the sinking rate for light ones. The model yields a very good qualitative and quantitative agreement with our experiments.