Magneto-vibratory separation of glass and bronze granular mixtures immersed in a paramagnetic liquid.

A fluid-immersed granular mixture may spontaneously separate when subjected to vertical vibration, separation occurring when the ratio of particle inertia to fluid drag is sufficiently different between the component species of the mixture. Here, we describe how fluid-driven separation is influenced by magneto-Archimedes buoyancy, the additional buoyancy force experienced by a body immersed in a paramagnetic fluid when a strong inhomogeneous magnetic field is applied. In our experiments glass and bronze mixtures immersed in paramagnetic aqueous solutions of MnCl2 have been subjected to sinusoidal vertical vibration. In the absence of a magnetic field the separation is similar to that observed when the interstitial fluid is water. However, at modest applied magnetic fields, magneto-Archimedes buoyancy may balance the inertia/fluid-drag separation mechanism, or it may dominate the separation process. We identify the vibratory and magnetic conditions for four granular configurations, each having distinctive granular convection. Abrupt transitions between these states occur at well-defined values of the magnetic and vibrational parameters. In order to gain insight into the dynamics of the separation process we use computer simulations based on solutions of the Navier-Stokes' equations. The simulations reproduce the experimental results revealing the important role of convection and gap formation in the stability of the different states.

Traveling waves in a water-immersed binary granular system vibrated within an annular cell.

It has been known since the time of Faraday that vertically vibrated fine grains may spontaneously form piles through their interaction with a fluid. More recently, it has been observed that a fine binary mixture may separate under vertical vibration through the differential influence of the fluid on the two granular components. Here, we report a detailed study of a system of water-immersed bronze and glass grains held between two coaxial cylinders. Under vertical vibration, the bronze separates to form a layer above the glass, which itself breaks symmetry to form a pile. Symmetry is broken a second time by the bronze forming layers of different thicknesses upon the two slopes of the glass pile. The pile then travels as a wave with the thicker bronze layer upon its leading surface. We examine the conditions for these traveling waves and determine how their speed varies with particle size, frequency, and amplitude of vibration. A model is developed which provides a semiquantitative account of the wave motion.

Faraday tilting of water-immersed granular beds.

Under low-frequency vertical vibration, a system of fine grains within a fluid is observed to tilt or to form piles, an effect studied by Faraday for grains in air. Here, we investigate the physical mechanisms behind Faraday tilting in a bed of vertically vibrated bronze spheres fully immersed in water. Experimental observations of surface tilting and bulk convection are compared with the results of molecular dynamics simulations in which the water is treated as an incompressible fluid. Our simulations reproduce the main features observed experimentally. Most tilt construction is shown to be due to horizontal fluid flow within the bed, principally occurring when the gap between the bed and the supporting platform is close to a maximum. Tilt destruction occurs by granular surface flow and in the bulk of the bed at times during each vibratory cycle close to and just later than bed impact. Destruction becomes more important for higher values of frequency and vibration amplitude, leading to lower tilt angles, partial tilting, or the symmetric domed geometry of Muchowski flow.