Abstract
Acoustic-gravity coupled fields have been investigated for particle characterization and separation. When a plane standing ultrasound wave is generated in a cell containing water as a medium, particles move to the node of the wave along the ultrasound force gradient. When the particles undergo a sedimentation force in addition to the ultrasound force, they aggregate at the equilibrium position, where these two forces are balanced. The equilibrium position, which is determined by the density and compressibility of a medium and particles, provides the physical properties of the particles. The local ultrasound energy, which is necessary to quantitatively discuss particle behaviors, is evaluated using a standard particle, the physical parameters of which are unambiguously determined; aluminum particles are used in the present study. The local ultrasound energy makes possible the determination of the compressibility of unknown materials. In this study, we have measured equilibrium positions of alumina, silica grass, polystyrene and poly(acrylonitrile-co-vinyl chloride-co-methyl methacrylate)(PAVCMM) particles, and determined their density and compressibility. These nonporous particles of inorganic and polymeric materials follow a derived model, suggesting that the local ultrasound energy and a derived model be valid. Separation of porous particles based on the porosity is also possible. The equilibrium positions of porous particles in the standing ultrasound wave field are determined by the porosity of the particles as well as by the density and compressibility of silica glass. The porosity of some silica gel particles has been determined as 22, 34, 55 and 63% by the present method, which agrees with that determined by a water-penetration method (22.7, 38.9, 69.6 and 72.5%). The proposed external field can be used for separation of particles having different acoustic natures.
