Particle characterization and separation by a coupled acoustic-gravity field.

A coupled acoustic-gravity field is proposed as a novel external field for particle separation and characterization. When a standing plane ultrasound wave is generated, particles move to the node of the wave along the ultrasound force gradient. If the particles also undergo a sedimentation 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, characterizes the particles. The local ultrasound energy, which is necessary for quantitative discussions, is evaluated by 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. Nonporous particles of inorganic and polymeric materials, the particle sizes of which range from 3 to 100 microm, follow a derived model, suggesting that the local ultrasound energy and a derived model be valid. The proposed external field can be used for separation of particles having different acoustic natures.