Separation of micron-sized particles in macro-scale cavities by ultrasonic standing waves

Abstract

The separation of micron-sized particles from a steady flow of water through the use of ultrasonic standing waves is discussed. An ultrasonic resonator has been designed with a flow inlet and outlet for the water stream. The resonator is typically about 0.15 m long with a cross-section of 0.0254 m×0.0254 m. The flow inlet and outlet ports have been designed to ensure laminar flow conditions into and out of the resonator. A PZT-4, 2-MHz, transducer is used to generate ultrasonic standing waves in the resonator. At fixed frequency excitation, particles are concentrated at the stable locations of the acoustic radiation force. Particle translation is achieved by a periodic sweeping of the frequency of excitation, which generates a slowly moving standing wave. The effect of the sweep period and the sweep frequency range on particle translation speed has been investigated for the separation of 6 micron polystyrene beads. A numerical model has been developed to compute the trajectories of particles subjected to the acoustic radiation force, fluid drag force, and buoyancy force. The acoustic radiation force is determined from a one-dimensional acoustic field model. A CFD model is used to calculate the mean fluid flow into and out of the resonator and is used to calculate the fluid drag force. The equations of motion are then integrated to yield the particle trajectories. Typical particle translation speeds are on the order of 5 mm/s. Successful separation of 6 micron polystyrene beads from a water inlet flow, with typical volume flow rates of 150 ml/min, has been achieved.