Phase separation technique for suspended particles in microchannel utilizing bilayered acoustic fields

A non-intrusive and continuous separation technique for suspended particles in a microchannel has been developed by utilizing acoustic radiation force with two ultrasonic transducers. The technique has two major advantages that the acoustic radiation force acts on particles in proportion to particle diameter, and collects particles to the nodal positions of the standing wave field perpendicular to the flow direction. Thus the large size particles have shorter time of transfer to the nodal positions than the small size particles. Particle velocities toward the nodal position within the sound field were measured by particle tracking velocimetry, and both the migration times of particle transfer to the nodal positions and the acoustic radiation force were evaluated from the particle images and velocity data in order to separate particles in the flow field. The ultrasonic transducers with 5 and 2.5 MHz were equipped parallel to the flow direction. Both large and small particles in the aqueous solution were trapped at the nodes of the upstream in 5 MHz sound field, and 2.5 MHz transducer was radiated to move only large particles toward a nodal position of its sound field. The exposure time of 2.5 MHz transducer was determined from the migration times of large and small particles transfer to the nodal positions. It is confirmed that the continuous and selective separation based on particle diameter was accomplished by the present technique.