Patent ID: 7674630

Claim:
A method for separation of particles of a particle mixture suspended in a medium, comprising the steps of: subjecting medium in a chamber to ultrasonic energy generating a first ultrasonic standing wave at a first frequency having first pressure nodes; placing the particle mixture in the chamber, so that a concentration of the particle mixture is placed at or in the vicinity of a first pressure node of the ultrasonic standing wave at said first frequency or anywhere between the first pressure node and a wall of the chamber ( 2 ; 22 ), wherein the medium and the suspended particle mixture are brought to flow in a laminar flow through a channel comprising the chamber and the ultrasonic standing wave is transverse to the flow direction; switching the frequency of the ultrasonic standing wave to a second frequency generating a second ultrasonic standing wave having at least one second pressure node, so that a first set of particles of the particle mixture is experiencing a smaller acceleration towards said second pressure node, and a second set of particles of the particle mixture is experiencing a greater acceleration towards said second pressure node; maintaining the second frequency during a time duration t 2 and with an amplitude a 2 , allowing the first set of particles of the particle mixture to travel a smaller first distance, and the second set of particles of the particle mixture to travel a greater second distance; switching back to the first frequency; wherein the smaller first distance is so small that the first set of particles of the particle mixture is experiencing a greater acceleration towards the first pressure node, and the greater second distance is so great that the second set of particles of the particle mixture is experiencing a smaller acceleration towards the first pressure node; maintaining the first frequency during a time duration t 1 and with an amplitude a 1 , allowing the first set of particles of the particle mixture to travel back towards the first pressure node, while the second set of particles of the particle mixture is not allowed to travel a significant distance; and repeatedly switching the ultrasonic standing waves between the first and second frequencies with a duty cycle involving their respective time durations t 1 and t 2 , and amplitudes a 1 and a 2 , so that a substantial part of the first set of particles of the particle mixture is collected at the first pressure node, and a substantial part of the second set of particles of the particle mixture is collected at the second pressure node.