Flow-through separation of microparticles of different sizes using ultrasound

Recently, many studies have been reported on the separation of microparticles in a fluid-flow using ultrasound since ultrasonic standing wave enables particles to align to its pressure nodal or anti-nodal plane (depending on their density and compressibility) due to its acoustic radiation force. However, the existing studies are showing that there still requires more stable and efficient method to separate particles of different sizes. This work concerns an alternative for fast and efficient flow-through separation method by using the frequency sweep technique. A modularized microchannel comprising steel and acrylic with a width of 1.5 mm was manufactured for this purpose. An ultrasound transducer installed at one side of the microchannel module generates ultrasonic standing wave field inside the channel, where the periodical increasing frequency sweep of the ultrasound can shift all particles nearly to one side of channel wall. Interestingly, in this phenomenon, it is observed that the response time of each particle is not the same due to the difference of the acoustic radiation force. This difference of the response time defines the minimum sweep period (MSP) of a specific particle, the frequency sweep period of the exciting ultrasound which can move only that type of particles to the last nodal plane close to the wall. As the different sized particle has different MSP, particles with two different sizes can be separated when the frequency sweep period is chosen between MSP's of two particles since only larger particles can move at that frequency sweep period. Polystyrene microspheres were used as test particles in this experiment. The experimental results confirm that by choosing the MSP of the operational frequency can be one of the efficient methods to separate microparticles in fluid-flow using ultrasound.