Particle manipulation in a microfluidic channel using acoustic trap

A high frequency sound beam was employed to explore an experimental method that could control particle motions in a microfluidic device. A 24 MHz single element lead zirconate titanate (PZT) transducer was built to transmit a focused ultrasound of variable duty factors (pulse duration/pulse repetition time), and its 1–3 piezocomposite structure established a tight focusing with f-number (focal depth/aperture size) of one. The transducer was excited by the Chebyshev windowed chirp signal sweeping from 18 MHz to 30 MHz with a 50% of duty factor, in order to ensure that enough sound beams were penetrated into the microfluidic device. The device was fabricated from a polydimethylsiloxane (PDMS) mold, and had a main channel composed of three subchannels among which particles flowed in the middle. A 60~70 μm diameter single droplet in the flow could be trapped near the channel bifurcation, and subsequently diverted into the sheath flow by releasing or shifting the acoustic trap. Hence, the results showed the potential use of a focused sound beam in microfluidic devices, and further suggested that this method could be exploited in the development of ultrasound-based flow cytometry and cell sorting devices.