Real time acoustic sensing of flowing microdroplets in a microfluidic device

Automated and rapid sample preparations have been considered as key components in developing micro total analysis systems (μTAS), for onsite monitoring and diagnosis of various pathogens. Fluorescence-activated cell sorting (FACS), in particular, has allowed fast screening of phenotypically different cells by scanning laser beams, but its implementation is rather complicated and costly. This paper presents recent results obtained on rapid acoustic sensing of flowing microdroplets in a PDMS microfluidic device. A 30 MHz lithium niobate single element transducer was built for the test. The transducer was positioned outside the channel, perpendicularly aiming at the channel wall. The echoes from moving droplets of 50 and 100 μm in diameter were measured, and their IB coefficients were calculated at the same time. The results show that those droplets can be readily identified by their echo amplitudes and IB values. The results showed that this sensing technique using a highly focused ultrasonic transducer was able to remotely identify flowing microdroplets in microfluidic channels, based on their echo signals and IB coefficients. Hence this acoustic approach may be used to develop much simpler devices than FACS systems and serve as a core element in μTAS.