We introduce a novel technique, which consists in dispensing individual picoliter to nanoliter drops or bubbles on demand directly in the liquid-filled channels of a polymer microfluidic chip. The technique involves a PDMS chip, which is fabricated using photolithography, with one or several microliter-size chambers driven by piezoelectric actuators. Individual particles are dispensed at kHz frequencies from a chamber to a main transport channel filled with an immiscible fluid, in a process analogous to atmospheric drop on demand dispensing. The particle formation process is characterized with respect to critical dispense parameters such as the shape and duration of the driving pulse, and the size of both the fluid chamber and the nozzle. Several features of this technique with direct relevance to lab-on-a-chip applications are presented and discussed, such as the rapid mixing of reagents or the encapsulation of a biological cell. Finally, we show that the same piezoelectric technique can be used to generate a single gas bubble on demand in a microfluidic chip.
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