  In this work, we introduce the novel technique of in-chip drop on demand,
which consists in dispensing picoliter to nanoliter drops on demand directly in
the liquid-filled channels of a polymer microfluidic chip, at frequencies up to
2.5 kHz and with precise volume control. The technique involves a PDMS chip
with one or several microliter-size chambers driven by piezoelectric actuators.
Individual aqueous microdrops are dispensed from the chamber to a main
transport channel filled with an immiscible fluid, in a process analogous to
atmospheric drop on demand dispensing. In this article, the drop 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 the in-chip drop on demand
technique with direct relevance to lab on a chip applications are presented and
discussed, such as the precise control of the dispensed volume, the ability to
merge drops of different reagents and the ability to move a drop from the
shooting area of one nozzle to another for multi-step reactions. The
possibility to drive the microfluidic chip with inexpensive audio electronics
instead of research-grade equipment is also examined and verified. Finally, we
show that the same piezoelectric technique can be used to generate a single gas
bubble on demand in a microfluidic chip.
