Electrohydrodynamic Generation and Delivery of Monodisperse Picoliter Droplets Using a Poly(dimethylsiloxane) Microchip

We developed a drop-on-demand microdroplet generator
for the discrete dispensing of biosamples into a bioanalytical unit. This disposable PDMS microfluidic device
can generate monodisperse droplets of picoliter volume
directly out of a plane sidewall of the microfluidic chip by
an electrohydrodynamic mechanism. The droplet generation was accomplished without using either an inserted
capillary or a monolithically built-in tip. The minimum
droplet volume was ∼4 pL, and the droplet generation was
repeatable and stable for at least 30 min, with a typical
variation of less than 2.0% of drop size. The Taylor cone,
which is usually observed in electrospray, was suppressed
by controlling the surface wetting property of the PDMS
device as well as the surface tension of the sample liquids.
A modification of the channel geometry right before the
opening of the microchannel also enhanced the continuous droplet generation without applying any external
pumping. A simple numerical simulation of the droplet
generation verified the importance of controlling the
surface wetting conditions for the droplet formation. Our
microdroplet generator can be effectively applied to a
direct interface of a microfluidic chip to a biosensing unit,
such as AMS, MALDI-MS or protein microarray-type
biochips.