Effects of Ultrasmall Orifices on the Electrogeneration of Femtoliter-Volume Aqueous Droplets

The ability to generate individual picoliter- and femtoliter-volume aqueous droplets on demand is useful for encapsulating and chemically manipulating discrete chemical and biological samples. This paper characterizes the effects of orifice dimensions and material choices on generating such droplets in an immiscible oil phase by using single high-voltage pulses with various amplitudes and durations. We have examined microfluidic orifices as small as 1.7 microm in equivalent radii and found that the electrohydrodynamic jet lengths and the subsequent formation of droplets are affected by the axial aspect ratios of the orifices (length of an orifice divided by its equivalent radius). As higher voltages were used to compensate for the increased capillary pressure and hydrodynamic resistance in ultrasmall orifices, we observed secondary jet protrusions and droplet formations that were not of classical electrohydrodynamic origin. The droplets generated from secondary jets traveled at relatively lower velocities as compared to those of electrohydrodynamic origin, and these slow individual droplets are potentially more useful for applications in microscale chemical reactions.