(182e) Monodispersed Droplet Generation Using AC Electric Field

We report a new microfluidic technology that uses an AC electric field to generate mono-dispersed water-in-oil droplets that range from 10 to 100 μm in diameter, without a conventional flow focusing design. The droplet size of this technolocy can be easily tuned by adjusting the voltage/frequency and is hence much easier to use then flow-focusing designs, which adjust water/oil flow rate ratios to vary the droplet size. As such, it is ideal for multiplex digital PCR and high-throughput emulsification applications that require frequent size tuning for a large dynamic range.

The droplet generation unit is a micropipette within a tubing with water being pumped through the micropipette and a co-flowing oil in the annular region to convect the water droplets away downstream. By using laser-assisted pulling, the tip of the micropipette is pulled into a conic shape with a tip diameter of about 10 μm¹. This conic geometry focuses the electric field so that the Maxwell pressure is sufficient to overcome capillary pressure even at small scales. The oil phase contains surfactants that stabilizes the emulsion but also allows droplets to pinch off at the tip of the micropipette.

Unlike flow focusing, our field focusing design does not produce a jet that breaks up by the Rayleigh instability or by a convective version of the instability that pinches off the tip of a long jet. We also avoid large droplets at the tip, like those pinched off by high oil shear rates at T-junctions, whose critical viscous drag for pinch-off is only reached for droplets much larger than the capillary tip. Instead, we tune the frequency of the AC field such that a Maxwell pressure is imparted over a large area of the first droplet, without producing a cone or jet². This large pressure accelerates the drop forward at a rate that cannot be matched by the water feed flow rate, much like inertial acceleration but at negligible feed Reynolds numbers. As in inertial acceleration of a finite-volume drop, a neck forms at the capillary tip that pinches off single droplets at a very high rate beyond 100Hz. Phase diagrams are constructed in the parameter space of dimensionless voltage (Bond number), frequency, flow rate (Weber number), viscous drag and surface tension (capillary number). Correlations for the droplet size and generation frequencies in terms of these dimensionless parameters will also be reported to collapse the measured data.

[1] Pan, Zehao, et al. "Universal Scaling of Robust Thermal Hot Spot and Ionic Current Enhancement by Focused Ohmic Heating in a Conic Nanopore." Physical Review Letters 117.13 (2016): 134301.

[2] Chetwani, N., Maheshwari, S. and Chang, H.-C."Universal cone angle of ac electrosprays due to net charge entrainment", Phys Rev Lett, 101, 204501(2008).