(349e) Generating Mono-Dispersed Femto/Pico-Liter Aqueous Droplets without External Oil Flow: AC Electrospray of Micro/Nanoemulsion

Water-in-oil and oil-in-water droplet microfluidics has seen wide applications in digital nucleic acid quantification, single cell analysis and nanoemulsion/particle synthesis. Currently, viscous shear droplet generation mechanisms are employed with a T-junction or a flow focusing geometry. Such shear-based droplet generation mechanisms require a high continuous-phase flow rate and caereful tuning of the relative water-oil flow rates (capillary numbers). As such, precise pressure and flow control in mechanically robust chips is necessary. Scaling up to multiple droplet generation nozzles is also problematic because of strong hydrodynamic interaction between different droplet generating nozzles at high continuous-phase flow rates. In this study, we report an AC electrokinetic droplet generation mechanism that does not require a flowing continuous phase. Instead, we use the Maxwell pressure of an AC field to extrude a long filament from the droplet at the nozzle. With polyethylene oxide (PEO) additives, the dispersed phase becomes viscoelastic and the elastic pressure at the droplet suppresses the formation of AC cones that generate highly dispersed nanodrolplets (Chetwani, Maheshwari and Chang, PRL, 2008) and favors the extrusion of a filament that breaks up by Rayleigh instability without satellite droplets (Chang, Kalaidin and Demekhin, Phys Fluid, 1999). We are hence able to generate monodispersed water-in-oil droplets (<1 % variation) with diameter from 1 to 100 microns at a frequency of 300 Hz without oil flow or micro-fabricated chips. The droplet size and generation frequency can be easily tuned by adjusting the voltage or the background pressure. A scaling model is developed to explain the dependence of droplet size on applied potential, field frequency and pressure. The same model captures the "phase diagram" for optimum operation in the parameter space. Massive parallelization using a multi-nozzle orifice plate allows generation of 1 million droplets in 15 minutes. We demonstrate the potential of this versatile technology by performing loop-mediated isothermal amplification and precise quantification of DNA templates, whose number varies by 4 orders of magnitude from hundreds to millions.