(160b) Focused DC and AC Electric Fields at Conic Pipettes: Nanoscale Thermal Hotspot and Nano-Droplet Generation with Universal Scalings

We report two new phenomena due to focused electric fields at a conic nanopipette and suggest how they can be harnessed for molecular assays:

(1) Ohmic heating due to a DC ionic current through a conic nanopipette or a conic nanopore is shown to produce a nanoscale thermal hotspot whose dimension is independent of the nanopipette length¹. As in DC Taylor cones, the conic geometry allows for a local analysis of the thermal temperature that is independent of far field conditions. The singular Ohmic heating rate in the bulk fluid at the tip is exactly balanced by the singular conductive heat loss, as the area per unit volume also blows up with the same power in the same limit. Using matched asymptotics, we obtain an estimate of the thermal hot spot temperature and size as a function of a single dimensionless parameter Z that scales quadratically with respect to voltage and cone angle. We use this self-similar local solution to collapse measured quadratic increase in the current at high voltages due to thermal effects on the fluid viscosity and ion conductivity. Temperature approaching 100 degrees C can be reached with modest voltages like 20 V. Nanothermophoresis of molecules is a possible application of this nanofluidic phenomenon due to focused DC field.

(2) We extend our early work on AC spray of liquid droplets into air² to generation of water droplets into an immiscible oil phase. The higher viscosity of the oil phase produces significant convection within the water phase. As a result, dynamic AC charge accumulation seen in our previous work was eliminated. Without charge polarization, charged AC cone, nanojets and nanosprays are not observed in oil. A conic pipette is also used to enhance capillary forces, such that the AC Maxwell pressure cannot pull out a jet but pinches off individual droplets at the tip. The frequency of the AC field is further tuned to the inverse RC time of a desirable pinching droplet size and the voltage is adjusted so that the electric Maxwell pressure matches the capillary pressure of the same droplet. Validated size calibrations and phase boundaries are offered in terms of the electric Bond number and a dimensionless frequency. Unlike conventional flow focusing design, which relies on oil/water flow rate ratio for size tuning, we can generate monodispersed water droplets from 1 to 100 μm in diameter by simply tuning the electric field. A digital droplet PCR technology based on this phenomenon is shown to allow nucleic acid quantification over a large dynamic range.

References:

[1] Pan, Zehao, Ceming Wang and Chang, H.-C. "Universal Scaling of Robust Thermal Hot Spot and Ionic Current Enhancement by Focused Ohmic Heating in a Conic Nanopore." , Phys Rev Lett, 117, 134301(2016).

[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).