(153a) Electric Field Induced Mixing and Separation in a Tiny Droplet

When a fluid is electrified, its surface forms a series of standing cones or spikes resembling a pincushion. This can trigger sparks, flashes of light, and sprays of droplets. This phenomenon is encountered in nature and in technology; in lightning, raindrop formation, electrostatic spray guns, plasma technology, inkjet printers, and medical diagnostics [1, 2]. In his seminal papers [3, 4], Taylor showed that surface tension and electric forces form a steady-state conical meniscus with a semivertex angle of 49.3°. However, field-driven meniscus evolution from a rounded shape to a cone was a long-standing puzzle in this well-studied phenomenon as it overlaps with spontaneous fluid ejection. In Ref. [5], we developed a method to control the cone-shaped spikes whose size is just shy of droplet ejection. Due to Earth’s gravity, the maximum possible cone size is small, near half a millimeter. Experiments on water were carried out on Earth and in space on the International Space Station (ISS). Microgravity on the ISS enabled us to expand the measured cone length to over 5 centimeters revealing that field-driven evolution from a rounded shape to a cone exhibits a universal self-similarity scaled by the fluid surface tension and density and strikingly insensitive to the forcing field while a 50% increase in applied voltage shortens the overall time for the meniscus to rise by more than an order of magnitude [5].

In this presentation, we will show that a dynamic cone offers a non-contact method to generate a controlled vortex flow inside a tiny droplet. We will demonstrate that the application of an AC voltage can be used to affect mixing and separation processes in the droplet. Specific examples include accumulation of entrapped air bubbles on the droplet surface to form a layer of dense froth, concentration or dispersion of particles suspended in a droplet, and intensification of precipitation processes in a droplet of a supersaturated aqueous solution.

The work is supported by National Aeronautics and Space Administration Grants NNX09AK06G and NNX13AQ53G.

References

1. J. Fernandez de la Mora, The fluid dynamics of Taylor cones. Annu. Rev. Fluid Mech. 39, 217–243 (2007).

2. J. Eggers and E. Villermaux, Physics of liquid jets, Rep. Prog. Phys. 71, 03660-1-79 (2008).

3. G.I. Taylor, Disintegration of water drops in an electric field. Proc. R. Soc. Lond. Ser. A 280, 383–397 (1964).

4. G.I. Taylor, Electrically driven jets. Proc. Roy. Soc. Lond. Ser. A 313, 453-475 (1969).

5. Ezinwa Elele, Yueyang Shen, Donald R. Pettit, Boris Khusid, Detection of a dynamic cone-shaped meniscus on the surface of fluids in electric fields, Phys. Rev. Lett. 114, 054501 (2015).