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Emission modes in electro co-flow

Josefa Guerrero and Alberto Fernandez-Nieves

We use glass-based microfluidic devices to study the emission regimes in electro co-flow. In contrast to classical electrospray, in electro-coflow a liquid is ejected through a nozzle into another co-flowing liquid. As a result, additional parameters provide control over the emission; these include the viscosity and flow rate of the outer co-flowing liquid. Below you can see a sketch of the experimental setup [1].


Fig. 1: Schematic of a microdevice.

In electro-coflow, depending on the inner and outer flow rates as well as the applied voltage, we can observe different emission modes: electro-dripping, microdripping, spindle, cone-jet and whipping modes, among others. Some of then have been described in a liquid air interface, and others are specific to electro co-flow. One of the goals of this project is to find the phase diagram respect all the variables that control our system: voltage, inner and outer flow rates and the viscosity of both liquids. These diagrams will help us to understand why we see certain modes in a well defined experimental window and no in others, and have control over the type and size of the emission.


Fig. 4: Phase diagram (V-Qi) using as outer medium 10cSt Silicone oil.

The present of a viscous co-flowing liquid changes the geometric properties of the modes. For example, the 3D whipping structure, that has a helicoidal shape when the outer medium is viscous, becomes chaotic when the experiments are performed in air or in low viscous media [2]. We have seen that the presence of the outer medium is crucial to get a wider parametric window where the whipping is steady.


Fig. 2: Steady state whipping.

Fig. 3: Chaotic whipping.



These two new variables also result in the observation of new modes that have not been reported before, like the 2D whipping you can see below.


Fig. 4: 2D whipping.

The droplet and microfibers generated using electro co-flow can be used in filtration, protective clothing, biomedical applications, pesticides, and composites materials, just to name a few.

References:
[1] V. R. Gundabala et al. Phys. Rev. Lett. 105 (2010).
[2] J. Guerrero et al. PNAS 111(38) (2014).

Soft Condensed Matter Laboratory, School of Physics, Georgia Institute of Technology
770 State Street NW, Atlanta, GA, 30332-0430, USA
Phone: 404-385-3667 Fax: 404-894-9958
alberto.fernandez [at] physics.gatech.edu