My research focuses on applying electric fields to coflowing liquids in a glass-based microfluidic channel. From a fundamental point of view, we investigate the mechanism of drop formation in the presence of electric fields. From an application point of view, we attempt to fill the gap in the droplet size ranges that can be achieved by the sole application of either hydrodynamic or electric forces.

While microfluidics offers a nice route to producing emulsions through the added flow-control achieved due to the low Reynolds numbers involved, existing microfluidic devices employing pure hydrodynamic forces such as T-junction, flow-focusing, and coflow geometries cannot produce sub-micron sized droplets due to either geometric or fluid-dynamic limitations. Alternatively, applying electric fields to a conducting liquid meniscus in a flow focusing device, a method reminiscent of classical electrospray, can result in the formation of drops which are in the few hundred nanometer size range. But steady generation of droplets is almost impossible using this method due to the fact that charged droplets would want to discharge onto the walls, thus destabilizing the liquid meniscus.

In this project, we designed a microfluidic device that allows the steady generation of droplets through the coupled action of electrical and hydrodynamic forces. Using a liquid collector as the counter-electrode, we overcome the discharge problems usually associated with the application of electric fields. In addition, the unique design also provides an easy way to extract the generated droplets from the microfluidic channel. In some of our experiments, we use ethylene glycol as inner conducting liquid, and polydimethylsiloxane (silicone oil) of various viscosities as outer insulating liquid. When a potential difference is applied in the device, the liquid meniscus at the end of the inner capillary tip deforms into a conical shape, with the apex of the cone opening into a charged microjet that breaks either directly into droplets, called cone-jet mode, or undergoes a 3-dimensional whipping motion before breaking into droplets, called the whipping mode. The transition from one mode to the other, the droplet size, the length of the jet are all affected by operating parameters such as inner flow rate, applied voltage, outer fluid viscosity, etc. For example, the effect of outer fluid viscosity on the length of the jet can be seen from the series of videos shown below. The videos from left to right show observations with increasing outer fluid viscosity, keeping the other parameters constant. Increasing outer viscosity leads to shorter jet lengths.

Contact Information:
Venkat Gundabala
Office: Boggs Building, Room B-55
Email address: venkata.gundabala [at] physics.gatech.edu
Phone: 404-385-3681