Electrical Control of Droplet formation in Microfluidic Devices

Abstract

This thesis presents both experimental investigations and demonstration on the use of an AC voltage induced electric field to actively control the size or frequencies of droplet generated in a microfluidic flow focusing device. In the first part, we demonstrate our patented technology where the concept is realized as a new method of controlling the droplet sizes or frequencies. We show that the concept can be applied for both the case of with and without the orifice in the microfluidic flow focusing geometry. Next, we systematically investigate the influences of various parameters such as frequencies of sinusoidal signal, conductivities of the dispersed phase fluid, different electrode configurations, ionic properties of the dispersed phase fluid and different volumetric flow rates. We present different electrical circuits to model the different electrode configurations and show that the voltage at the tip of the dispersed phase fluid is the underlying cause for the experimental observations seen in the different conditions. Using the voltage divider rule, the voltage at the tip of the dispersed phase fluid is estimated using the ratio of the frequency (f) of the signal to the conductivity (к) of the dispersed phase fluid. In the case of electrode in configuration A, we observed that the system behaves akin to a high pass filter around the value of about f/к = 4 x 105 m/F when the experimental data are rescaled. Phase diagrams illustrating the effect of both the frequencies and conductivities of the different electrode configurations are also presented. In electrode configuration A and C, we observed a transition from the unstable jetting to the dripping mode at a similar value of about f/к = 5 x 105 m/F. This result agrees to our hypothesis where the voltage at the tip of the dispersed phase fluid decreases significantly behaving akin to a high pass filter below the stated ratio of f/к. We also present an electrohydrodynamic model to account for both hydrodynamic and Maxwell stresses using an effective capillary number Caeff. This non-dimensional number takes into account a capillary number Ca and an electrical bond number Be. Using the model, the diameters of the droplets can also be estimated by different power laws when formed in either the dripping or jetting regime. However, this model does not explain the transition between the droplet formation regimes. In the dynamical studies, we present the concept of using this AC voltage induced electric field to play musical sound tracks. A standard epifluorescence optical setup is used to detect the frequencies of the generated droplets. The frequencies of the generated droplets are then modulated by changing the applied AC voltage to play different musical notes. We examine the frequency range of the system and show that the range allows access to play all the different musical notes within one octave. We also examine the response time of the system using an amplitude modulation signal at different voltages and frequencies. Experimental results show that the system is capable of modulating the frequencies of the generated droplets in the order of milli-seconds which is suitable for playing musical sound tracks. Lastly, we use this high speed of control in the droplet formation frequencies as a microfluidic jukebox. Different musical song tracks namely Ode to Joy and The flight of the bumblebee are played by rapidly changing the droplet generation frequencies using different AC voltages.