Electric control in droplet-based microfluidics

INTRODUCTION Droplet-based microfluidics is an attractive tool for many biochemical applications as it allows discrete volume of fluids to be individually handled and manipulated. Each generated droplet can act as an independent micro-reactor in which bio-chemical reactions can be processed within the droplets at high throughput. However, the full exploitation of the potential is currently limited as only a single droplet size can be produced at fixed flow rates [1]. Hence, greater control of droplet sizes is needed to realize the full potential and the usefulness of droplet-based microfluidics, for material synthesis or biotechnology applications. Electrical control of droplets offers a good way to manipulate droplet sizes due to the fast and robust response of the droplets to the electric field. It was first reported by Link et al [2] using a microfluidic flow focusing device. It demonstrated that by charging the fluids, electrical forces can be used to decrease the size of the droplets. Subsequently, using a similar geometry but by inserting metals directly into the microchannels, Kim et al [3] also show a decrease in the size of the droplets with the increase in voltage. Following these works, Malloggi [4] and Gu et al [5] used an electrowetting-enhanced system to decrease the size of the droplets. This is achieved by tuning the wetting contact angle of the dispersed phase fluid through the increase in voltage. However in the above, several underlying drawbacks limit the use of them in biological applications or using droplets as microreactors. The direct contact of the metal with the fluids raised fears of possible contamination and the charging of the fluids produces unwarranted electrochemical reactions which interfere and disrupt the stability of the content of the droplets. In addition, the above reported methods also do not address the manipulation and control at single droplet level. To address the above issues, we developed a new method of droplet size control using electrodes which are not in contact with the fluids. The system is able to tune the size of the droplets in the order of milliseconds and also control the size of the droplets at single droplet level.