Novel Electrical Control in Droplet Microfluidics Using an AC Electric Field

We report a novel electrical control in droplet microfluidics using an ac electric field. The method can be used to actively control and tune the size of water-in-oil droplets formed in a microfluidic flow focusing device. Electrodes fabricated using microsolidics technique are not in contact with the fluids and hence prevent undesirable electrochemical reactions. The proposed electrical control is able to change the size of the droplets in the order of millisecond and also to manipulate the size of the droplets formed at single droplet level through the change in applied voltages and signal. An electro-mediated droplet formation regime transition from dripping to jetting is also observed due to the changes in the Maxwell stress arising from the application of the ac electric field. At applied frequency of between 50 to 5 kHz, the change in droplet sizes follow similar trend for the case of pure DI water but is different for the case of NaCl solution.