Time-resolved electrochemical measurement device for microscopic liquid interfaces during droplet formation

An electrochemical measurement system with a high-speed camera for observation of dynamic behavior of ionic molecules at a water-in-oil interface during microfluidic droplet formation is described. In order to demonstrate the usefulness of the system, a liquid interface between 1 M sodium chloride aqueous solution and 0.02 M tetrabutylammonium tetraphenylborate 1,2-dichloroethane solution was investigated. During aqueous droplet formation in a microfluidic device, averaged and dynamic currents between the two phases were measured under potential control. The measured current corresponded to the transport of electrolyte ions to form the electrical double layer at the liquid interface. When an 18-μm-sized droplet was formed in each 1.2 ms, the amount of charge on each droplet was measured to be 20 pC at 0.4 V and negligible at the potential of zero charge (0.19 V). In addition, the high-speed camera observations revealed that the charge affects the stability of the droplet during and/or just after the generation process. This measurement system is expected to facilitate a fuller understanding of the droplet formation process.