Measuring the Dynamic Nanometric Contact Radius of a Single Microdroplet on an Electrified Microinterface.

Aqueous microdroplets have received attention due to their peculiar physicochemical properties, such as their ability to drive unfavorable chemical reactions orders of magnitude more quickly than in the bulk phase. However, very few techniques can probe microdroplets, one-at-a-time. Even fewer techniques can also provide real time information on the physical properties of the microdroplet reactor at the nanoscale. Such properties are highly important for rigorous mechanistic investigations. Here, we demonstrate a simple electrochemical method to quantify the nanometer contact radius that forms between a colliding droplet and an electrified surface, as a function of time. We address the limitations in the previous model used for sizing the nanometric contact area and offer a new quantitative framework. These new analyses give access to nanoscale wetting dynamics of individual microdroplets on an electrified micro-interface. We demonstrate control over the microdroplet wetting dynamics using electrostatics. Finally, we use this platform to drive reactions within individual adsorbed microdroplets. We track the oxygen reduction reaction in real time at the well characterized microdroplet|microelectrode contact, extracting the actively partition-controlled oxygen concentration in single microdroplets. These results have high sensitivity, allowing us to decipher both physical properties of droplets far below the diffraction limit of light and measure reactions at nanoscale, multiphase surfaces.