Use of electrowetting to measure dynamic interfacial tensions of a microdrop

The adsorption of surface active species to liquid–liquid and to solid–liquid interfaces can have dramatic effects in microfluidics. In this paper we show how electrowetting on dielectric can be used to monitor a dynamic liquid–liquid interfacial tension (IFT) with a time resolution of O(1 s) using amplitude modulation of the AC voltage. This straightforward method, which requires less than a microlitre of sample, is demonstrated for aqueous drops containing Triton X-100 surfactant on a Teflon AF-coated substrate and with heptane as the immiscible oil ambient. Under these conditions, next to extracting the oil–water IFT (γow), also the effective water–substrate IFT difference (Δγws) can be obtained from the oil–water IFT and the Young's angle. Both γow and γws decrease over time due to adsorption. The measured dynamic oil–water IFT compares well to results of pendant drop experiments