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 (γ(ow)), also the effective water-substrate IFT difference (Δγ(ws)) can be obtained from the oil-water IFT and the Young's angle. Both γ(ow) and γ(ws) decrease over time due to adsorption. The measured dynamic oil-water IFT compares well to results of pendant drop experiments.