Interfacial tension measurements with microfluidic tapered channels

We developed a microfluidic chip that is capable of measuring interfacial tensions between two liquids directly from a force balance. This functionality is obtained by using a tapered microchannel connected to a straight channel containing the second immiscible liquid. The equilibrium position of the liquid–liquid interface along the tapered channel depends on the interfacial tension, the contact angles with the channel walls, and externally controllable pressures. Using a device of PDMS with a channel height of 50 μm and a width tapering from 300 to 20 μm, equilibrium interfacial tensions (IFTs) could be measured for various systems with and without surfactant that display IFTs between 3 and 38 mN/m. Quantitative agreement with macroscopic measurements was obtained. In the absence of surfactant the resolution of our approach is limited by contact angle hysteresis and pinning. We demonstrate the potential of our device for measuring IFTs that change over time, by progressively exposing the W/O interface to surfactant. This microfluidic method offers interesting perspectives for measuring static or dynamic interfacial tensions on chip, combined with a flexible control over the composition of the continuous phase.