16th EUFOAM Conference

European Conference on Foams

Abstract of the talk by Dr. Andrew Mellor:

Different Method Same Result: How to ensure truly comparable data when measuring dynamic interfacial tension

The dynamic interfacial tension (DIFT) between two immiscible liquids is commonly measured using drop volume tensiometry (DVT) or drop shape analysis (DSA), in configurations such as pendant/falling and rising drops. Such instrumental flexibility is a valuable tool, but rests on the assumption that different configurations yield equivalent, directly comparable results. Here we test that assumption by systematically comparing various parameters of dynamic interfacial tension measurements such as instrument configuration, oil viscosity, droplet formation time, and the location of surfactants (aqueous phase, oil phase or both). Studying model surfactant systems, Triton X-100 and Tween-20, at concentrations well below the CMC. We find the apparent rate constant 𝑘 for interfacial-tension reduction decreases with increasing viscosity of the surfactant-laden phase and with increasing surfactant molecular size. While initial surfactant location has little or no effect on the apparent rate constant for DIFT, we do observe measurable differences in absolute steady-state interfacial tension depending on surfactant location. These observations are consistent with diffusion-limited adsorption where transport in the bulk phase and partitioning between phases sets the timescale for adsorption and final interfacial coverage. Finally, based on this work, we offer recommendations for reporting of data when using such methods of interfacial characterisation.