Drop-based microfluidic devices are becoming more common and molecular mass transfer and drop circulation are issues that affect the performance of such devices. Moreover, interfacial properties and surfactant mass transfer rates govern emulsion behavior. Since these phenomena depend strongly on drop size, measurement methods using small drops and flow typical of applications are desired. Using mineral oil and water, with alcohol surfactant, we demonstrate here a microfluidic approach to measuring interfacial tension, surfactant mass transfer and interfacial retardation. Interfacial flow is influenced markedly by adsorption of surfactant. Severe interfacial retardation (by a factor of 30) is observed at low surfactant concentrations and interface remobilization is observed at higher surfactant concentrations. The interfacial tension is described well by Langmuir kinetics and the parameters for interfaces with mineral oil (studied here) match those previously found at air interfaces. For the conditions explored, the surfactant mass transfer is shown to be kinetically limited, and the desorption rate coefficient (from the interface to the oil) is measured to be approximately 50 s-1. This experimental approach (i.e. adjustable geometry and drop size and height) can therefore probe interfacial dynamics in simple and complex flow.
and Hudson, S.
Mass transfer in two-phase microchannel flows, New Journal of Physics, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=902708
(Accessed December 3, 2023)