The Dynamic Effects of Surfactants on Droplet Formation in Coaxial Microfluidic Devices

Abstract

Droplet emulsification in microfluidic devices involves the constant formation of fresh interfaces between two immiscible fluids. When the multiphase system contains surfactant, dynamic mass transfer of the surfactant onto the interface results in a dynamic interfacial tension different from the static interfacial tension measured in an equilibrium state. In this work, we have systematically investigated the effects of surfactant concentration and type on the dynamic interfacial tension of two different liquid-liquid two phase systems [N-hexane/water-sodium dodecyl sulfate (SDS) and N-hexane/water-cetyltrimethylammonium bromide (CTAB)] rapidly producing relatively small droplets in coaxial microfluidic devices. Dynamic interfacial tension experiments using the pendent drop method and a tensiometer were conducted, and a semiempirical equation was developed to put into context the effects of surfactants and the experimental conditions on droplet formation and dynamic interfacial tension in dynamic microchannel flows. The results presented in this work provide a more in-depth understanding of the dynamic effects of surfactants on droplet formation and the precise controllable preparation of monodispersed droplets in microfluidic devices.

The dynamic mass transfer of surfactants upon droplet formation in coaxial microfluidic devices

“…In our previous work (Xu et al, 2012;Xu et al, 2008), it was shown that the droplet diameter in coaxial microfluidic devices can be estimated by a model at low values of the continuous flow Reynolds number, which is expressed as…”

Microfluidic methods to study emulsion formation

“…An overview of droplet sizes and time-scales in these measurements is given in Table 1.2. (Steegmans, Warmerdam, et al 2009), (B) (Wang, Lu, Xu & Luo 2009), and (C) (Xu et al 2012); pressure drop (D) (Wang et al 2014); and droplet deformation (E) (Brosseau et al 2014). processes.…”

“…Microfluidic devices can also be used for interfacial tension measurements (Wang, Lu, Xu & Luo 2009;Brosseau et al 2014;Xu et al 2012;Wang et al 2014;Steegmans, Warmerdam, et al 2009). The tensiometric measurements of and Steegmans et al (2009) were both conducted in cross-flow microfluidic devices and were based on the relation between droplet size and interfacial tension; the former is able to measure in the millisecond range and the latter in the sub-millisecond range, due to differences in geometry.…”

“…The tensiometric measurements of and Steegmans et al (2009) were both conducted in cross-flow microfluidic devices and were based on the relation between droplet size and interfacial tension; the former is able to measure in the millisecond range and the latter in the sub-millisecond range, due to differences in geometry. The method of Xu et al (2012), with a coaxial microfluidic device, is based on the same principle as those of Steegmans et al (2009) and of . Other microfluidic methods to measure the interfacial tension are based on droplet deformability after formation (Brosseau et al 2014) and on the pressure drop during droplet formation (Wang et al 2014).…”

“…Recently, we have shown that it is possible to measure the interfacial tension during droplet formation in the sub-millisecond to millisecond time-scale with a microfluidic Y-junction . Others have also used microfluidics for this purpose, albeit exploring slightly longer time-scales (Brosseau et al 2014;Wang et al 2016;Xu et al 2012;Wang et al 2014). The steps beyond initial emulsifier adsorption can also be investigated by microfluidics, for example whether emulsifiers efficiently stabilise the oilwater interface and prevent droplet coalescence (Krebs et al 2012b;Krebs, Schroën, et al 2013;Baret et al 2009).…”