Abstract
Dynamic interfacial tension was studied experimentally during drop formation in a flow-focusing microchannel. A low viscosity silicone oil (4.6 mPa s) was the continuous phase and a mixture of 48% w/w water and 52% w/w glycerol was the dispersed phase. An anionic (sodium dodecylsulfate, SDS), a cationic (dodecyltrimethylammonium bromide, DTAB) and a non-ionic (Triton™ X-100, TX100) surfactant were added in the dispersed phase, at concentrations below and above the critical micelle concentration (CMC). For SDS and DTAB the drop size against continuous phase flowrate curves initially decreased with surfactant concentration and then collapsed to a single curve at concentrations above CMC. For TX100 the curves only collapsed at surfactant concentrations 8.6 times the CMC. From the collapsed curves a correlation of drop size with capillary number was derived, which was used to calculate the dynamic interfacial tension at times as low as 3 ms. The comparison of the surfactant mass transport and adsorption times to the interface against the drop formation times indicated that surfactant adsorption also contributes to the time required to reach equilibrium interfacial tension. Criteria were proposed for drop formation times to ensure that equilibrium interfacial tension has been reached and does not affect the drop formation.
Highlights
By considering the various time scales relevant to the surfactant transport, we show that the surfactant adsorption times can be significant and should be considered during drop formation
The droplet size and formation time in the dripping regime for all three surfactants at different concentrations will be discussed which will be used to determine the dynamic interfacial tension (DIT)
It was found that drop size decreased as the continuous oil phase flowrate increased and as the surfactant concentration increased
Summary
Flow focusing microfluidic devices are commonly used to produce droplets and exist in two configurations: hydrodynamic flow-focusing, where two liquid streams of the continuous phase surround the dispersed phase stream and break it to drops at the inlet [22,26] and geometrical focusing where the breakup of the dispersed phase happens at an orifice of size smaller than the main microchannel width [27,28] The latter configuration has previously been used to obtain DIT [21,29] when surfactants are dissolved in the continuous phase, while the former configuration, is yet to be investigated.
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