Surfactant effects on droplet formation in microfluidic systems

Multiphase flows in microscale channels allow manipulation of the flow patterns and offer high heat and mass transfer rates. Surfactants are commonly used in a wide variety of applications from emulsification to DNA extraction and inkjet printing, and their effects on droplet formation is of great importance. In this study, the formation of aqueous droplets in an organic continuous phase was studied experimentally using a flow-focusing microchannel in the presence of a range of surfactants. A two-colour PiV system was used to identify changes in the interface during the drop formation and to obtain velocity fields in both phases. A low viscosity silicone oil (0.0046 Pa s) was used as the continuous phase and a mixture of 48% w/w water and 52% w/w glycerol was the dispersed phase. Two cationic surfactants, C12TAB (50 mM) and C16TAB (5 mM) were added in the aqueous phase, at concentrations above the CMC values. An ionic surfactant, SDS (1 mM) was also used to compare the effects on the drop formation. Four regimes of drop formation were identified, namely squeezing, dripping, jetting and threading, whose boundaries changed when the surfactants were present. For all solutions studied, three distinct drop formation stages were identified, expansion, necking and pinch-off. Smaller drops were observed at higher concentrations of surfactant, as expected. The dynamics of surfactant adsorption can play a vital role in the drop formation process and were studied. The effect of dynamic interfacial tension at the characteristic time scale of drop formation becomes more significant at for surfactants with low CMC values, which suggests increased surfactant activity. Using this information the differences between surfactant systems during the transitions between drop formation stages could be explained.