Effect of surfactants during drop formation in a microfluidic channel: A combined experimental and CFD approach

The effect of surfactants on the flow characteristics during rapid drop formation in a microchannel is investigated using a combination of high-speed imaging, micro-particle image velocimetry (µPIV), and numerical simulations; the latter are performed using a three-dimensional multiphase solver that accounts for the transport of soluble surfactants in the bulk and at the interface, and adapted to the context of a hybrid Front-Tracking/Level-set method. Drops are generated in a flow-focusing, cross-junction microchannel, using silicone oil (4.6 mPa s) as the continuous phase and an aqueous mixture of 52% w/w glycerol as the dispersed phase. A non-ionic surfactant (Triton X-100) is dissolved in the dispersed phase at concentrations below and above the critical micelle concentration (CMC). Excellent agreement between the numerical predictions and the experimental data is found for the drop size, drop formation time, velocity and circulation patterns. The results reveal strong circulation patterns in the forming drop in the absence of surfactants, whose intensity decreases with increasing surfactant concentration, while a stagnation point is found just at pinch-off. The surfactant concentration profiles in the bulk and at the interface are shown for all stages of drop formation. It is found that the surfactant interfacial concentration is large at the front of the forming drop and at the back, while the neck region is almost surfactant-free. Within the forming drop, the surfactant is mainly concentrated in the middle. Marangoni stresses develop but away from the neck, contributing mainly to changes in the velocity profile inside the drop