(381b) Surfactant Effects on Droplet Formation in Microfluidic Systems

Droplet formation in microscale channels allows controlled drop size and low polydispersity indexes. 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 different surfactants. A low viscosity silicone oil (4.6 mPa 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 values¹. An ionic surfactant, SDS (11 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. The different regimes observed with SDS surfactant-laden drops are shown in Figure 1. For all solutions studied, three distinct drop formation stages were identified, expansion, necking and pinch-off. Smaller drops were observed at higher surfactant concentrations. The dynamics of surfactant adsorption can play a vital role in the drop formation process and thus were studied. The effect of dynamic interfacial tension at the characteristic time scale of drop formation becomes more significant 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. An innovative two-colour μPIV system² was then used to identify changes in the interface during the drop formation and to obtain velocity fields in both phases. From the velocity profiles in the squeezing regime, an internal circulation pattern was observed at the center of the forming droplet during the expansion stage, which gradually faded as the droplet moved further into the main channel. Upon addition of surfactants, a weaker circulation was seen, attributed to the accumulation of surfactants at the drop tip. During pinch–off, a flow inversion at the thinnest part of the neck was detected, which caused the dispersed phase to pull back into the inlet channel after the drop had detached. On the other hand, addition of surfactants increased the local velocity difference between the two phases compared to the surfactant-free case, for the same phase flowrates. Based on the geometric characteristics of the forming drops and the velocity fields obtained with μPIV, the forces acting on the drop were estimated. It was found that drag forces overcome the surface tension ones between the expansion and the necking drop formation stages.

References:

(1) Roumpea E, Kovalchuk N, Chinaud M, Nowak E, Simmons M, Angeli P, (2019), Experimental studies on droplet formation in a flow-focusing microchannel in the presence of surfactants, Chemical Engineering Science; 195: 507–518

(2) Chinaud M, Roumpea E, Angeli P, (2015) Studies of plug formation in microchannel liquid–liquid flows using advanced particle image velocimetry techniques. Experimental Thermal and Fluid Science; 69: 99-110.