Pressure drop-based determination of dynamic interfacial tension of droplet generation process in T-junction microchannel

The dynamic interfacial tension caused by the lower surfactant adsorption rate than the droplet generation rate is an important issue in microfluidic study. A pressure drop measurement method is proposed to determine the dynamic interfacial tension during the droplet generation process at a T-junction microchannel. Experimental results show that in the transitional flow regime between squeezing and dripping, the maximum pressure drop appears at the end of filling stage defined by Glawdel et al. (Phys Rev E 85(1):016322, doi:10.1103/PhysRevE.85.016322, 2012a), as the liquid-liquid interface starts to leave the side channel. This pressure drop is composed of Laplace pressure and flow resistance. Using the pressure drop measurements, the contribution of fluctuated Laplace pressure is confirmed, and the instantaneous interfacial tension at the maximum pressure drop is calculated. Although the final droplet volume shows that almost no interfacial tension difference exists at the breakup moment, the pressure drop result unveils some discrepancies. Experimental results reflect the variation of dynamic interfacial tension is a quasi-surfactant diffusion-controlled process when the main channel is totally blocked by the growing droplet at the end of filling stage. However, this quasi-diffusion model no longer works as the droplet does not block the channel, due to the strong convection of two-phase flow. Surfactant sodium dodecyl sulfate and Tween 20 also show different micelle dissociation rates in the variation test of concentrations.