Microfluidic emulsification with a surfactant and a particulate emulsifier: Dripping-to-jetting transitions and drop size scaling

We report an experimental investigation on the effect of emulsifiers on the dripping-to-jetting transition and droplet size scaling in a co-flow microcapillary device. The emulsifiers studied include an oil-soluble surfactant Span 80, a water-soluble nanosheet, and their combination. We characterize the dispersed phase behaviors in the state space comprising the Weber number of the inner phase Wein and the capillary number of the outer phase Caout. Results show that the dripping-to-jetting (D-J) transition occurs at Wein ≈ O(1) for the no-emulsifier case and Wein ≈ 65 for the Span 80 system. In the case of the nanosheet, the D-J transition is dependent on concentration with Wein ≈ 0.08–5.6. For the combination system, depending on the nanosheet concentration, the critical Wein varies from 0.3 to 21. We explain that these emulsifier-dependent critical Wein result in terms of additional interfacial stresses arising due to surfactant transport and adsorbed nanosheets. Combining droplet size data from this study and literature, we find that in the dripping regime, the droplet diameter decreases with increasing Caout, yielding a scaling exponent of ≈−0.33 and a pre-factor that could be dependent on the type of emulsifier. In the jetting regime, the droplet diameter increases with the flow rate ratio of the two phases, with a scaling exponent of ≈ 0.37 that is independent of the type of emulsifier. The results from this work help to better understand the influence of surfactants and particulate emulsifiers in microfluidic emulsification.