Investigation of viscous effects on droplet generation in a co-flowing step emulsification device

In the previous articles, we proposed a co-flowing step emulsification method to generate microdroplets. Various influencing factors, including interfacial tension, continuous and dispersed phase flow rates, and critical device dimensions, were analysed, however, without considering the effects of viscosities. Here, the volume-of-fluid (VOF) based CFD simulation method is used to investigate the effects of viscosities on the co-flowing step emulsification in this paper. The continuous and dispersed phase viscosities researched vary from 0.05 Pa s to 2.0 Pa s and 0.001 Pa s to 30 Pa s, respectively. The resulting droplet diameter ranges from 35.6 µm to 522.6 µm. It is found that the continuous phase viscosity has a significant effect on the droplet diameter, frequency and generation regime. As the continuous phase viscosity increases, the droplet diameter decreases, and the droplet generation regime changes from dripping to tip-streaming gradually. Whereas the dispersed phase viscosity increase nearly does not cause droplet diameter change, besides satellite droplets generation. The results are corresponding to the droplet diameter prediction model we proposed. This work effectively provides fundamental insights into microfluidic droplet generation characteristics in co-flowing step emulsification. [Display omitted]