A computational approach to regime mapping ultra-low interfacial tension microfluidic droplet flows

Droplet microfluidics studies the systematic production of micrometer-scale droplets contained inside microfluidic devices, with fundamental biomedical applications such as drug delivery and point-of-care diagnostics. The most common systems use a non-biocompatible oil as the continuous phase, often preventing successful implementation. One fully-biocompatible alternative to this is an Aqueous Two-Phase System (ATPS) consisting of two thermodynamically separated polymeric phases to create droplets. However, despite the promising trend of ATPS microfluidics in realizing biomedical applications, computational analysis of the system’s mechanics has not been extensively conducted. Thus, we have designed a 2D Volume-of-Fluid CFD model to systematically map and study the flow regimes of ATPS droplet flows.

We base the simulation initial and boundary conditions on our in-house experiments. Our model’s droplet size and frequency vary from ~4% to ~20% respectively when compared with experimental results, demonstrating good agreement. To study droplet formation, we systematically vary the continuous phase capillary number and flow rate ratio of both phases. We export key results such as velocity, pressure and volume fraction from our simulations for analysis. Our current results demonstrate a clear transition from a monodisperse dripping to a polydisperse jetting regime as capillary number is increased for a fixed flow rate ratio. Furthermore, we identify transitional regimes where satellite droplets cause a bi-modal distribution of droplet sizes.

Currently, we are completing a comprehensive flow regime map across a wide range of capillary numbers and flow rate ratios. We are also statistically analyzing our exported results to devise a quantitative method to differentiate between flow regimes. Ultimately, we aim to develop novel methods for controlled ATPS droplet production in microfluidics, bringing the technology closer to a commercial and point-of-care stage.