Dispersion of magnetic beads in flowing droplets.

Microfluidic droplets have proven useful for the manipulation and sample preparation of single-cells. However, the droplet toolkit lacks a robust and high-throughput purification module that can maintain partitions. We have demonstrated a proof-of-principle of magnetic purification from flowing droplets using an external magnetic field to partition beads within droplets before asymmetric splitting. Here, we study the interplay between the opposing effects of the magnetic and viscous forces by systematically reporting bead aggregation as a function of design parameters and droplet velocity. We used image-processing to measure aggregate count, size, and shape, allowing us to discover how the distributions of these parameters correlates with changes in experimental conditions. Data have revealed distinct aggregation regimes, in which either the magnetic attraction or the viscous dispersion forces predominate. We further explored the effect of design parameters on the transitions between these regimes both experimentally and via modeling using the dimensionless Mason number (ratio of viscous to magnetic forces). This knowledge will drive the optimization of our droplet purification module with increased throughput.