Expansion-mediated breakup of bubbles and droplets in microfluidics

This paper reports a breakup regime of bubbles and droplets that is caused by a sudden channel expansion in a microfluidic device. In this regime, bubbles or droplets generated at a flow-focusing geometry periodically breakup into smaller bubbles or droplets, respectively, upon entering an expansion. In addition to Capillary number Ca, which is previously shown to govern the dispersion breakup in such geometries, we find that, at a high-inertia regime, the Weber number We also plays a significant role in specifying the transition from non-breakup to breakup regimes. Furthermore, we identify different periodic breakup modes, for example, symmetric and asymmetric breakup, which are dictated by the Ohnesorge number. A power law of f(0.5) proportional to W e(0.1)Ca(0.2), where f denotes the frequency at which the dispersions arrive at the expansion region, governs when the droplets and bubbles breakup. This power law highlights the importance of inertia to the dispersion breakup in an expansion-mediated geometry. Our results demonstrate that, without modifying the geometry and by only tuning several dimensionless parameters related to the fluid flow, a microchannel expansion region can produce mono-, bi-, or tri-disperse bubble or droplet populations. These discoveries may find utility in the design of multi-disperse bubble or droplet populations using microfluidics.