High inertial microfluidics for droplet generation in a flow-focusing geometry.

A numerical study of aqueous droplet generation in a high Reynolds (Re) number air flow was performed in a microfluidic flow-focusing geometry. Droplet breakup mechanisms, flow regime mapping, droplet morphology, and droplet generation frequency were studied in a high initial air flow under various flow conditions. Several flow regimes were identified including dripping, unstable dripping, plugging, stratified flow, multi-satellite droplet formation, and unstable jetting. Unstable dripping, multi-satellite droplet formation, and unstable jetting have been observed as new flow regimes in our study. We found that the high inertial air flow remarkably induces the formation of these new flow regimes by retaining unique droplet generation mechanisms and morphology. In particular, the polydisperse spray of tiny droplets is formed at the junction in the multi-satellite droplet formation regime, while at the end of a jet in the unstable jetting regime. On the other hand, stable droplet generation occurs in the dripping and plugging regimes, while generated droplets in the unstable dripping, unstable jetting, and multi-satellite droplet formation regimes are unstable. The maximum generation frequency of ~ 1900 Hz was obtained under the unstable dripping regime. It was found that increasing Re number results in droplet size reduction, while higher capillary (Ca) number leads to bigger droplets.