flowfocusing

Flow focusing, also called flow-focusing, is a droplet generation technique used in microfluidics to create uniform droplets or threads by constricting a core fluid with a surrounding sheath of another fluid. In a typical device, a central channel carries the inner (dispersed) fluid and a surrounding annulus delivers the outer (continuous) phase. As the fluids exit the outlet, the outer flow compresses the inner jet, increasing shear and reducing the cross-section until interfacial tension causes the core to pinch off into droplets.

The mechanism relies on the interplay of viscous, inertial, and capillary forces. The resulting droplet size and production rate depend mainly on the flow rate ratio between the inner and outer fluids, as well as fluid properties such as viscosities, interfacial tension, and the geometry of the device. Flow focusing can operate in different regimes, producing monodisperse droplets when tuned to a dripping regime, or smaller droplets under jetting conditions.

Materials systems typically involve an inner aqueous phase and an outer oil phase, though many immiscible liquid pairs can be used. The method is widely applied to produce uniform droplets for encapsulation, emulsions, single-cell handling, material synthesis, and biomedical assays.

Advantages of flow focusing include precise control over droplet size, high monodispersity, and compatibility with rapid production and parallelization. Limitations can include sensitivity to fluid properties, potential clogging, and the need for careful device fabrication and surface chemistry to maintain stable interfaces.