To generate droplets in a flow focusing configuration, the continuous phase is injected through two outside channels and the disperse phase is injected through a central channel into a narrow orifice to create a strong extensional flow.
The simulation below shows a flow focusing geometry integrated into a microfluidic device. One liquid flows into two inlet channels surrounding a center channel containing a second immiscible liquid. The two liquids are forced to flow through a small orifice downstream. Pressure and viscous stresses from the outer fluid force the inner fluid into a narrow thread, which breaks downstream of the orifice.