Figure 2

(a) High-speed video frames showing the droplet generation with different excitation frequencies. f _F  = 0 Hz is the natural state without excitation, f _F  = 1 Hz is the unstable state, f _F  = 2–9 Hz is the stable synchronized regime, f _F  = 10–19 Hz shows the unstable output, and f _F  > 19 Hz shows the output of droplets unaffected by the excitation. (b) A cycle of the suction-generation process with f _F  > 9 Hz. (c) Droplet diameter versus droplet generation frequency. P = 200 mbar. Points with cross relate to the natural states without active control. (d) Plot of the variation of maximum synchronized frequency with different applied pressures, and the inset showing the droplet size generated in the synchronized regime with different applied pressures. (e) Plot of the variation of maximum synchronized frequency with the outer flow rate change, and the inset showing variation of maximum synchronized frequency with the inner flow rate change. P = 200 mbar in both figures. (f) Plot of CV value as a function of the excitation frequency.