Oscillating dispersed-phase co-flow microfluidic droplet generation: jet length reduction effect.

Microdroplet generation methods are assessed by two important criteria of droplet throughput and size dispersity. The widely-used co-flow droplet generation technique is bottlenecked with droplet polydispersity at high throughputs due to transition to an unstable jetting regime at high dispersed-phase (d-phase) flow rates. In this paper, we introduce a novel technique to oscillate the d-phase nozzle inside the continuous phase (c-phase) channel to suppress the jetting effect. The effect of the nozzle oscillation frequency (0-15 Hz) on the jet length was studied at different d-phase (Qd = 1.8, 2.4 and 3.0 ml min-1) and c-phase (Qc = 6, 12 and 18 ml min-1) flow rates and d-phase viscosities (1, 2.5, and 6 mPa s). The jet length was directly proportional to the d-phase flow rate and inversely proportional to the oscillation frequency. Oscillation-induced jet length reduction was more significant at high jet velocities, but a less steep jet length reduction was always observed at oscillation frequencies higher than 10 Hz. A maximum jet length reduction of 70.8% was obtained at the highest d-phase and lowest c-phase flow rates. Increasing the viscosity of the d-phase resulted in diminishing the effect of oscillation on jet length reduction. Moreover, we observed that nozzle oscillation could disintegrate the long jet into droplets of various sizes that were mostly smaller than the stationary-mode droplets. We hypothesize that oscillating the dispersion nozzle at lower flow rates, without the jetting effect, can simultaneously generate multi-size monodisperse droplets. This active technique can also be implemented into aqueous two-phase systems (ATPSs) in which droplet generation is a difficult task.