Droplet generation in a flow-focusing microchannel with continuous phase perturbations

The droplet generation of the droplets with desired size and frequency is of great importance to many applications, e.g., drug delivery, microreactor and digital PCR, etc. Active droplet generation has attracted many attentions, due to the advantages of on-demand, rapid response and high accuracy of the droplet size control. This work numerically investigates droplet formation in a flow-focusing microchannel with the continuous phase perturbation. The perturbations are exerted on the continuous phase at the upstream of the cross junction. The dependence of excitation frequency and magnitude on droplet generation size and frequency are explored in detail. It is found that the droplet generation frequency f_d equals the excitation frequency f_e and its subharmonics. And a step increase of the droplet generation frequency is observed with the increase of the excitation amplitude. Several regimes of droplet generation frequency are found, which is determined by the excitation frequency and amplitude. The physical mechanism behind the excited droplet generation is unveiled. Compared with the excited droplet generation at f_d = f_e, the additional retracting phase and the second squeezing phase are found during the droplet generation process at the regime of f_d = f_e/2. The droplet radius under the excitation can be predicted by the function of the flow rate of the dispersed phase and the excitation frequency R_d=α(Q_d/f_e)^1/3, with the value of α depends on the excitation frequency and amplitude.