Oscillating dispersed-phase co-flow microfluidic droplet generation: Multi-droplet size effect.

Controllable generation of microdroplets at desired sizes and throughputs is important in many applications. Many biological assays require size-optimized droplets for effective encapsulation of analytes and reagents. To perform size optimization, different-size droplets must be generated from identical sources of samples to prevent potential cross-sample variations or other sources of error. In this paper, we introduce a novel alteration of the co-flow droplet generation technique to achieve multi-size generation of monodispersed droplets. Using a custom-made mechanism, we oscillate the disperse-phase (d-phase) flow nozzle perpendicular to the continuous phase (c-phase) flow in a co-flow channel. Oscillation of the d-phase nozzle introduces an additional lateral drag force to the growing droplets while exposing them to various levels of axial drag owing to the parabolic velocity distribution of the c-phase flow. Superimposing both effects results in simultaneous and repeatable generation of monodispersed droplets with different sizes. The effect of nozzle oscillation frequency (f = 0-15 Hz) on droplet generation at different d-phase (Qd  = 0.05, 0.10, and 0.50 ml/min) and c-phase (Qc  = 2, 5, and 10 ml/min) flow rates was studied. A wide range of monodispersed droplets (4nl-4  μ l) were generated using this method. Droplet sizes were directly proportional to the We number and inversely proportional to the Ca number and oscillation frequency. Our technique is promising for applications such as aqueous two-phase systems, where due to inherently low interfacial tension, the d-phase flow forms a long stable jet which can be broken into droplets using the additional oscillatory drag in our device.