Synchronized generation and coalescence of largely dissimilar microdroplets governed by pulsating continuous-phase flow

Abstract

The effects of pulsating continuous-phase flow on droplet generation at a T-junction are investigated. The pulsating perturbation produced by a microfluidic oscillator is found to govern the droplet formation. In this way, the droplet size, the generation frequency, and the fluid properties become uncorrelated. Within a wide viscosity (1–60 cP) and flow rate range [Qd,max/Qd,min is on the order of O(102)] of the discrete fluid, the droplet volume increases linearly with Qd and hence can be easily tuned. Using a single perturbation source, microdroplets of largely different viscosities and volumes can be synchronously generated, facilitating subsequent precise control and manipulations such as one-to-one coalescence. Passively, monodisperse droplets can be formed following the squeezing, dripping, or tip-streaming mode.1–3 But the droplet size and generation frequency cannot be independently controlled as they correlate with each other. In addition, they are further influenced by the channel geometries, fluid properties, and flow rates (and their ratio) of the continuous and discrete phase.4,5 In comparison, active modulation of droplet generation provides a better control.6,7 Relevant methods include applying electric or magnetic fields8–13 and using mechanical/piezoelectric vibrators,14–16 microvalves, and micropumps.17–19 The required actuators and controllers usually raise the fabrication and operational complexity.