Effect of flow rates of the continuous phase on droplet size in dripping and jetting regimes in a simple fluidic device for coaxial flow

We studied the water-in-oil (W/O) emulsion droplet formation in a simple coaxial microfluidic device with varying the flow rate of continuous phase and investigated the effect of the flow rate on the size and coefficient of variance of the emulsion droplets. The emulsion droplets prepared in the dripping regime at a low flow rate of the continuous phase exhibited a uniform size distribution. The increase in the flow rate of the continuous phase resulted in a transition from the dripping regime to the jetting regime. The emulsion droplets produced in the jetting regime exhibited multimodal size distributions, which is mainly attributed to the formation of satellite droplets. Based on the calculated Capillary number (C_a) and the Weber number (W_e), we suggest that the flow rate of the continuous phase and the viscosity of the discontinuous phase are the key factors for determining the regime of droplet formation. We believe that these results can serve as a guideline for the determination of flow rates for the production of uniform emulsion droplets in coaxial fluidic devices.