Experimental study of double emulsion formation behaviors in a one-step axisymmetric flow-focusing device

Formation behaviors of double emulsions in the one-step axisymmetric flow-focusing microfluidic device are studied experimentally based on a visualization system. Typical droplet formation regimes are examined and the effects of the flow rates on droplet sizes and morphologies are discussed. Especially, a comparative study of the performance between the flow-focusing and co-flowing device is conducted. The results indicate that the dripping and jetting regimes appear during the double emulsion formation under one-step flow-focusing, in which the dripping regime includes two distinct generation modes namely synchronized dripping and asynchronized dripping. Squeezing from the outer interface leads to the collapse of the inner interface in the synchronized dripping regime. The formation regime, droplet sizes as well as their polydispersities are determined by the flow rate of the outer fluid, while the shell thickness depends on the flow rate ratio between the middle fluid and the inner fluid. Compared with the co-flowing system, deformation of the interfaces is enhanced by the hydrodynamic focusing effect in the flow-focusing system where faster production of double emulsions with smaller sizes is achieved.