Micro-droplet formation utilizing microfluidic flow focusing and controllable moving-wall chopping techniques

The formation of micro-scale droplets in liquids is crucial for many applications. This paper reports a new microfluidic chip capable of generating tunable micro-droplets in liquids by using the combination of two microfluidic techniques, microfluidic flow focusing and a controllable moving-wall chopper. The microfluidic chip can generate droplets with tunable sizes. Dispersed phase sample flow is first hydrodynamically focused into a narrow stream by using neighboring sheath flows containing continuous-phase samples. A new chopping microstructure called 'controllable moving walls', which is a pair of side chambers orthogonally placed next to the sample flow channel, is used to generate micro-droplets. The moving-wall structures can be deformed by external air pressure to cut the pre-focused stream into segments. By controlling the air injection frequency and air pressure of the side chambers, tunable micro-droplets with a relatively uniform diameter can be formed accordingly. Experimental data show that a maximum deformation of 62.5 mu m could be achieved at a pressure of 30 psi for a moving wall with a cross section of 100 mu m x 50 mu m. By controlling the flow rate between the dispersed and continuous phases with a range from 2 to 20, micro-droplets with a relatively uniform size ranging from 10 mu m to 120 mu m can be successfully generated. The micro-droplets have a much more uniform size compared to previous studies. This new microfluidic device can be promising for a variety of applications such as emulsification, nano-medicine and droplet-based microfluidics.