Abstract
This paper presents a droplet generation scheme that employs constant pumping sources to periodically digitize a continuous liquid flow into nanoliter segments with desired volumes. In this scheme, a constant-pressure air flow assisted by channels with certain geometries is used to break a constant-rate liquid flow and dispense the separated segments. Simply by adjusting the air pressure and liquid flow rate, droplets with desired volumes can be generated. In the prototype demonstration, the droplet generators are made of PDMS microfluidic channels with preferred geometries and surface properties. 3-way junctions integrated with converging/diverging channels are utilized to guide the flows. Through the discretization sequence, liquid is temporally held by the channel to facilitate the cutting executed by air flow. Based on this scheme, it has been demonstrated that a continuous liquid flow can be split into segments with volumes ranging from 0.5 to 50 nanoliters. In addition to air pressure and liquid flow rate, the resulted droplet volumes are also found to be affected by channel geometries. As such, this adjustable droplet generation scheme opens up a new class of metering and manipulation prospects for microfluidic applications.