Easy-to-operate fabrication of tapered glass capillaries for microdroplet generation

Tapered glass capillaries play an important role in microdroplet generation and related microfluidic applications such as particle synthesis, digital PCR and microrectors. However, its controlled production poses a challenge. Specifically, there is difficulty controlling the tip sizes of capillaries with good repeatability at an affordable cost, which is critical to produce standard droplets for use by research groups. To produce tapered capillaries, micro needle geometries are first pulled by a commercial pipette puller, which yields a sharp tip (i.e. 1-3 mu m). Those tips need to be enlarged and standardized by post microfabrication. Although larger tips could be achieved by advanced pulling-forging, sanding-forging or breakingforging methods, the uniformity and repeatability do not meet the requirements for mass production. In this paper, several easy-to-operate fabrication approaches, including mechanical and chemical fabrication methods, are studied. Chemical etching fabrication method shows good repeatability and robust control of dimensions. A capillary droplet microfluidic device assembled by tapered glass capillaries fabricated by chemical etching method is also presented, which shows good performance for droplet generations. This non-labor intensive fabrication is straightforward and be readily adopted for industrial scale mass production.