High aspect ratio induced spontaneous generation of monodisperse picolitre droplets for digital PCR

Droplet microfluidics, which involves micrometer-sized emulsion droplets on a microfabricated platform, has been demonstrated as a unique system for many biological and chemical applications. Robust and scalable generation of monodisperse droplets at high throughput is of fundamental importance for droplet microfluidics. Classic designs for droplet generation employ shear fluid dynamics to induce the breakup of droplets in a two-phase flow and the droplet size is sensitive to flow rate fluctuations, often resulting in polydispersity. In this paper, we show spontaneous emulsification by a high aspect ratio (>3.5) rectangular nozzle structure. Due to the confinement and abrupt change of the structure, a Laplace pressure difference is generated between the dispersed and continuous phases, and causes the thread thinning and droplet pinch-off without the need to precisely control external flow conditions. A high-throughput droplet generator was developed by parallelization of a massive number of the basic structures. This device enabled facile and rapid partition of aqueous samples into millions of uniform picolitre droplets in oil. Using this device, on-chip droplet-based digital polymerase chain reaction (PCR) was performed for absolute quantification of rare genes with a wide dynamic range.