Using a circular groove surrounded inlet to generate monodisperse droplets inside a microfluidic chip in a gravity-driven manner

A new approach to generate uniform-sized nanoliter droplets inside a simple microfluidic chip was developed by using a circular groove to break off the liquid infused from the surrounded inlet. We characterized the droplet formation by using circular grooves with different diameters ranging from 1.2 to 2.4 mm. Monodisperse droplets with a very narrow size distribution (RSD < 0.56%, n = 70 droplets) can be reliably produced and the volume of droplets varied between 74 and 576 nl by using different circular grooves. Influences of liquid properties on droplet formation were also investigated, including surface tension, viscosity and density. Moreover, the transport of droplets and reliable coalescing of two neighboring droplets were realized by using Y-shaped guiding lanes with two different designs. Controlling the velocities of each droplet based on their different physical properties or slowing down the first droplet relying on a pinched segment of the guiding lane has been developed. These methods have been used to control the contact conditions of two reagents coalescing in a 'head-to-tail' mode.