Droplet Formation by Confined Liquid Threads inside Microchannels

A confined liquid thread can form monodisperse droplets near the exit of a microchannel, provided the continuous phase is able to enter the microchannel. A general model that accurately predicts the droplet size including the breakup position inside the microchannel is presented and is verified with experimental observations; breakup occurs as long as the capillary number (Ca) of the liquid thread is below a critical capillary number (Ca_cr); for cylindrical microchannels, it is derived that Ca_cr = 1/16. Below Ca_cr, the formed droplets at the exit of the microchannel have a diameter approximately two times the diameter of the liquid thread; around and above Ca_cr, the liquid thread remains stable and the formed droplets grow infinitely large. The presented controlled droplet generation method is a useful tool for producing monodisperse emulsions and has great potential for the food and pharmaceutical industry.