A co-flow-focusing monodisperse microbubble generator

Here we report the design and fabrication of a simple and inexpensive microfluidic device based on micro-scope glass slides and two tapered glass capillaries, for generating monodisperse microbubbles. The first capillary that used for transporting gas, was heated and pulled to have a sharp tapered tip around 2 microns, and was inserted into the second capillary, with its sharp tip aligned to the converging-diverging throat of the second capillary. This configuration provides a smooth, small gas flow rate, as well as a high velocity gradient at the gas outlet. By varying liquid flow rates and viscosities, highly monodisperse microbubbles with diameter range from 3.5 to 50 microns have been successfully produced, at a rate up to 50 kHz. A simple scaling law based on capillary number and flow rate ratios, used to predict bubble size, is also proposed.