A microfluidic device for generating microbubbles includes a substrate and a microfluidic channel embedded in the substrate. The microfluidic channel includes a plurality of fluid inlets, at least one bubble formation outlet having a nozzle with an adjustable diameter, and a flow focusing junction in fluid communication with the plurality of fluid inlets and the bubble formation outlet. A method for mass producing monodisperse microbubbles with a microfluidic device includes supplying a flow of dispersed phase fluid into a first fluid inlet of a microfluidic channel, supplying a flow of continuous phase fluid into a second fluid inlet of the microfluidic channel, and adjusting a diameter of a nozzle to obtain a plurality of monodisperse microbubbles having a specified diameter.