A microfluidic device incorporating three coaxial capillaries set in a single block of ultra-high molecular weight polyethylene (UHMWPE) for the rapid formation of monodisperse double emulsions and polymersomes is described. The device utilizes easily interchangeable, coaxial capillaries whose geometry is maintained by a single UHMWPE block. Water-in-oil droplet formation using the device is characterized by measuring the diameter of the drops produced as a function of varying Reynolds, Weber, and capillary numbers. These parameters are used to characterize the dripping to jetting transition across a range of channel sizes. Double emulsions incorporating solutions of amphiphilic block copolymers are then processed to form large numbers of monodisperse polymersomes in an effective and controlled manner.