This article presents an active, two-step emulsification scheme that is capable of producing double emulsions with desired geometries and compositions on demand. Three-layer PDMS micro-devices with pneumatically actuated membrane-valves constructed on top of specially designed fluidic-channels are utilized to meter and shape immiscible fluids into double emulsions. By intermittently squeezing a fluid into another one, controlled emulsification is realized, and successive emulsification steps result in the formation of multiple emulsions. In the prototype demonstration, a three-layer PDMS molding and bonding process was employed to fabricate the proposed microfluidic devices, whose channel surfaces were selectively modified into hydrophilic by a photo-grafting process. A governing computer program cooperating with a set of control hardware was employed to coordinate the actuation of the prototype system. It has been demonstrated that: (1) both water-in-oil-in-oil and water-in-oil-in-water double emulsions can be produced; (2) the sizes of inner aqueous droplets and outer oil drops can be controlled independently; and (3) adjacent oil drops with varying overall sizes, and both diameters and numbers of inner aqueous droplets can be produced on demand. As such, the demonstrated emulsification scheme could potentially fulfill the real-time controllability on emulsion formation, which is desired for a variety of chemical and biological applications.
