Microdroplets: Double Emulsification
Double emulsions are droplets that have smaller droplets contained within them. They can be composed of water droplets encapsulating oil drops, or vice versa. They are formed in microfluidic devices using a serial emulsification technique in which a drop maker is used to form the inner drops, which are then encapsulated in the outer drops in drop maker downstream, as shown in this movie of a double emulsion maker operated at four different flow rates:
Double emulsions are useful in biological application of microfluidics in which it is important to have an aqueous carrier fluid. This is needed, for example, when sorting droplets by FACS, which requires an aqueous carrier fluid, or when the environment within the drop must be modulated by changing the properties of the carrier fluid, such as osmolarity or pH. This is possible because small molecules can usually partition into the shell of the double emulsions and diffuse into the core, allowing the core and continuous fluid to maintain a state of chemical equilibrium.
The double emulsions shown above are formed in a twp-step drop formation process, because the inner drops are formed first in one drop maker, and then the outer drops later in a second drop maker. It is also possible to form double emulsions in a single-step process in which the inner and outer drops are created simultaneously. This is achieved by creating a coaxial "double jet" of the inner and middle fluids, and then breaking it into double emulsions in a single pinching process, as shown in this movie:
One-step double emulsification is useful for forming double emulsions with large inner drops and very thin shells, which are optimal as microcapsules. It is also useful for emulsifying fluids with non-Newtonian flow characteristics, very high viscosity, or in which the wetting properties make single emulsification difficult. This is because the inner fluid is entirely surrounded by the middle fluid, which can then be used to drive breakup of the inner fluid. In the above movie, for example, the inner fluid is a non-Newtonian polymer solution, which are normally impossible to emulsify in a controlled manner with direct single emulsification.
More elaborate multiple emulsions can also be formed with microfluidics, including triple emulsions. In this example, triple emulsions are formed by breaking a triple coaxial jet into drops:
Even higher-order multiple emulsions can be formed by using additional drop makers.