Microdroplets: Higher-Order Emulsification
High-order multiple emulsions are emulsions containing a large number of nested inner drops. A quintuple emulsion, for example, is a drop that encapsulates four nested drops of different miscibility, as shown in the above image. To form quintuple emulsions with microfluidics, a device consisting of five sequential drop makers is used, as shown in the following movies:
Each quintuple emulsion is the product of five sequential emulsifications: the first drop maker forms the innermost drop, which is then encapsulated in a larger drop in the next drop maker forming a double emulsion, which is the encapsulated in the third drop maker forming a triple emulsion, and so on, forming the quintuple emulsion, as shown above.
As the quintuple emulsions exit the drop maker, they enter a region of the channel that widens. The streamlines in this region diverge, stretching the quintuple emulsions horizontally and compressing them vertically. Occasionally, the stresses are sufficient to break pieces off of the double emulsions, as shown in the middle movie above. Downstream of this they flow through a large "basin" channel, as they exit the device and are collected.
High-order multiple emulsions are useful for performing elaborate reactions in the drops involving different solutes with distinct solubility in the hydrophilic, lipophilic, or fluorophilic phases, or for creating capsules with especially impermeable shells, since the shells can be composed of several layers, each with different chemical properties, and thus different solubility for encapsulated actives.