Monodisperse Double Emulsions Generated from a Microcapillary Device
When you mix two immiscible liquids, you get a dispersion of droplets of one liquid inside the other. This is defined as an emulsion. Double emulsions are dispersions of a third liquid inside the droplets of the first emulsion. Thus we have three liquids: the inner, the middle, and the outer. Double emulsions have a high potential for applicability within the commercial field, because the middle liquid shields the inner liquid from the outer and so allows for effective encapsulation and separation. However, controlled creation of double emulsions has been difficult to achieve. This paper reports a new method that controls both the size and the number of double emulsion droplets through a microcapillary technique.
The double emulsion device created by the lab works as follows: a cylindrical glass capillary injection tube with a tapered end is nested into one side of a square capillary tube. The inner fluid is flown through this injection tube. The middle fluid is flown into the square tube around the injection tube, going in the same direction as the inner fluid. The outer fluid is flown through the square tube as well, but coming in from the other side of the tube and flowing in the opposite direction as the other two liquids. A collection tube is nested into the square tube, on the side opposite the injection tube. All three fluids meet at the collection tube and flow out. It is in the collection tube that the double emulsion droplets are created.
The group was able to use this method to control the size of the double emulsions by controlling the size of the tubes as well as the rate of flow of the fluids. They found that by varying the parameters in the device, they could create two different qualitative cases in emulsion formation: dripping and jetting. The picture below shows these two cases as investigated by the group.
The group could predict, via a theoretical model based on the Rayleigh Plateau instability, for which parameters of the experiment, the emulsion would take place via jetting or via dripping. And for both of these two regimes, they can theoretically determine both the size of the inner and outer drops along with the number of inner drops in the double emulsion.
The group was able to use this device in order to fabricate new materials. They created rigid spherical shell capsules and polymer vesicles (polymerosomes). The polymerosomes were created in a clever way. They chose water-oil-water fluids for the inner-middle-outer fluids, with the middle fluid carrying a polymer. After the double emulsions are created, they let the solvent evaporate, and the polymers self assemble assemble into layers on both interfaces. This creates a polymerosome.
Conclusion and Future Work
The group has created a very valuable tool, allowing for the formation of double emulsions in both hydrophobic and hydrophilic liquids. However, the rate of formation of the emulsions is limited by the fluid flow and cannot be sped up more than 5000 Hz. Therefore, in order to make this a viable tool for industry, the micro channels must be scaled up by fabricating parallel devices.