Nonspherical Colloidosomes with Multiple Compartments from Double Emulsions
by Professor Weitz
As an introduction, colloidosomes are hollow capsules whose walls are composed of densely packed colloidal particles. Traditional ways to prepare colloidosomes are to create particle-covered water-in-oil (W/O) emulsion droplets. Subsequently, these particle shells in the oil phase are transferred into an aqueous phase to generate the colloidosomes. A new approach to fabricate monodisperse colloidosomes by using double emulsions as templates is demonstrated. Water-in-oil-in-water (W/O/W) double emulsions with a core–shell structure are created using a glass capillary microfluidic device. Hydrophobic SiO2 nanoparticles, suspended in the oil phase, become the wall of colloidosomes upon removal of the oil. The functionality and physical properties of colloidosomes such as permeability, selectivity, and biocompatibility can be precisely controlled by suitable choice of colloidal particles and processing conditions. Such versatility makes these colloidosomes attractive candidates for applications in encapsulation and delivery of foodstuffs, fragrances, and active ingredients.
Researchers demonstrate the generation of nonspherical colloidosomes with multiple compartments. They use glass capillary microfluidics to prepare W/O/W double emulsions with different morphologies. These double emulsions have a different number of internal aqueous drops in the oil drop. The nanoparticles in the oil phase eventually become the shell of colloidosomes upon the removal of the oil. During the oil removal, the internal W/O interface retains their spherical shapes whereas the outer O/W interface deforms; this process leads to the generation of nonspherical colloidosomes with multiple compartments.
Briefly discribing the preparation of glass microcapillary devices. Cylindrical glass capillary tubes with an outer diameter of 1mm and inner diameter or 580 mm were pulled using a Sutter Flaming/Brown micropipette puller. The dimension of tapered orifices was adjusted using a microforge. The glass microcapillary tubes for inner fluid and collection were fitted into square capillaries that had an inner dimension of 1 mm. By using the cylindrical capillaries whose outer diameter match the inner dimension of the square capillaries, an axisymmetric alignment could be easily achieved to form a coaxial geometry. The distance between the tubes for inner fluid and collection was adjusted to be 5–30 mm. A transparent epoxy resin was used to seal the tubes where required. Solutions were introduced to the microfluidic device through polyethylene tubing attached to syringes that were driven by positive displacement syringe pumps. The drop formation was monitored with a high-speed camera attached to a Leica inverted microscope. For the generation of W/O/W double emulsions, three fluid phases are delivered to the glass microcapillary devices. The outer aqueous phase comprises 0.2–2 wt% PVA solution and the inner aqueous phase comprises 0–2 wt% PVA solution. The middle phase typically consists of 7.5 wt% hydrophobic silica nanoparticles suspended in toluene. Double emulsion droplets were converted to nanoparticle colloidosomes by either exposing them to vacuum or to atmosphere overnight and were then washed with a copious amount of deionized water to remove the remaining oil phase.
As the inertial force of the middle and inner phases becomes comparable to the interfacial tension, the droplet breakupoccurs in the dripping-to-jetting transition regime and n (the number of internal aqueous drop) increases above one. As shown in the first picture on top, by increasing the flow rate of the middle phase (Qm), n increases gradually. In general, n increases with increasing Qi and Qm and with decreasing Qo.
The morphology of double emulsions with the same n can also be controlled by varying the ratio of Qi and Qm. As the second picture shows, double emulsions with n=2 with spherical structure can be created by adjusting Qm to be greater than Qi by threefold threefold. The third picture shows that when the ratio of Qm to Qi is smaller than 3, ellipsoidal double emulsions are formed. The deformity of outer W/O interface can be easily explained by the fact that smaller droplets have a higher Laplace pressure acting across the interface given that the interfacial tension is the same. Thus the internal aqueous drops are less deformable than the encapsulating oil drop. All in all, this approach provides a new way to generate nonspherical emulsion droplets without relying on the jamming of the particles in solid-stabilized emulsion drops.