Nonspherical Colloidosomes with Multiple Compartments from Double Emulsions

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by Professor Weitz

Abstract

Because I came from a technology background, this article deeply interested me because of its relevance and potential for numerous technological applications. As an introduction and also by reading other texts, 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. In the article, the authors carried out research experiments and developed a new approach to fabricate monodisperse colloidosomes by using double emulsions. 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 author also demonstrated that 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. All in all, such versatility makes these colloidosomes attractive candidates for medical and bio-applications.

Experiment

Generation of nonspherical colloidosome

The experiments procedures are easily understood. First and foremost, the authors demonstrated 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 ideas is that the nanoparticles in the oil phase eventually become the shell of colloidosomes upon the removal of the oil. However, the interesting fact is that during the oil removal, the internal W/O interface retains their spherical shapes whereas the outer O/W interface deforms; this process is how nonspherical colloidosomes are generated.

As for the actual experiment, I will briefly discribe 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. When I was in my undergraduate studies, I have worked with a Professor in the bioengineering departments and have done very similar fabrication work as this experiment. Moreover, 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. 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 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. Lastly, how double emulsion droplets were converted to nanoparticle colloidosomes was 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.

Findings

Generation of double emulsions with varying number of internal drops
Optical microscopy images of W/O/W double emulsions


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.

SEM images of nonspherical colloidosomes

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.