(373b) Methods for Encapsulating Mobile Microparticles

Particle containing composite microcapsules have become increasingly prevalent and are widely employed in a wide range of applications including pharmaceutical, personal care, nanocomposite self-healing barrier films and coatings industries. Most commonly, small particles are held at the interface (as seen in colloidosomes), or physically arrested in a solid core. Microencapsulation through polymerization or precipitation on the interface of emulsion droplets, has been widely successful in creating capsules of fluids that can be then incorporated in other media. However, due to surface energy, Pickering stabilization, or adsorption of particles at interfaces, it is difficult to synthesize microcapsules of suspensions of freely diffusive or movable particles. Various examples exist, however they typically lack robustness in their material and physical properties.

We present various approaches to encapsulating spatially manipulatable microparticles, over a broad particle size range (1-50 microns in diameter), within the core of a microcapsule, avoiding substantial Pickering stabilization and adsorption. One approach uses an emulsified yield stress suspensions encapsulated through a standard polyurea interfacial polymerization. This yield stress material is tuned to match the physical properties of the encapsulated microparticles to hinder particle diffusivity to the capsule walls during polymerization and to balance gravity induced settling, whilst also not permanently fixing particle locations. Particles are actively released through either a temperature change or by manipulation of particles through external forces. This was demonstrated with a magnetic field for Janus particles functionalized with a ferromagnetic coating of iron oxide to directly visualize particle translation and rotation. A second approach looks at the use of density matching between binary solvents and microparticles in polyurea based microencapsulations to achieve a similar effect without the need for an external activating force. Both methods prove viable over a range of particle sizes and could be applicable to a number of composite microcapsule applications.

This work was supported by the Applied Physics Laboratory at Johns Hopkins University.