Polymer-Wrapped Oil Droplets and Polymer Particles with Complex Curvilinear Polyhedral Geometries by Surfactant-Driven Elastocapillary Buckling of Polymer Capsules.

Surfactants have been widely used to tune interfacial tensions in colloidal dispersions, resulting in phenomena such as wetting/dewetting, emulsification, and foaming. In this study, we report that adding micromolar concentrations of surfactants to aqueous dispersions of thin, deformable polymer microcapsules partially filled with droplets of oil induces complex shape changes in both the capsules and the encapsulated oil droplets. Using video microscopy and confocal microscopy, we observed the evolution of complex, anisotropic shapes with apparent six-, five-, four- and 3-fold symmetries in the presence of added surfactant and identified several factors influencing these shape changes and the resulting distribution of shapes, including surfactant concentration and charge, polymer capsule diameter and wall thickness, and the size of the encapsulated oil droplets. Our results suggest that these shape changes are driven by elastocapillarity, in which the addition of surfactant changes the balance between elastic energy and interfacial energy. Furthermore, we show that photopolymerization of deformed droplets of vinyl monomer oils "caged" in these capsules can be used to template the synthesis of polymer particles with complex anisotropic shapes similar to those of the folded and polymer-wrapped oil-filled capsules prior to polymerization. Finally, we demonstrate that the use of degradable polymer cages enables the isolation of "cage-free" polymer particles with complex shapes and geometries.