Unloading and Reloading Colloidal Microcapsules with Apolar Solutions by Controlled and Reversible Buckling

We introduce a new experimental method to encapsulate and release oils and fluorescent molecules into preformed elastic colloidal microcapsules of polydimethylsiloxane (PDMS)-filled siloxane shells, which are cross-linked with tetraethoxysilane. The method uses controlled buckling, where the volume of the capsule is reduced by dissolving the PDMS oil inside the capsule by surfactant micelles. This results in a change in the morphology of the capsule that depends on the ratio of shell thickness to total particle radius (d/R-t). Microcapsules of d/R-t in the range 0.007-0.05 formed microbowls upon decreasing the inner volume. The amount of oil released or dissolved by the micelles can be directly related to the concentration of surfactant. By tuning the amount of oil released, we can make microbowls of variable depth. In addition, we demonstrate that the microbowls can be further used to load different oils like silicone oil, hydrocarbons, and apolar dyes. The elasticity of the capsule wall and the leftover PDMS oil inside the capsule provide the principal driving forces by which one can promote the uptake of different oils, including dissolved dye molecules.