Stimuli-responsive particle-stabilised (Pickering) emulsions are of interest as triggerable encapsulation systems with key applications in drug delivery, catalysis and cosmetics. However, the design of reversible Pickering emulsions is extremely challenging as the hydrophobic-hydrophillic balance of the particle stabilisers must be sufficiently modified by the applied stimulus to induce a phase change.

In this work, we investigate a model system based on silica particles that are functionalised with azobenzene photoswitches to prepare light-responsive Pickering emulsions. The particle hydrophobicity was systematically changed by varying the separation between the photoswitch and the surface, the grafting density and using light irradiation to change the molecular conformation and packing. We show that combination of surface energy analysis and optical microscopy studies on these systems enable reliable prediction of the stability and expected light-response of a given emulsion. From these results we identify a set of design rules to facilitate the rational design of light-responsive Pickering emulsions for the first time.

Figure: Light-responsive azobenzene-silica stabilised Pickering emulsions. A reversible transition between emulsified water-in-oil droplets and demulsified water and oil phases is observed with the application of either UV or blue light, which can be repeatedly cycled.