Silica microbubble powders from freeze-dried Pickering emulsions

Microbubbles are unique materials widely used in food formulations, as well as biomedical and pharmaceutical applications. However, their thermodynamic instability limits practical use. To enhance their stability, we studied the production of microbubble powders using Pickering emulsions as templates, in combination with freeze-drying to remove both the aqueous solvent and the oil inside the emulsion droplets. Hydrophobic silica nanoparticles stabilized O/W Pickering emulsions with cyclooctane as a volatile oil phase, while nanoparticles were dispersed either in the water phase alone or in both water and oil phases. After freeze-drying, SEM microscopy revealed two distinct microbubble structures: single-layer microbubbles when nanoparticles were present only in the water phase, and network microbubbles with an internal particle network, when nanoparticles were present in both the aqueous and oil phases. Both microbubble types exhibit long-term stability. The stability of redispersed bubbles was examined in water and protein dispersions. We found that the presence of a particle network inside significantly improved bubble stability. Microscopy showed that SL microbubbles persisted for less than 3 days, whereas NW bubbles remained stable for over 1 week; correspondingly, 50 % air loss occurred after 5 days (SL) and 7 days (NW). For SL bubbles, faster freezing and the presence of protein significantly enhance stability, with many bubbles remaining after 7 days. Our method of using freeze-drying to create microbubble powders from Pickering emulsion templates solves the problems of low production efficiency and scalability that exist when attempting to directly stabilize air-water interfaces using Pickering stabilization. • Scalable method for stable microbubbles via freeze-drying Pickering emulsions • Internal particle network greatly improves microbubble stability • Expected to be useful for pharmaceutical applications