Destabilization mechanism of (W+W)/O reverse Janus emulsions.

Hypothesis: Reverse Janus emulsion, with droplets composed by "two rooms" of water phases, is a novel multiple emulsion attributed to excellent integration capability and biocompatibility. However, significant instability compared with normal Janus emulsions renders the stability issue of great importance. Moreover, the ultra-low aqueous-aqueous inner interfacial tension, the anisotropic nature of the droplets with distinct lobe composition, and the random orientation in the continuous phase endow the complicated and various demulsification mechanisms.

Experiments: Reverse Janus emulsion of (W+W)/O, employing typical salt-alcohol aqueous two-phase system (ATPS) as inner phases, is prepared in batch scale by conventional one-step vortex mixing. The demulsification process is detected by multiple light scattering technique, which provides real-time, in-situ, and quantitative information of emulsion evolution. Moreover, the fusion pattern of the anisotropic droplets is illustrated by the combination with light microscopy and size distribution measurement.

Findings: Coalescence and sedimentation are found to be two main demulsification processes. Two salt "body" lobes of the "snowman" shaped Janus droplets combine first resulting in an intermediate Cerberus topology with two alcohol "heads" on one salt "body". Subsequently, two "head" lobes coalesce resulting in a larger Janus droplet. Ultimately, the Gibbs free energy leads to a final state with three separated liquids. In addition, the variation in lobe viscosity, density, and properties of interfacial film greatly affect the demulsification rate and fusion pattern. A critical alcohol/surfactant mass ratio of 2 is found, beyond which a completely different fusion pattern occurs. Two alcohol "body" lobes combine first resulting in an intermediate Cerberus topology with two salt "heads" on one alcohol "body". Subsequently, two "head" lobes coalesce resulting in a larger Janus droplet. The findings are instructive in the stability of aqueous based multiple emulsions with advanced morphologies and meanwhile, promote the future application of this novel emulsion in food science, pharmacy, and biomimetic compartmentalization.