Droplet-to-particle levitation transition in acoustic levitation: spontaneous breath figure formation and impact on dissolution

We report the drying of isolated polymer solution droplets, employing acoustic levitation, and demonstrate the spontaneous generation of breath figures (BF) on the resulting polymer particles and capsules with controlled surface pore arrays. By contrast with supported polymer thin films, the evaporative cooling experienced by suspended droplets suffices to yield ubiquitous BF formation, owing to their thermal insulation and the synchronous condensation and self-assembly of water microdroplets, accompanied by capsule skin formation and kinetic arrest. A simple model describes simultaneously the radius and temperature evolution along the droplet-to-particle transformation, and the scaling of surface pore dimensions, with environmental parameters. The generality of the approach is demonstrated with a range of model polymers, and the coupled roles of solution thermodynamics and droplet environment are shown to permit the facile design of capsules with tunable transport and dissolution kinetics.