Drying of polymer solution droplets in levitation: pattern formation, buckling and kinetic arrest

We investigate the drying of isolated polymer solution droplets under controlled environmental conditions, reminiscent of spray drying. Optical, thermography and scattering data are acquired in acoustic levitation to examine the spatio-temporal transformations of dilute polymer solution droplets towards the formation of polymers capsules and particles. In addition to expected demixing, skin formation, buckling and kinetic arrest associated with this complex directional solidification process, we find that large thermal excursions associated with evaporative cooling can lead to complex feedback and unexpected outcomes, such as the breath figure (BF) formation and assembly, in turn resulting in controlled surface pore arrays. A simple model is found to account for the dimensions, shape 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 surfactants, 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.