Stains from Freeze-Dried Drops

The evaporation of droplets of colloidal suspensions onto a surface is a common tool to achieve surface coatings and self-assembly. However, because of the spontaneous flow developing within an evaporating drop, the deposit is difficult to control, and an unwanted ring-like structure often forms, with particles aggregating along the drop edge. Here, by freezing the drops before sublimating them in dry air we propose a new approach that produces a different kind of stain where most particles are clustered in the center of the drops instead. We demonstrate that these deposits can be continuously tuned from wide but thin to concentrated and thick by varying the droplet's aspect ratio. Unlike evaporated liquid drops, stains from freeze-dried drops do not depend on the drying conditions or substrate roughness and possess a porous and branched microstructure somewhat reminiscent of freeze-casted ceramics. With these stains being governed by the freezing process rather than the drying, this opens alternative ways to control colloidal deposits.