Controlling Self-Assembly and Topology at Micro-Nano Length Scales Using a Contact-Free Mixed Nanocolloid Droplet Architecture.

Spatially varying the ordering of colloids of multiple sizes at micro-nano scales finds application in different industrial processes including manufacturing of photonic crystals. In this work, we showcase a unique physics-based architecture through which we have been able to control the morphology of the precipitates evolving out of the drying of a contact-free droplet at micro to nano length scales. We show that by varying the relative concentration of the larger sized colloids, one can modulate evaporation, subsequent particle transport, and particle ordering at the droplet interface, thereby controlling the rates of certain instabilities like buckling. In this way, we have produced evaporation-induced self-assembly structures (devoid of any substrate effect) with striking topological and surface features. Furthermore, we proved that these instabilities can be further tuned using a measured amount of external heating through the alteration of the evaporation rates. Notwithstanding, we also quantified that the ordering of the mixed colloids varies, in a spatial sense, across the droplet surface, exhibiting unique patterns, porosity, and lattice arrangements, all at the nanoscale. The results assure that the fine-tuning of the macroscale parameters like heating rate and particle loading can be used to fine-tune the micro-nanoscale features in a droplet-based high-throughput bottom-up framework.