From the mesmerizing intricacy of an ammonite shell to the chilled elegance of a snow flake, nature articulates langue of shape and geometry fluently and delivers complex patterns and structures on all length scales with ease and panache via self-assembly and self-organisation. In nanoscience, we aspire to harness such linguipotence of geometry to create hierarchical nanostructures with tailored geometry and enhanced functionalities. A widely studied system for spontaneous pattern formation is evaporative drying of a sessile drop containing non-volatile particles. The most familiar pattern is the “coffee ring”, due to an outward capillary flow that shuttles dispersed particles towards the peripheral contact line where they get trapped. Marangoni effects may counteract this capillary flow, and the residual pattern may be further influenced by instabilities triggered by a temperature gradient across the solvent layer that manifest in different convective patterns, e.g. the Bénard-Marangoni (BM) convection. By controlling parameters such as evaporation rate, substrate chemistry, particle shape, size and concentration, droplet confinement, and surfactant addition, a plethora of patterns can be obtained, such as concentric rings, polycrystalline dendrites, uniform deposits, and polygonal particle networks. The coffee ring effect has also been exploited in applications, e.g. inkject printing and fabrication of sensors and transparent conductors.
In these previous studies, the dispersed non-volatile particles were inert; mechanistically, the pattern formation resulted from a competition between inter-particle forces and capillary and convective solvent flows. It remains little understood how reactive particles may alter evaporation induced patterns, for in situ generated molecular and particulate species can affect the solvent flows and thus the residual pattern.
The overall objectives for this project are:
1) To elucidate a mechanism for the formation of complex patterns from the evaporation of a reactive ZnO nanofluid droplet
2) To study a plethora of physical parameters (such as particle size and morphology, substrate chemistry, evaporation rate, etc.) on the ultimate pattern formation
3) To explore potential functionalities of such surface patterns.
