Self-assembly of highly ordered structures enabled by controlled evaporation of confined microfluids

Recent progress in nano- and microscale self-assembly as a lithography- and external field-free means to construct well-defined, often intriguing structures has received much attention for producing complex, large-scale structures with small feature sizes. Evaporative self-assembly in a sessile drying droplet containing nonvolatile solutes (e.g., polymers and nanoparticles) on a solid substrate generally produces stochastic irregular structures (e.g., multiple concentric "coffee rings", polygonal network structures, and fingering instabilities) due to the lack of control over the evaporation process and the temperature gradient-induced convection. To utilize the evaporative selfassembly process as a micro-fabrication technology for developing wellordered, complex multi-dimensional structures, the evaporation process and the broad range of parameters, including the evaporation rate, solution concentration, and interfacial interactions among the solvent, solute, and substrate, should be precisely and systemically controlled. Irreversible solvent evaporation of the solution containg polymeric materials and nanocrsytals confined in a restricted geometry renders the organization of these nonvalotile solutes into concentric microstructures of high fidelity and regularity, which offer tremendous potential for applications in microelectronics, optoelectronics, and biotechnology, among other areas.