Droplet microfluidics, colloidal assembly and nanoscale processing: Synergistic control and properties of colloid-based photonic microobjects.

Colloidal photonic crystals have drawn wide attention in a number of realms due to their manyapplications. Photonic microobjects can be processed by a combination of droplet-based microfluidics and the subsequent different post-processing approaches in a precisely controlled manner in terms of compositions, geometries, and functionalities, offering a wide range of properties for the resulting products. In this review, we provide a summary of colloidal-based photonic microobjects that have evolved from droplets produced by microfluidic devices with different configurations and designs. The colloidal building blocks can be either inert or responsive to external stimuli, which impart the colloidal photonic microobjects with tunable properties. By leveraging a number of post-processing strategies, including evaporation of solvents from the droplet templates, external field-guided assembly, selective sputter coating, controlled etching, osmosis regulating, etc., the obtained photonic microobjects eventually possessed diverse microstructures with different fashions, featuring the photonic microobjects with demanded photonic performances in sub-microscale or can be further organized for bulk applications. Finally, we analyze the challenges and present outlooks on future development trends regarding the construction of colloid-based photonic microobjects, including current issues, critical needs, and promising emerging photonic applications. Also, we propose some emerging scientific questions and engineering limitations may be worthy of exploration based on the combination of microfluidics processing, colloidal assembly, and post-treatments.