Sessile Microdroplet‐Based Writing Board for Patterning of Structural Colored Hydrogels

The patterning of structural colored materials has a significant impact on various applications such as flexible displays, anti‐counterfeiting patches, colorimetric sensors, etc. Herein, a sessile microdroplet‐based writing board is presented to pattern magnetochromatic hydrogels with abundant structural colors and improved optical performance. It is demonstrated that predesigned hydrophilic patterns on a hydrophobic writing board can capture a mixture of polymer and Fe3O4@SiO2 magnetic nanoparticles inks with a spatial resolution of ≈100 pin per 1 cm2 while retaining magnetic field responsibility to the lower limit of 84 Gs. The inks are self‐partitioned into microdroplet arrays, which would in situ transform into structural colored hydrogels within a short time via thiol‐Michael addition. In contrast to conventional evaporation induced assembly of colloidal photonic crystals in sessile droplets, the resulting structural colored hydrogel microarrays show not only good stability and optical adjustability but tunable morphologies. In addition, the introduction of the microfluidic mixing and ink dispensing system greatly shortens the time interval from the polymer mixing to sessile droplet generation, circumvents the challenge of short operation time for the self‐crosslinking ink components, and enables the direct handwriting of high quality structural colored patterns.