Instantaneous, Simple, and Reversible Revealing of Invisible Patterns Encrypted in Robust Hollow Sphere Colloidal Photonic Crystals. Issue 25 (4th May 2018)

• Record Type: Journal Article
• Authors: Zhong, Kuo; Li, Jiaqi; Liu, Liwang; Van Cleuvenbergen, Stijn; Song, Kai; Clays, Koen
• Is Part Of: Advanced materials. Volume 30:Issue 25(2018)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2018-05-04
• Subjects: anticounterfeiting -- hollow spheres -- invisible patterns -- responsive colloidal photonic crystals -- structural color; Materials -- Periodicals; Chemical vapor deposition -- Periodicals; 620.11
• DOI: 10.1002/adma.201707246
• Languages: English
• ISSNs: 0935-9648

Abstract

Abstract: The colors of photonic crystals are based on their periodic crystalline structure. They show clear advantages over conventional chromophores for many applications, mainly due to their anti‐photobleaching and responsiveness to stimuli. More specifically, combining colloidal photonic crystals and invisible patterns is important in steganography and watermarking for anticounterfeiting applications. Here a convenient way to imprint robust invisible patterns in colloidal crystals of hollow silica spheres is presented. While these patterns remain invisible under static environmental humidity, even up to near 100% relative humidity, they are unveiled immediately (≈100 ms) and fully reversibly by dynamic humid flow, e.g., human breath. They reveal themselves due to the extreme wettability of the patterned (etched) regions, as confirmed by contact angle measurements. The liquid surface tension threshold to induce wetting (revealing the imprinted invisible images) is evaluated by thermodynamic predictions and subsequently verified by exposure to various vapors with different surface tension. The color of the patterned regions is furthermore independently tuned by vapors with different refractive indices. Such a system can play a key role in applications such as anticounterfeiting, identification, and vapor sensing. Abstract : Invisible patterns encrypted in hollowsphere colloidal photonic crystals are instantaneously and reversibly revealed by a simple method. These encrypted patterns remain invisible under static environmental humidity, even up to near 100% relative humidity, while they are immediately (≈100 ms) revealed by human breath. Such a system can prove highly useful in anticounterfeiting and sensing applications.