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Photonics

Capillary confinement produces organized photonic films

Asymmetric drying promotes alignment of cellulose nanocrystals

by Kerri Jansen
November 18, 2018 | APPEARED IN VOLUME 96, ISSUE 46

Credit: Nano Letters
Researchers used a polarized light microscope to track the stages of thin-film formation inside a rectangular capillary. A suspension of cellulose nanocrystals, exposed to air on the right side, dries asymmetrically and the refracted color changes from blue to red as the nanocrystals form a liquid-crystalline phase, take on their preferred alignment, and solidify.
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Inside a rectangular capillary, a suspension of cellulose nanocrystals dries asymmetrically, refracting different colors of light as it solidifies.

Photonic thin films—in which ordered nanostructures interact with light to produce color—can be used to make lightweight and flexible color filters, sensors, display components, and more. But ensuring that the nanocrystals within them align uniformly to produce well-defined color can be difficult and time consuming. Casting such a film in a dish, for example, can take weeks and still result in poorly controlled nanostructures and colors. Now, Georgia Institute of Technology’s Vladimir V. Tsukruk and colleagues at the Air Force Research Lab and Kent State University have developed a capillary-based method that makes highly ordered photonic thin films in a matter of hours (Nano Lett. 2018, DOI: 10.1021/acs.nanolett.8b02522). The team puts an isotropic suspension of cellulose nanocrystals in a 50-µm-thick rectangular capillary that is open on one end to allow water to evaporate. Within the confined space of the capillary, the solution dries asymmetrically and the crystals organize in a uniform pattern as the film solidifies. The film can be peeled away after breaking the capillary. The highly ordered nanostructures, which take on left-handed chirality, refract light in a predictable, uniform way. Tsukruk says his team has scaled up the technique to make films of up to 25 cm2. Mark MacLachlan at the University of British Columbia, who works with cellulose nanocrystals, says he thinks the work is an elegant contribution to the field, and it could be important for developing new microdevices, including ones that use light for biological or chemical detection.

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