Droplets of cellulose nanocrystal suspensions on drying give iridescent 3-D “coffee-stain” rings
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Films prepared from aqueous suspensions of cellulose nanocrystals (CNC) often display iridescent colors due to the reflection of light from the helicoidal orientation of the rod-like CNC in the film. Droplets of CNC suspension deposited on a plane surface evaporate to give films that display iridescent rings. Profilometry measurements across the rings show that the outer edge of the films is much thicker than the region in the center. This is ascribed to a “coffee-stain” effect; the CNC are transported to the outer edge for droplets where the contact line is pinned during evaporation. The gradient in concentration across the ring results in a color gradient, with the longer wavelengths decreasing towards the center of the sample, in accord with the hypothesis of a two-stage process for CNC chiral nematic color formation.
KeywordsCellulose nanocrystals Iridescent films Chiral nematic structure Coffee-stain effect Film topology and color
We thank Dr. Tiffany Abitbol for preparation of the CNC suspension, and Dr. H.P.T. Nguyen and the McGill Nanotools Microfab facility for the profilometry measurements. Funding from the Natural Sciences and Engineering Research Council Canada, and support from Fonds Québecois de la Recherche sur la Nature et les Technologies through the Center for Self-Assembled Chemical Structures is gratefully acknowledged.
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