Photonic printing on transparent substrates using emerging synthetic photonic crystals is in high demand, especially for antifraud applications. However, photonic printing is faced with grand challenges including lack of full invisibility of printed patterns before stimulation or after stimulus removal and absence of the long-lasting stability. Natural anisotropic crystal structures and artificially molecularly arranged polymers show an optically anisotropic property known as birefringence. Crystalline cellulose is the most abundant birefringent biocrystal on the earth. Here, we introduce a printing method based on using a cellulose nanocrystal/polymer ink that is governed by surface evaporation phenomenon and divided surface tension forces to direct the self-assembly of ink components at the nanoscale and print three-dimensional birefringent microfigures on transparent substrates. This type of printing is from now on referred to as birefringent printing. Unlike previously reported photonic crystal printing methods, this method is accurate, has high contrast, is virtually impossible to forge, and is very simple, inexpensive, and nontoxic.
Keywords: birefringence; cellulose nanocrystal; directed self-assembly; invisible printing; polymer ink; surface tension.