Photonic Papers and Inks: Color Writing with Colorless Materials
(Under Construction: Ian Burgess, Spring 2012)
H. Fudouzi, Y.N. Xia, Photonic Papers and Inks: Color Writing with Colorless Materials, Advanced Materials, 15, 892-896 (2003).
This paper describes a technique to exploit solvent swelling of an elastomeric matrix as a photonic ink, capable of writing color patterns while each component material is "colorless" (i.e. does not absorb visible light).
The ink/paper system is based on an elastomeric matrix (polydimethylsiloxane, PDMS) that contains an embedded regular lattice of polystyrene (PS) microshperes. A scanning electron micrograph (SEM) of the structure is shown in the figure below (left figure, B). This dielectric superstructure with wavelength-scale periodicity acts as a photonic crystal, exhibiting color due to a strong Bragg reflection peak in the visible region of the spectrum. Since the apparent color is directly due to Bragg reflection, it is very sensitive to physical properties of the system that control the optical pathlength of periodicity. Swelling the PDMS matrix with a solvent (e.g. silicone oil, aliphatic compounds, etc.) pushes the PS spheres, increasing the lattice constant and redshifting teh optical Bragg reflection peak (shown schematically in the figure below, left A). This results in a visible color change (shown in the figure below, right). By blotting the photonic paper with a sufficiently small amount of solvent (i.e. with a pen, stamp or printer), the color change can be effectively confined to a user-defined pattern (shown in the figure below, right). Unlike traditional inks, this photonic-paper/ink system can be readily made eraseable-rewriteable by using sufficiently volatile solvents. The authors show that in this case, after sufficient time for the solvent to evaporate, the color of the paper returns to its dry state and messages can be erased. However they show that permanent messages can also be written by infiltrating fluids that can be later cured into the matrix (the authors use silicone liquids with vinyl groups attached which facilitate thermal grafing to the PDMS network). The authors also demonstrate how multicolor printing can be achieved in this platform by varying the composition of the swelling liquid to change the final degree of swelling. They demonstrate a qualitative correlation between decreasing molecular weight of the swelling solvent and increasing degree of redshift of the reflection peak.