Photonic Papers and Inks: Color Writing with Colorless Materials
1. H. Fudouzi, Y.N. Xia, Photonic Papers and Inks: Color Writing with Colorless Materials, Advanced Materials, 15, 892-896 (2003).
2. H. Fudouzi, Y.N. Xia, Colloidal Crystals with Tunable Colors and Their Use as Photonic Papers, Langmuir, 19, 9653-9660 (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 of the elastomeric matrix. They demonstrate a qualitative correlation between decreasing molecular weight of the swelling solvent and increasing degree of redshift of the reflection peak when using silicone liquids. Further redshifting was possible when using hexanes and cyclohexane as infiltrating fluids due to their higher capacity to swell PDMS. The authors also report a spatial edge resolution of approximately 50 microns.
This is an example of a potentially very useful application of solvent swelling as a monolithic platform for reversible or permanent reflective color printing. At the same time, from a soft-matter perspective, it is an example of an interesting problem with real-world application that would benefit from more quantitative predictive modeling. While a more detailed experimental characterization of this system by the same authors can be found in reference 2, there is no theoretical analysis of the swelling in nanostructured polymer. As shown by the authors qualitatively, the degree of swelling is expected to be controllable by varying the chemical affinity between the matrix and the solvent. Also qualitatively, the edge resolution should be controllable through both the molecular weight of the solvent and the amount that is added. A quantitative look at these phemonena might be a very interesting (and complicated) soft-matter problem to study, but as shown in this paper, may lead to better design of some very practical technologies.