Figure 2.

(a) Image of a sponge-written “TU/e” logo that appeared in wet state on exposure to water or exhaled breath but remained hidden in the dry state. (b) Wavelength of light reflected by the “TU/e” logo in wet and dry state. Error bar indicates mean ± standard deviations for five measurements at different places on the pattern. (c) Height profile of the “TU/e” logo in wet state showed swelling for the slash and shrinkage for the letters with respect to the green background and nearly flat surface in the dry state. (d) FT-IR spectra of the patterned CLC polymer coating. Slash consisted of only K⁺ ions. Background consisted of both K⁺ and Ca²⁺ ions. Letters consisted of only Ca²⁺ ions. Polarized optical microscopy image (in incidence light mode) of (e) slash and (f) letter “T”. Image of the “TU/e” pattern on wet coating as seen through (g) right-handed and (h) left-handed circular polarizer.