Scientists at Tongji University, Shanghai, China, have developed a method of creating patterns on photonic paper that are only visible when the paper is wet. The siloxane-containing photonic paper is fabricated by using lithography to create a contrast of crosslinking or degree of hydrophobization. When the invisible prints are soaked in water for several minutes, the small difference in reflection wavelength (λ) between the pattern and the background is significantly amplified due to their different swelling speeds, which leads to a visible image. The process is fully reversible and soaking and drying will repeat the cycle of showing and hiding the patterns.
This article was also featured in Chemistry World. The Chemistry World story is available here.
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