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Watermark Ink

August 8, 2011

When I was a child, I was given a book that contained interesting projects to keep children busy. The publisher had a good idea. What parent wouldn't buy a book that promised to provide a modicum of quiet around the house, if only for an hour at a time?

One of the projects was about secret writing. If you write something on paper with lemon juice, the message is invisible when the liquid dries. The instructions in the book were to heat the paper by holding it over a candle, and the invisible message would appear. The candle option, or any other heat source, is certainly not something that would be published, or recommended to children, today.[1]

The lemon juice invisible ink works because the ascorbic acid in lemon juice decomposes at about 375°F, a little below the memorable combustion point of paper, 451°F. The presence of other organic chemicals in the juice helps as well. Milk will work, also, since its various proteins and fats will decompose when heated. As can be seen from the results of my experiment, below, milk works a little better than lemon juice.

Figure caption

Invisible ink test.

The top line was written with lemon juice, the bottom with milk. The writing was exposed by holding the paper a few inches above an electric stove heating element)

A group of scientists from Harvard University's School of Engineering and Applied Sciences have developed a high-tech version of secret writing, although that wasn't the intended application of their technology.[2-3] Their nanostructured surfaces are designed as a means to measure the surface tension of liquids. Patterning the nanostructures allows the selective display of messages, also. The message will be different, depending on the surface tension of the liquid applied to the surface.

The technology, called "W-Ink", or "Watermark Ink" is based on the properties of the material called inverse opal. An opal is essentially an ordered array of tiny spheres, and an inverse opal is what you get when you backfill the space between the spheres with another material and then etch away the original spheres. When you do this, you have a new material with an interconnected network of pores.

The way the material acts when it's exposed to a liquid depends on the pore structure and the surface chemistry. Liquid will enter the pores and change the color of the material if it wets the surface and the surface tension allows it to be pulled into the pores.

The natural color of the inverse opal will only appear in regions devoid of liquid. Since there's a color change, the Harvard scientists see their material as "litmus paper" for surface tension; or, more scientifically speaking, a colorimetric method for surface tension measurement.

Figure caption

(A) Appearance of a prepared surface upon exposure to various ethanol concentrations

(B) Distinguishing different liquids

(Harvard University Image))

There are quite a few applications for this concept. It can be used as a validation tag on items to detect counterfeits, verify gasoline grade at the pump, and test for the presence of adulterants in foodstuffs. One advantage of this technology is that no power is required.

A report on this research was published in the Journal of the American Chemical Society,[3] and there's a short YouTube video demonstration.[4] The research was supported by the Air Force Office of Scientific Research, the Natural Sciences and Engineering Research Council of Canada, and the US Department of Homeland Security.

There's a book (Gever Tulley and Julie Spiegler, "50 Dangerous Things (You Should Let Your Children Do)," NAL Trade Reprint edition, April 20, 2011, 144 pages) and a companion web site (fiftydangerousthings.com) that contain activities much like the invisible ink experiment at the head of this article. As they say, "Adult supervision is required."


  1. I didn't let my children do 90% of the things I did when I was a child! The government concurs, but perhaps a little too strongly, as I wrote about in the article, Baking Soda and Vinegar, October 8, 2010.
  2. Caroline Perry, "'Watermark Ink' device identifies unknown liquids instantly," Harvard University Press Release, August 3, 2011.
  3. Ian B. Burgess, Lidiya Mishchenko, Benjamin D. Hatton, Mathias Kolle, Marko Lonar and Joanna Aizenberg, "Encoding Complex Wettability Patterns in Chemically Functionalized 3D Photonic Crystals," J. Am. Chem. Soc., (Online publication, DOI: 10.1021/ja2053013), July 18, 2011.
  4. YouTube video, W-Ink: An invisible ink displaying multiple levels of encryption.

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Linked Keywords: Steganography; secret writing; lemon juice; candle; ascorbic acid; paper; 451°F; organic chemical; milk; protein; fat; experiment; scientist; Harvard University; School of Engineering and Applied Sciences; nanostructure; surface tension; liquid; inverse opal; opal; sphere; pore; surface chemistry; hydrophilic; litmus paper; colorimetric; counterfeit; gasoline; adulterant; foodstuff; Journal of the American Chemical Society; YouTube; Air Force Office of Scientific Research; Natural Sciences and Engineering Research Council of Canada; US Department of Homeland Security; Gever Tulley; Julie Spiegler; 50 Dangerous Things (You Should Let Your Children Do; fiftydangerousthings.com.