A variety of interesting coloring books are available on the market that use water as a coloring agent.¹ When water is applied to the pages of these books, colors seem to appear like magic. When the water evaporates, the colors disappear. This allows the pages of these books to be colored over and over again. I’ve been curious about the science behind how these books work for quite some time, so I did some experiments and internet searching to try to figure this out. Spoiler alert: there’s chemistry involved!

In my internet searching I found a patent for “Water Surface Indication”.² The properties of the materials described in this patent are similar to the characteristics of the pages I observed during my experiments with water reveal coloring books (I used Melissa & Doug’s “Water WOW!” Water Reveal Pads in my tests).

The patent describes colored images on a surface (Figure 1, red rectangle) that is coated with a porous polymer (Figure 1, blue circles). The polymer coating is opaque, hiding the colored surface below. The polymer has an index of refraction (n = 1.4) that is quite different than air (n = 1.0) that is trapped in the pores of the polymer (Figure 1, white space between blue circles). This difference in refractive index causes any light incident on the polymer layer to be scattered, making the polymer opaque and causing the colored images to remain hidden.

Figure 1: Left: Diagram of red surface coated with a porous polymer (blue circles) with air pores (white space surrounding blue circles). Right: Light (red arrows) scatters off the polymer, hiding the red dye below.

Any water that is added fills the pores, displacing the air (Figure 2). Because the index of refraction of water (n = 1.33) is close to that of the polymer, light incident on the page isn’t scattered when it is wet. Rather, light penetrates the wet polymer layer and reflects off the colored surface, allowing the images to be seen.

Figure 2: Left: Diagram of red surface with a porous polymer (blue circles) with water filling the pores (blue space surrounding blue circles). Right: Light (red arrows) penetrates the water/polymer surface and reflects off the red dye, allowing it to be seen.

Below you can see some experiments I did to test whether such a porous polymer system is used in water reveal coloring books (Video 1).

Video 1: The Science of Water Reveal Coloring Books, Tommy Technetium YouTube Channel, July 15, 2024.

Interestingly, a product called the Bloody Bathmat,^3,4 which is used to play pranks and tricks on people, also seems to use the same type of polymer system seen in water reveal books (Video 2).

Video 2: How Does the Bloody Bathmat Work?, Tommy Technetium YouTube Channel, June 24, 2024.

What do you think? Is it likely that such a polymer system is used in these two products? Be sure to let me know your thoughts in the comments. Also, if you have other ideas about how these products might work, please be sure to share these ideas as well. Further, if you have suggestions for other experiments to try with water reveal coloring books, be sure to let me know. As one example, did you notice in these videos that you could easily tell that some liquids (acetone and hexane) evaporate off the color reveal surfaces much easier than others (ethanol and water)? Thus, these books might provide a colorful way to demonstrate different rates of evaporation of various liquids, which could lead to a nice tie-in to a discussion of intermolecular forces.

Happy Experimenting!

References:

1. See for example: https://www.melissaanddoug.com/collections/activity-books/products/water-wow-dinosaurs-water-reveal-pad-on-the-go-travel-activity (accessed July, 2024).

2. Dobson, P. J., Pincher, L. US Patent No. 10,964,236 B2, 2021.

3. https://bloodybathmat.com?sca_ref=6144308.hTAQ7GxfC4 Note: The author receives a commission if this link is used to purchase the Bloody Bathmat. If the reader desires to use a link that does not send commission, use the link in reference 4.

4. https://bloodybathmat.com/