Transparent printer ink can create a whole spectrum of colors when printed in precise, microscopic patterns. Researchers used this technique to print vibrant butterflies (pictured) and other colorful pictures.

K. Li et al/Science Advances 2021

You may have seen or used disappearing ink. One minute you’re writing, and the next — poof! Your words are gone. It’s almost like magic. Now, scientists have performed perhaps an even more impressive trick. Using transparent ink, they have printed images in a full rainbow of colors. The key to this innovation: They print the clear liquid in precise microscopic patterns.

Those tiny patterns create what’s known as structural color.

Unlike the dyes and pigments used to color most products, structural colors won’t fade. And they don’t use chemicals that can pollute the environment. Researchers in China wanted to use those features to create longer-lasting, more eco-friendly printing.

Structural colors arise from the way different colors — wavelengths of light — bounce off microscopic structures. “In nature, there are many beautiful structure colors,” says Yanlin Song. He’s a materials chemist at the Chinese Academy of Sciences. That’s in Beijing. Some bird feathers and butterfly wings sport vibrant structural colors, he notes. This phenomenon also colors chameleons’ skin and some flower petals.

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Song’s team printed structural colors on transparent, flexible sheets of silicone. To do this, they used an ordinary ink-jet printer and a clear, nontoxic polymer ink. The printer studded the silicone sheets with millions of microscopic ink domes. Adjusting the size of each dome determined what wavelengths of light it would reflect. That, in turn, controlled the dome’s color. Increasing a dome’s width from 6.6 to 11 micrometers (260 millionths to 433 millionths of an inch) shifted its hue along the spectrum from blue to red and back again. Each dome acted like a single pixel, adding a tiny bit of color to a larger image.

The denser the ink domes were packed, the brighter an image became. Printing domes of several different colors across a single area created blended shades. For instance, combining red, yellow and green domes made brown. Adding domes with cooler tints, such as blue, tinted a picture gray. The researchers made portraits of physicist Isaac Newton and the Mona Lisa. They also printed pictures of Marilyn Monroe and other celebrities.

The team shared its results online September 22 in Science Advances.

“The printing speed needs to be improved,” Song says. An image 2 by 2 centimeters across (0.8 by 0.8 inches) took about five minutes to make. The next step, he says, is to build faster printers that can quickly create large images whose hues come from structural color.

“I was excited to see that somebody had used [structural color] for this purpose,” says Lauren Zarzar. She’s a materials chemist who has studied structural colors cast by water and oil droplets. She works at Pennsylvania State University in University Park. “They had some nice examples that I think illustrated the versatility of this mechanism.”

Zarzar imagines several uses for this technology. Printing many domes of different hues could create complex color signatures that would be hard to replicate, she says. Those signatures could be printed on ID cards or money. That might make it harder to counterfeit IDs or paper currency. “There’s also a lot of decorative applications,” Zarzar adds. “Cosmetics, decorations on clothing, car paints and decals. Things like that. It also can be used in architecture.”  

This is one in a series presenting news on technology and innovation, made possible with generous support from the Lemelson Foundation.

chemical: A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.

environment: The sum of all of the things that exist around some organism or the process and the condition those things create. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature and humidity (or even the placement of things in the vicinity of an item of interest).

hue: A color or shade of some color.

Isaac Newton: This English physicist and mathematician became most famous for describing his law of gravity. Born in 1642, he developed into a scientist with wide-ranging interests. Among some of his discoveries: that white light is made from a combination of all the colors in the rainbow, which can be split apart again using a prism. Newton died in 1727.

micrometer: (sometimes called a micron) One thousandth of a millimeter, or one millionth of a meter. It’s also equivalent to a few one-hundred-thousandths of an inch.

microscopic: An adjective for things too small to be seen by the unaided eye. It takes a microscope to view objects this small, such as bacteria or other one-celled organisms.

phenomenon: Something that is surprising or unusual.

physicist: A scientist who studies the nature and properties of matter and energy.

pigment: A material, like the natural colorings in skin, that alter the light reflected off of an object or transmitted through it. The overall color of a pigment typically depends on which wavelengths of visible light it absorbs and which ones it reflects. For example, a red pigment tends to reflect red wavelengths of light very well and typically absorbs other colors. Pigment also is the term for chemicals that manufacturers use to tint paint.

pixel: Short for picture element. A tiny area of illumination on a computer screen, or a dot on a printed page, usually placed in an array to form a digital image. Photographs are made of thousands of pixels, each of different brightness and color, and each too small to be seen unless the image is magnified.

polymer: A substance made from long chains of repeating groups of atoms. Manufactured polymers include nylon, polyvinyl chloride (better known as PVC) and many types of plastics. Natural polymers include rubber, silk and cellulose (found in plants and used to make paper, for example).

rainbow: An arc of color displayed across the sky during or just after a rain. It’s caused when water droplets in the atmosphere bend (or diffract) white sunlight into a number of its component hues: usually red, orange, yellow, green, blue, indigo and violet.

spectrum: (plural: spectra) A range of related things that appear in some order. (in light and energy) The range of electromagnetic radiation types; they span from gamma rays to X rays, ultraviolet light, visible light, infrared energy, microwaves and radio waves.

technology: The application of scientific knowledge for practical purposes, especially in industry — or the devices, processes and systems that result from those efforts.

transparent: Allowing light to pass through so that objects behind can be distinctly seen.

wavelength: The distance between one peak and the next in a series of waves, or the distance between one trough and the next. It’s also one of the “yardsticks” used to measure radiation. Visible light — which, like all electromagnetic radiation, travels in waves — includes wavelengths between about 380 nanometers (violet) and about 740 nanometers (red). Radiation with wavelengths shorter than visible light includes gamma rays, X-rays and ultraviolet light. Longer-wavelength radiation includes infrared light, microwaves and radio waves.

Journal:​ K. Li et al. Facile full-color printing with a single transparent ink. Science Advances. Vol. 7, published online September 22, 2021. doi: 10.1126/sciadv.abh1992.

Maria Temming is the assistant editor at Science News for Students. She has bachelor's degrees in physics and English, and a master's in science writing.

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