The color of most everyday objects is determined by the way in which chemicals within the objects absorb and reflect different wavelengths of light. But this isn't always the case. The iridescent colors of the wings of several species of butterfly are the result of a qualitatively different type of color known as ‘structural color’, which is caused by the interaction of light with periodic structures within or on the surface of an object. Not only do structural colors tend to be more vivid and visually striking than those of conventional chemical pigments, they are usually more stable and longer lasting.

Structural colors occur regularly in nature, yet conventional synthetic techniques for producing them artificially are time consuming and costly. Now, an international collaboration between Sunghoon Kwon and colleagues at the Seoul National University in Korea and Yadong Yin and colleagues at the University of California Riverside in the USA1 have developed a structurally colored ink that is not only quick and easy to produce, but also changes color according to the strength of an applied magnetic field.

The ink — referred to as ‘M-Ink’ — consists of a colloidal suspension of superparamagnetic nanocrystal clusters (CNCs) in a mixture of a solvation liquid and a photocurable polymer resin. When a magnetic field is applied to the ink in its fluid state, the CNCs align in regularly ordered linear arrays. The interference that occurs when light hits and is scattered from these periodically arranged CNCs results in structural color.

Fig. 1: Multicolor image produced with M-Ink. Subjecting unfixed ink to a magnetic field alters the structural color. By selectively curing individual regions, mutlicolored images can be formed.

Changing the strength of the magnetic field alters the spacing between the CNCs in these arrays, which in turn changes the tone of the observable color. Exposing the ink to ultraviolet light while it is subjected to a magnetic field cures the resin and locks the CNCs into position, thereby permanently fixing the color. By selectively exposing different parts of the resin to ultraviolet light and repeating the process under different magnetic fields in different regions, the research team was able to form stable, complex multicolored images within a matter of seconds (Fig. 1).