Colour change

A research team led by a chemist at the University of California, Riverside has fabricated microscopic polymer beads that change colour instantly and reversibly when external magnetic fields acting upon the microspheres change orientation.

The beads, or ‘magnetochromatic microspheres’, have excellent structural stability.

They are also compatible with various types of dispersion media such as water, alcohol, hexane and even polymer solutions, allowing them to retain their magnetically tuneable colours in a variety of chemical environments.

'The instantaneous colour change occurs with no change in the structure or intrinsic properties of the microspheres,' said Yadong Yin, an assistant professor of chemistry, who led the development that brought together chemists at UCR and engineers at Seoul National University, South Korea.

To fabricate the microspheres, the researchers first mixed magnetic iron-oxide particles into a resin, which is initially in liquid phase but later turns solid on exposure to ultraviolet curable resin.

They then dispersed the resin solution in oil (mineral oil or silicon oil), whereupon the resin transformed into spherical droplets in the oil.

Next, the researchers applied an external magnetic field to organise the iron-oxide particles into periodically ordered structures that display a reflective colour if viewed along the direction of the magnetic field.

Finally, they exposed the liquid system to ultraviolet radiation to polymerise the resin droplets and make them solid microspheres.

Yin explained that the colour observed in the new materials is ‘structural colour’ because it is caused by interference effects rather than pigments.

Such colour effects, as seen in feathers of many birds, butterfly wings and beetle shells, are produced when microstructures in these objects are aligned in periodic arrays.

‘The colour is tuned by changing the relative orientation of the periodic arrays in the microspheres by using external fields,’ he said.

‘The use of magnetic fields as external stimuli has the additional benefits of instant action, contactless control and easy integration into electronic devices already in the market.’

Next, Yin and his colleagues plan to work on the specific applications for the magnetochromatic microspheres.

‘Rewritable energy-saving display units such as papers and posters are our main interests,’ Yin added.

‘We will also try to develop similar materials for chemical and biological sensors.’

The UCR Office of Technology Commercialization has filed a patent application on the technology and is currently seeking partners in industry interested in developing the technology commercially.