Reported January 2007

BACKGROUND: Materials chemists at the University of Toronto have developed a new elastic light-sensitive material that changes color based on pressure and could be used to capture data-rich fingerprints in multiple colors. The material could also be used in pressure sensors in consumer products, such as consumer electronics, airbag deployment, strain and torque sensors in high-rise buildings, or even in children's toys, where kids would press or squeeze the item to see it change color in front of their eyes.

HOW IT WORKS: Traditional fingerprinting methods involve treating samples with powders, liquids or vapors to add color to the print, so it can easily be photographed. This process is known as contrast enhancement. The Toronto scientists engineered their new material into thin, elastic foam that can be transferred onto any surface, such as glass, metal or plastic. If the foam is compressed, the internal structure changes, altering the wavelength (color) of light it produces and further enhancing contrast. The resulting images capture detailed information about pressure patterns and surface ridges that may not be visible to the naked eye.

WHERE THE COLOR COMES FROM: A peacock's brightly colored feathers don't get their color from pigments. Pigment molecules create colors by absorbing or reflecting certain wavelengths of light, depending on the chemical composition. Peacock feathers only have brown pigment (melanin). The bright colors we see arise from the inherent structure of the feathers, which have arrays of tiny holes neatly arranged into a hexagonal (lattice) pattern. This causes the light to refract off the surface in such a way as to produce the perception of color in the human eye; the colors one sees depends on the angle of reflection. Physicists call these structures photonic crystals.

ABOUT PHOTONIC CRYSTALS: Photonic crystals are materials with an arrangement of atoms in a precise lattice pattern that repeats itself identically and at regular intervals. But Nature doesn't produce crystalline structures with the level of precision we need, so scientists learned to make their own version of these materials, atom by atom, to control and manipulate light. Light generally travels in a straight line, but if the atoms are organized precisely enough, certain wavelengths of light will be blocked and reflected in new directions, even turning corners. The spacing of the atoms in the lattice structure determines which wavelengths will be blocked.

If you would like more information, please contact:

Andre Arsenault
Ph.D. Student
Department of Chemistry
University of Toronto
Toronto, Canada
(416) 978-4735
(416) 978-1018
aarsenau@chem.utoronto.ca

DID YOU KNOW...?

Iridescence appears not only in peacocks, but also in beetles, dragonflies, the wings of kingfishers and certain species of butterfly, and in the colorful spines of a marine worm called the sea mouse. Opals are another naturally occurring form of photonic crystal.

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