Ultrasound Leads to Intense Mechanoluminescence

The next time you pull off that old party trick of turning off the lights, then crunching into a wintergreen-flavored Life Savers candy until the sparks fly, you may have a deeper appreciation for the effect known as triboluminescence, or mechanoluminescence.

Scientists have observed the phenomenon for more than 400 years, but it mostly has been relegated to party games because the luminescent effect caused by crushing or scraping crystals (in the case of hard candy, sugar crystals) is very weak. However, investigators Nathan C. Eddingsaas and Kenneth S. Suslick of the University of Illinois at Urbana-Champaign have developed a method that greatly enhances the luminescent effect.

These photographs illustrate the principle of mechanoluminescence. The left image shows the light from loose N-acetylanthranilic acid crystals crushed between two transparent windows; the right image, light from similar crystals shaped into a logo, then crushed between two windows. Courtesy of Nathan C. Eddingsaas and Kenneth S. Suslick.

As they report in an ASAP article published May 5 by the Journal of the American Chemical Society, they induced mechanoluminescence in an organic slurry comprising the sugar resorcinol, suspended in an organic liquid, dodecane, under nitrogen gas. To create the mechanical motion, they sonicated the slurry with pulses of ultrasound, at 6 W/cm² and 20 kHz. For comparison, they also induced mechanoluminescence in the slurry by grinding portions between a pair of glass plates. The researchers captured light from the agitated slurry via a fiber optic bundle that led to an Acton monochromator with a 300-groove-per-millimeter diffraction grating and a CCD camera made by Princeton Instruments.

They found that the intensity of mechanoluminescence induced by sonication was 1000 times greater than that created by simple grinding. Moreover, they observed unexpected emission lines from carbon monoxide and other chemicals, indicating the existence of chemical reactions within the slurry during the sonication process.

The researchers expect that using sonication to induce mechanoluminescence in materials will lead to a better understanding of mechanoluminescence in general and of the chemical reactions that result in particular.