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 CHAMPAIGN -- Researchers at the
University of Illinois report that a new study of
mechanoluminescence revealed extensive atomic and molecular spectral
emission not previously seen in a mechanoluminescence event. The
findings, which appeared online in May in the Journal of the
American Chemical Society, also include the first report of gas
phase chemical reactions resulting from a mechanoluminescence event.
Mechanoluminescence is light generated
when a crystal, such as sugar or quartz, is fractured by grinding,
cleaving or via other mechanical means. Sir Francis Bacon wrote
about this phenomenon as early as 1605. Others have used the effect
to impress, if not enlighten, others.
"You may, when in the dark, frighten
simple people only by chewing lumps of sugar, and, in the meantime,
keeping your mouth open, which will appear to them as if full of
fire," Father Giambattista Beccaria wrote in "A Treatise Upon
Artificial Electricity," in 1753.
Scientists believe mechanoluminescence
occurs as a result of the generation of opposite charges along the
fracture plane of an asymmetrical or impure crystal. When the
charges recombine, the surrounding gas is ionized and emits light.
Mechanoluminescence that results from
simple grinding or cleavage of a crystal can be quite weak and
difficult to study. Late last year, U of I
chemistry
professor Kenneth Suslick and graduate student Nathan Eddingsaas
reported in the journal Nature that a new technique, the sonication
of crystal slurries, produced a much more intense
mechanoluminescence than grinding. Sonication, the use of sound
energy to agitate particles or other substances, causes high-intensity collisions of crystal particles in liquid slurries.
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The resulting mechanoluminescence is
an order of magnitude brighter than that produced by grinding.
Sonication of liquids causes acoustic
cavitation: the formation, growth and implosion of bubbles. This
generates tremendous heat, pressure and shock waves within the liquid
that can exceed the speed of sound. Crystal particles suspended in a sonicated liquid collide and fracture, causing intense
mechanoluminescence. The new
study involved the sonication of a slurry of recorcinol (sugar)
crystals in the liquid paraffin, dodecane. When nitrogen or oxygen
was bubbled through the sonicated slurry, the resulting emission
spectrum was more than a thousand times more intense than that
produced by grinding. The researchers also saw emission lines not
previously reported in a mechanoluminescence event. These peaks on
the mechanoluminescence spectra are evidence of gas phase chemical
reactions during the event.
"When oxygen is present, chemical reactions take place that are
similar to those that occur in the production of diamond films using
an electrical discharge," Suslick said. "The intense
mechanoluminescence and chemical reactions produced by ultrasound
give us a better understanding of mechanoluminescence,
mechanochemistry and the effect of ultrasound on solids within a
liquid."
[Text copied from
University
of Illinois news release]
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