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"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.
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 shockwaves 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 time 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." |
