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Credit: Spadoni & Daraio/Caltech
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Taking
inspiration from a popular executive toy ("Newton's cradle"), researchers at the
California Institute of Technology (Caltech) have built a device—called a
nonlinear acoustic lens—that produces highly focused, high-amplitude acoustic
signals dubbed "sound bullets."
The acoustic lens
and its sound bullets (which can exist in fluids—like air and water—as well as
in solids) have "the potential to revolutionize applications from medical
imaging and therapy to the nondestructive evaluation of materials and
engineering systems," says Chiara Daraio, assistant professor of
aeronautics and applied physics at Caltech and corresponding author of a recent
paper in the Proceedings of the National Academy of
Sciences (PNAS) describing the
development.
Daraio and
postdoctoral scholar Alessandro Spadoni, first author of the paper, crafted
their acoustic lens by assembling 21 parallel chains of stainless steel spheres
into an array. Each of the 21 chains was strung with 21 9.5-millimeter-wide
spheres. (Daraio says particles composed of other elastic materials and/or with
different shapes also could be used.)
The device is
akin to the Newton's
cradle toy, which consists of a line of identical balls suspended from a frame
by wires in such a way that they only move in one plane, and just barely touch
one another. When one of the end balls is pulled back and released, it strikes
the next ball in line and the ball at the opposite end of the cradle flies out;
the balls in the middle appear to remain stationary (but really are not,
because of the nonlinear dynamics triggered in the system).
The chains of
particles in Daraio's and Spadoni's acoustic lens are like a longer version of
a Newton's
cradle. In the lens, a pulse is excited at one end by an impact with a striker,
and nonlinear waves are generated within each chain. These chains, Daraio says,
"are the simplest representation of highly nonlinear acoustic waveguides,
which exploit the properties of particle contacts to tune the shapes of the
traveling acoustic signals and their speed of propagation, creating compact
acoustic pulses known as solitary waves."
Solitary
waves—unlike the rippling waves produced by dropping a pebble into a pond—can
exist in isolation, neither preceded nor followed by other waves.
"The
solitary waves always maintain the same spatial wavelength in a given
system," she adds, "and can have very high amplitude without
undergoing any distortion within the lens, unlike the signals produced by
currently available technology."
The chains are
squeezed closer together—or "precompressed"—using fishing line. By
changing the amount of precompression, Daraio and Spadoni were able to vary the
speed of the solitary wave. When a series of those waves exit the array, they
coalesce at a particular location—a focal point—in a target material (which can
be a gas, like air; a liquid; or a solid). This superposition of solitary waves
at the focal point forms the sound bullet—a highly compact, large-amplitude
acoustic wave. Varying the parameters of the system can also produce a
rapid-fire barrage of sound bullets, all trained on the same spot.
In the current
design, the spheres are assembled in a two-dimensional arrangement, with each
row independent of its neighbors. "Three-dimensional arrangements will be
just as easy to create and will allow 3-D control of the sound bullets'
appearance and travel path," Spadoni says.
"Our lens
introduces the ability to generate compact, high-amplitude signals in a linear
medium, and also allows us to dynamically control the location of the focal
point," Daraio says. That means it isn't necessary to change any of the
geometric components of the lens to change the location of the focal point.
"All we do
is adjust the precompression for each chain of spheres," she says.
This simple
adjustment should make the sound bullets easy to adapt to a variety of
applications. "Anybody who has had an ultrasound exam has noted that the
operator switches the probes according to the characteristics and location
within the body of what is being imaged," Daraio says. "The acoustic
lens we propose would not require replacement of its components, but rather
simple adjustments of the precompression on each chain."
The acoustic lens
created by Daraio and Spadoni was intended to be a proof of concept, and is
probably many years away from being used in commercial applications. "For
practical uses," Daraio says, "an improved design for controlling the
application of static precompression on each chain would be required-based, for
example, on electronics rather than on mechanical impacts as is currently done
in our lab."
Still, the
instrument has the potential to surpass the clarity and safety of conventional
medical ultrasound imaging. The pulses produced by the acoustic lens—which are
an order of magnitude more focused and have amplitudes that are orders of
magnitude greater than can be created with conventional acoustic
devices—"reduce the detrimental effects of noise, producing a clearer
image of the target." They also "can travel farther"—deeper
within the body—"than low-amplitude pulses," Daraio says.
More
intriguingly, the device could enable the development of a non-invasive scalpel
that could home in on and destroy cancerous tissues located deep within the
body.
"Medical
procedures such as hyperthermia therapy seek to act on human tissues by locally
increasing the temperature. This is often done by focusing high-energy acoustic
signals onto a small area, requiring significant control of the focal
region" so that healthy tissue is not also heated and damaged, Daraio
explains. "Our lens produces a very compact focal region which could aid
further development of hyperthermia techniques."
Furthermore,
sound bullets could offer a nondestructive way to probe and analyze the
interior of nontransparent objects like bridges, ship hulls, and airplane
wings, looking for cracks or other defects.
"Today the
performance of acoustic devices is decreased by their linear operational range,
which limits the accuracy of the focusing and the amplitude achievable at the
focal point," Daraio says. "The new nonlinear acoustic lens proposed
with this work leverages nonlinear effects to generate compact acoustic pulses
with energies much higher than are currently achievable, with the added benefit
of providing great control of the focal position."
The paper, "Generation and control of
sound bullets with a nonlinear acoustic lens," was funded by the Army
Research Office and the National Science Foundation.
SOURCE
