Tech Briefs Magazine - October 2021 - 36a

RESEARCH SPOTLIGHT
they used to characterize the vibrational
behavior of the single scale. The LDV
results showed good agreement with the
calculated resonances for the first and
third modes, which differed by just 2.9%
and 1.0%, respectively. The calculated
resonances were 28.4, 65.2, and 153.1
kHz, compared to LDV results of 27.6,
90.8, and 152.3 kHz. The 28% deviation
for the second mode can be explained
by the simplified curvature of the moth
scale, the fact that the perforation
rate of the scale was modeled as
constant when it actually varies, and
inconsistencies in the incident sound
wave during the LDV measurements.
It is interesting to note that the
calculated modes of the moth scale
overlap and span the biosonar range
used by bats for echolocation (typically
20-180 kHz). To see if this was just a
coincidence, the researchers repeated the
analysis for a similar unit cell that mimics
the structure of a butterfly wing scale.
This time, the modes fell outside of the
bats' biosonar range at 88.4, 150.9, and
406.0 kHz. It makes sense, evolution-wise:
Moths are nocturnal and are often within
bats' crosshairs, whereas butterflies are
active during the day and do not need
to protect themselves from the fanged
creatures. This comparison supports the
theory that moths may have evolved to
acoustically camouflage themselves from
bats.
FIGURE 3           
of the scale. They used the Periodic
boundary condition to model the single
unit cell instead of an entire scale array,
which saved computational effort and
memory. " We can simplify our model
to a few scales and use the Periodic
boundary condition to expand the
structure into an array. If we made an
actual array model, it would be too
big for the computer to handle, " said
Shen. The team then modeled the scale
vibrations at ultrasonic frequencies using
a macroscale FEM model, allowing them
to calculate the vibration of the scale.
" COMSOL®
is really good at coupled
problems. We needed both acoustics and
solid mechanics to understand how the
ultrasonic wave is coupled with the scale
36a COMSOL NEWS
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structure, " said Shen.
The team also built two models to
analyze the damping effect of the moth
scale and the ultrasonic properties of an
entire moth wing made up of such scales.
The former consisted of a single scale with
one end fully clamped, while the latter
added Rayleigh damping to the material
and was used to calculate the absorption
coefficient of the scaled array.
» COMPARING CALCULATIONS
TO MEASUREMENTS
In order to see how the calculated
vibrations of the moth scale compare to
the real-world behavior of moth scales,
the team then turned their attention to
a laser Doppler vibrometer (LDV), which
» INSPIRING NEW USES FOR
ACOUSTIC CAMOUFLAGE
This research project marks the first
effort to characterize the biomechanics
and vibrational behavior of moth scales,
both numerically and experimentally.
The results demonstrate that
multiphysics modeling software can be
used to accurately capture moth scale
behavior, paving the way for further
simulation-driven analysis in this area. In
the future, the University of Bristol team
aims to expand the current periodic
model into a full 3D model of an array of
moth scales.
The research itself also has far-reaching
implications outside of the animal
kingdom. By being able to understand
the vibroacoustic behavior of moth
scales, researchers can start to develop
macroscopic structures with the same
acoustic camouflage capabilities. " If we
can make materials mimicking moth scales,
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Tech Briefs Magazine - October 2021

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