Tech Briefs Magazine - October 2021 - 35a

RESEARCH SPOTLIGHT
their hypothesis, the group sought
to capture the governing physical
phenomena of the wing scales and
show that moth scales can achieve high
absorption coefficients at resonance.
To do so, they turned to numerical
modeling...
FIGURE 2       
Researchers from the University of
Bristol used numerical modeling to study
this wing scale phenomenon and see how
we can potentially apply these acoustic
camouflaging capabilities to other areas.
» ECHOLOCATION
MEETS ITS MATCH
For more than 65 million years, bats have
sought out moths as a source of food.
Some moths can detect the signals of
approaching bats, while others defend
themselves with poison, or clicking sounds
that can startle the bats enough to fly
away. The cabbage tree emperor moth
is both deaf and nontoxic, but it is not
helpless. It simply relies on a more passive
defense strategy: acoustic camouflage
(also called acoustic cloaking).
How do moths use acoustic
camouflage to fend off bat attacks? To
find out, we can take a closer look at
their wings. Moth wings are solid, thin
membranes made up of chitin, a longchain
polymer derived from glucose.
Stiff wing veins hold these membranes
in place. Looking even closer, the upper
and lower surfaces of the moth wing are
covered in arrays of overlapping scales,
like the tiles on a roof. Each scale is
porous and of a complex structure. " The
highly sculptured scale structure implies
sophisticated evolutionary adaptations,
analogous to the highly organized
nanoscale photonic structures for visual
signaling, " said Zhiyuan Shen, researcher
at the University of Bristol.
These wing scales are less than 0.25
mm long, making them smaller than
1/10th
of the wavelength that bats
use for echolocation, using signals of
frequencies from 11 kHz to 212 kHz (Ref.
1). The University of Bristol researchers
hypothesized that moth wings can
be categorized as ultrathin absorbers
with subwavelength thickness, acting
as resonant absorbers, in their paper
" Biomechanics of a Moth Scale at
Ultrasonic Frequencies " . To investigate
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TECHNIQUES MEET
NUMERICAL SIMULATION
The project started with a few moth
pupae, which were cultivated in the lab
until they reached maturity. Researchers
collected samples of the moth wings,
which then underwent two types of
advanced imaging techniques: scanning
electron microscopy (SEM) and confocal
microscopy. The SEM technique involved
mounting sections of the moth wing to
adhesive carbon tabs, which were then
coated with a thin, 5-nm layer of gold.
The scales were imaged under highvacuum
and variable-pressure modes and
magnified to get a large, clear image. For
the confocal microscopy process, the team
immersed a single moth scale in glycerol
and sealed it between two microscopy
slides. Then they used autofluorescence to
get ultraclear images.
Once the clear, high-quality images of
the moth wings had been produced, the
team was able to extract 3D data from
the images into a 3D isosurface model,
which they saved in the STL format in the
MATLAB®
COMSOL Multiphysics®
software and imported into the
simulation software
using LiveLink™ for MATLAB®
. Using the
COMSOL Multiphysics model, the team
identified the ideal unit cell of the moth
wing scale and parameterized it to study
the effective material properties.
Next up, the team was ready to
perform their vibroacoustic analysis
" 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
structure. "
- ZHIYUAN SHEN, UNIVERSITY OF
BRISTOL
COMSOL NEWS 35a
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Tech Briefs Magazine - October 2021

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