Vibroacoustic Properties in Moth Wings - Pro Physik
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SA 4.0
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Lsadonke
Fig. 1 The Bunaea
alcinoe or cabbage
tree emperor moth
Vibroacoustic Properties in Moth Wings
Some special scaled wings exhibit acoustic camouflaging properties. Modelling this effect
helps to better understand the vibroacoustic phenomena at play.
Brianne Christopher
Have you ever looked at the
ground or watched a tree branch
or a leaf on a bush – and all of
H owever, this type of visual ca-
mouflage is a moot point when
trying to avoid one of the main pre-
For over 65 million years, bats
have sought out moths as a source
of food. Some moths can detect the
a sudden, it moves? Plenty of in- dators of insects: Bats do not see with signals of approaching bats while
sects and arachnids camouflage their eyes, but instead navigate and others defend themselves with
themselves from predators by search for food using echolocation. poison or clicking sounds that can
blending in with their surroun- So what is a bug to do? As it turns startle the bats enough to fly away.
dings. For instance, the orchid out, certain types of moths, like the The cabbage tree emperor moth is
mantis has wings that look just Bunaea alcinoe (Fig. 1), also called both deaf and nontoxic but it is not
like the delicate buds of an or- the cabbage tree emperor moth, ha- helpless. It simply relies on a more
chid flower; the Phasmatodea, ve scaled wings that provide acoustic passive defense strategy: acoustic
also known as the “stick bug”, camouflage, protecting them from camouflage or cloaking.
has arms and legs that bear an bats’ advanced sonar detection. But how do moths use acoustic
uncanny resemblance to little Researchers from the University of camouflage to fend off bat attacks?
brown twigs; and the Luna moth Bristol used numerical modelling to To find out, we can take a closer look
has fluorescent green wings that study this wing scale phenomenon at their wings. Moth wings are solid,
perfectly match bright leaves on and see how we can potentially apply thin membranes made up of chitin,
a tree. these acoustic camouflaging capabi- a long-chain polymer derived from
lities to other areas. glucose. Stiff wing veins hold these
4 Physics’ Best, April 2021 © 2021 Wiley-VCH GmbH
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Fig. 2 SEM images of B. alcinoe
scales (top row), confocal microscopy
of the scale (middle row) and the top
lamina (bottom row, left) and bottom
lamina (right) with longitudinal ridges
and cross-ribs
100 µm 2 µm 2 µm
membranes in place (Fig. 2). Look- ridges
ing even closer, the upper and low-
er surfaces of the moth wing are
covered in arrays of overlapping crossribs
scales, like the tiles on a roof. Each
scale is porous and of a complex
100 µm 6 µm 6 µm
structure. This highly sculptured
scale structure stems from sophis-
ticated evolutionary adaptations,
analogous to the highly organized
nanoscale photonic structures for
visual signaling. basal
These wing scales are less than
0.25 mm long, making them smal-
apical
ler than one tenth of the wavelength
that bats use for echolocation: the
frequencies of the signals range X+
from 11 to 212 kHz [1]. The Univer- Z+
sity of Bristol researchers hypothe-
sized that moth wings can be cate-
gorized as ultrathin absorbers with
subwavelength thickness, acting as
resonant absorbers. To investigate
their hypothesis, the group sought X–
to capture the governing physical Y+
Y– Z–
phenomena of the wing scales and y
z
x
show that moth scales can achieve 1 0.3 0.5
high absorption coefficients at re- 0
sonance. To do so, they turned to
numerical modelling.
Advanced Imaging Techniques
Meet Numerical Simulation
0 Unit [GPa] 0 0
The project started with a few moth
pupae which were cultivated in the
0.1 0.1 0.5
Fig. 3 The parametrized single unit
with different boundary conditions (top
row) and the simulation results of stress
distribution (middle row) and deformati-
on (bottom row), each in the single unit 0 0 0
under different boundary conditions
© 2021 Wiley-VCH GmbH Physics’ Best, April 2021 5
Software
lab until they reached maturity. produced, the team was able to ex- The team then modelled the scale
Researchers collected samples of tract 3D data from the images into vibrations at ultrasonic frequencies
the moth wings which then under- a 3D isosurface model. They saved using a macroscale FEM model in
went two types of advanced ima- this model in the STL format in the COMSOL Multiphysics® allowing
ging techniques: scanning electron MATLAB® software and imported them to calculate the vibration of
microscopy (SEM) and confocal it into the COMSOL Multiphysics® the scale. To understand how the
microscopy. The SEM technique simulation software with the Live- ultrasonic wave is coupled with the
involved mounting sections of Link™ for MATLAB®. Using the scale structure they had to consider
the moth wing to adhesive carbon COMSOL Multiphysics® model, both acoustics and solid mechanics.
tabs which were then coated with a the team identified the ideal unit The team also built two models
thin layer of gold with a thickness cell of the moth wing scale and pa- to analyze the damping effect of
of 5 nm. The scales were imaged rameterized it to study the effective the moth scale and the ultrasonic
under high-vacuum and variable- material properties (Fig. 3). properties of an entire moth wing
pressure modes and magnified to The next step consisted of per- made up of such scales. The former
get a large, clear image. For the con- forming a vibroacoustic analysis of consisted of a single scale with one
focal microscopy process, the team the scale. The team used the Peri- end fully clamped while the latter
immersed a single moth scale in odic boundary condition in COM- added Rayleigh damping to the ma-
glycerol and sealed it between two SOL Multiphysics® to model the terial and was used to calculate the
microscopy slides. Then they used single unit cell instead of an entire absorption coefficient of the scaled
autofluorescence to get ultraclear scale array which saved computati- array.
images. onal effort and memory. The perio-
Once the clear, high-quality dic boundary condition allowed ex-
images of the moth wings had been panding the structure into an array.
Calculations vs. Measurements
In order to compare the calculated
vibrations of the moth scale to their
real-world behaviour, the team used
20
a laser Doppler vibrometer (LDV)
to characterize the vibrational be-
10 haviour of the single scale (Fig. 4).
The LDV results showed good
agreement with the calculated re-
0 sonances for the first and third
modes: they differed only by 2.9 %
and 1.0 %, respectively. The calcula-
–10
ted resonances were 28.4, 65.2, and
153.1 kHz compared to LDV results
[pm] –20
of 27.6, 90.8, and 152.3 kHz (Fig. 5).
y
The simplified curvature of the
1 moth scale might explain the 28 %
0 deviation for the second mode. In
x
addition, the perforation rate of
0.5 the scale was modelled as constant
when it actually varies and inconsi-
stencies in the incident sound wave
0
during the LDV measurements
were neglected.
–0.5 The calculated modes of the
moth scale overlap and span the bio-
sonar range used by bats for echolo-
clamped edge [1] –1 cation (typically 20 – 180 kHz). To
crosscheck if this was just a coinci-
Fig. 4 Modelled and measured resonances of the moth scale: Scanning LDV results dence, the analysis was repeated for
of the first three resonances of the scale (top row) vs. simulation of mode shape of a a similar unit cell that mimics the
single scale with curvature radius of 700 μm (bottom row). structure of a butterfly wing scale.
6 Physics’ Best, April 2021 © 2021 Wiley-VCH GmbH
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While moths are nocturnal and of-
ten within bats’ crosshairs, butter- a b
12 1.0
flies are active during the day and
do not need protection. The modes 10 Displacement spectra
Measured results 0.8
Coefficient value in 1
at 88.4, 150.9, and 406.0 kHz fell Simulation results
Displacement in pm
8 Scaled wing
outside of the bats’ biosonar range 0.6 Reflection coefficient
6 Transmission coefficient
supporting the theory that moths Absorption coefficient
Wing membrane only
0.4
may have evolved to acoustically 4 Absorption coefficient
camouflage themselves from bats. 2 0.2
0 0.0
Acoustic Camouflage Reused 20 40 60
Frequency in kHz
80 20 40 60
Frequency in kHz
80
This research project marks the first
effort to characterize the biome- Fig. 5 Comparison between the modelled modes of a moth scale and the measured
chanics and vibrational behaviour modes using a laser Doppler vibrometer
of moth scales, both numerically
and experimentally. The results
demonstrate that multiphysics mo- include high-efficiency ultrasonic MATLAB® is a registered trademark
delling software helps to accurately acoustic absorbers. It would be a of The MathWorks, Inc.
capture moth scale behaviour, pa- big improvement for acoustic de-
ving the way for further simulation- sign if the thickness of a material
driven analysis. One future aim is to would only be one hundredth of the Author
expand the current periodic model wavelength it operates at.
into a full three-dimensional model In future, enhanced noise miti- Brianne Christopher, Senior Content
Manager, COMSOL, Inc.
of an array of moth scales. gation materials with acoustics ca-
The research is also applicable mouflaging capabilities might be
beyond the animal kingdom. Being used in building design and defense
able to understand the vibroacoustic technology.
behaviour of moth scales allows
developing macroscopic structures [1] G. Jones and M. Holderied, Proc. R.
Soc. B: Biological Sciences 274, 905
with the same acoustic camouflage
(2007), DOI: 10.1098/rspb.2006.0200
capabilities. Possible applications
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