Biomedical Engineering Reference
In-Depth Information
in the fully wet state. From cross-sectional ESEM images we can determine that the
thickness of the homogeneous layers in the fully wet state swells to about 120 nm,
while the thickness of the inhomogeneous layers remains nearly unchanged. The
swelling of the period of the multilayer can cause a color change from golden in
the dry state to red in the wet state. Furthermore, it could be proved from calculated
reflection spectra that the air voids in the wet state are filled with water, since the
intensity of reflection in the wet state is much smaller than that in the dry state.
In the biological world, many animals can change their coloration in response to
environmental stimuli. The adaptive values of color changes are usually regarded
as camouflage, signal communication, conspecific recognition, and reproductive
behavior. In principle, color changes can occur via the change in pigment,
microstructure, or their combination. Although the precise functionality of the
tunable structural coloration in the beetle
T. isabellae
is not clear, the reversible
structural color change may certainly render more options for both adaption and
function. Owing to its unique properties, tunable structural colors could open new
avenue to the applications in display and imaging technology, printing and painting,
textile industry, sensing, and photonic devices [
54
,
55
].
In butterflies, there exist also structures closely similar to multilayers which
give rise to structural colors. The most significant example may be the
Morpho
butterflies, one of the most representative animals that display brilliant structural
colors. In most of the
Morpho
butterflies, the dorsal wings of the male display a
brilliant blue color, while their ventral side is usually dark brown in color. Along
the direction perpendicular to the wing veins, the blue color shows a weak viewing-
angle dependence. Only at large oblique viewing angles the blue color will change
rather abruptly to dark blue or violet. Along the direction of the wing veins,
however, the blue color will disappear abruptly with increasing viewing angles. This
fascinating iridescence in the
Morpho
butterflies has drawn considerable attention
of scientists [
10
,
11
,
16
,
20
,
56
-
65
].
The wings of the
Morpho
butterflies are covered with rows of partially over-
lapping scales. Various forms of scales are present depending on the species
and the wing positions. In the blue iridescent region of the wings, there exist
usually two kinds of scales: cover scales (glass scales) and ground scales (basal
scales) that lie beneath cover scales. Typical ground scales are rectangular and
their dimensions are of the order of 0.1
0.2 mm
2
. Figure
8.14
shows a
Morpho
butterfly,
Morpho rhetenor
, its scales, and scale microstructures [
60
]. The wings of
M. rhetenor
have one layer of highly iridescent ground scales on its dorsal wing
surface. The top surface of the ground scales exhibits periodic rows of ridges that
extend longitudinally from one end of the scale to the other. Each ridge contains
a bookshelf-like multilayer, responsible for the blue structural color. Structural
coloration in the wings is determined by many factors such as the geometrical
structure of the discrete multilayer and the arrangement of the ridges. The discrete
multilayer can produce interference while the ridges and the discrete multilayer
itself can give rise to diffraction. As a result, the blue structural color in the
Morpho
butterflies stems from the interplay of interference and diffraction. The parallel
ridges can offer a strong anisotropic reflection, while the irregularity in the ridge
