Biomedical Engineering Reference
In-Depth Information
the inverse opal,
a
, according to
D
va
(
v
: a function of the refractive index of the
material) [
152
-
158
].
Furthermore, multifunctional silk fabrics may also be obtained by controlling the
band gaps of the photonic crystals. For example, one can acquire the UV protective
clothing by creating photonic crystals or inverse capable of reflecting UV light. On
one hand, the thermal insulating performance due to a reflection peak in the IR range
will create cooling textiles in a hot summer. On the other hand, it will preserve to
some extent our body heat in a cold winter if the IR reflecting structure is generated
in the inner layer of textiles.
7.4.3.2
Double Reflection of Structural Color by Surface Structural
Engineering
Although great accomplishment has been made, the structural colors produced by
the animal kingdom are much richer and more effective than what we can produce
so far. Furthermore, it is especially difficult to mimic some unique optical properties
of natural structural color, such as polarization, colors mixing, etc. In the following,
we will describe the double reflection and polarization effect produced by
Papilio
butterfly and a method utilizing a combined colloidal crystal and surface deposition
technique to mimic these effects.
Apart from the iridescent visibility, nature structural color produced by some
animals also gives rise to additional unique optical properties and benefits. For
instance, the brown barbules in male peacock tail feathers adopt mixed structural
coloration [
140
], Some breeds of
Papilio
butterflies reveal the double reflection
effect [
141
,
172
].
The color mixing mentioned above is created by in nature in different ways
[
173
]. Figure
7.16
a depicts the double reflection and iridescent visibility created
by the blue wing scales of
Papilio ulysses
. The surface of its wings is composed of
millions of scales. The scales of
P. ulysses
are of a size around 150
m
and consist of a fairly regular array of concavities. Under optical microscope, the
concavities reveal a green reflection light (Fig.
7.11
a(i)). When illuminated and
observed at normal incident light, the concavities in
P. ulysses
appear to be green
(Fig.
7.16
a(i,ii)). However, upon crossing an input linear polarizer with an exit
analyzer, the green reflected light in
P. ulysses
almost disappear while the deep
purple (near-UV) color reflected by ridges in
P. ulysses
reflect back (Fig.
7.16
a(iii)
& (iv)). This implies that the purple reflected light is not altered by the polarizers.
By further characterization of the microstructures of the wing scales, it was found
that the profile of the concavities was almost flat, the ridges run through the full
length of the scales with a periodicity of 4-5
m
90
m(Fig.
7.16
a(iv)). The configuration
of the ridges is a 2D array of 70 nm
100 nm rectangular air squares surrounded
by organic cuticle layers (the main and sub-ribs) with a periodicity of
140 nm
(
D
1
C
d
2
) along the main ribs. This
long-range ordered structure with a very small periodicity can be considered to be
a 2D photonic crystal slab tilt about 30
o
d
1
) along its length direction and 160 nm (
D
2
C
with respect to the surface of the scales.
