Chromogenic film having a diffraction pattern similar to an opal

The present invention provides a high-luminance opal-like diffraction chromogenic film having no dead angle, which comprises a hexagonally close-packed grating monolayer film of micron-order particulates.

The file of this patent contains at least one drawing executed in color. 
Copies of this patent with color drawing(s) will be provided by the Patent 
and Trademark Office upon request and payment of the necessary fee. 
FIELD OF THE INVENTION 
The present invention relates to an opal-like diffraction chromogenic film. 
More particularly, the present invention relates to a novel diffraction 
chromogenic film which is useful as a material for esthetics and 
decoration purposes in the form of an opal-like diffraction chromogenic 
film with few dead angles, and a method for manufacturing same. 
PRIOR ART 
Various diffraction color developing methods using diffraction phenomenon 
have conventionally been known in decoration and esthetics areas. 
These conventional methods include, for example: 
(1) Diffraction color development utilizing a diffraction grating from 
mechanical linear streaks (holography, for example); 
(2) Diffraction color development based on linear and spot diffraction 
gratings from development of a photograph; 
(3) Diffraction color development through preparation of a high-accuracy 
diffraction grating by the LSI pattern preparation technology; 
(4) Weak diffraction color development based on preparation of 
convex-concave irregularities of a solid surface; and 
(5) Preparation of irregular gratings through application of an enameling 
agent containing pigment particles, and weak color development occurring 
along with such application. 
However, all these conventional methods had the following problems: 
(1) The diffraction light from mechanical linear streaks, while showing a 
high luminance, exhibits a strong diffraction phenomenon only in a 
direction at right angles to the line running direction, and contains dead 
angles. 
(2) Linear streaks and spot gratings based on photographic development 
permit preparation of high-accuracy diffraction gratings for a size larger 
than 1 .mu.m, whereas it is difficult to achieve a high luminance since 
the gratings are developed in a film. Because of the size limitation of 
photosensitive particles, furthermore, variations of lines and spots in 
size intrinsic to development occur at a diffraction grating period of 
under 1 .mu.m, resulting in a dim diffraction light. 
(3) LSI pattern technology, while permitting preparation of linear or spot 
diffraction gratings with a high resolution (0.3 .mu.m), is very 
expensive. 
(4) Irregular gratings (spots) obtained by granulating with sand paper or 
by etching give only a very weak diffraction light and lead to diffuse 
reflection. 
(5) When a solvent (enameling agent) containing pigment particulates is 
applied, these particulates produce irregular gratings and the resulting 
diffuse reflection leads to a very weak diffraction light. 
SUMMARY OF THE INVENTION 
The present invention has therefore an object to solve the defects in the 
conventional diffraction color development as described above, and to 
provide a high-luminance gratings which comprise a film composed of 
particulates and have few dead angles. 
As means to solve the above-mentioned problems, the present invention 
provides an opal-like diffraction chromogenic film which comprises a 
hexagonally close-packed grating monolayer film of micron-order 
particulates.

Now, the present invention is described in further detail by means of 
examples. 
EXAMPLES 
A. Color development of hexagonally close-packed grating monolayer film of 
micron-order particulates (monoparticulate film): 
Color development of the monoparticulate film based on wavelength-order 
particulates (polystyrene balls) and the corresponding texture are 
described below. FIG. 3A is a whole view of color development of the 
monoparticulate film. FIG. 3B is a photomicrograph as observed with an 
angle of no diffraction: there is no color development because polystyrene 
balls are colorless. FIG. 3C is a photomicrograph of the same object as 
viewed with a proper angle showing a CD diffraction in a dark field: clear 
diffraction color development is observed. The monoparticulate film is not 
totally monocrystalline, but is of a polycrystalline texture in which 
crystal orientations are slight different among them, thus resulting in 
colors dependent upon crystal orientation. In FIG. 3C, many streak defects 
are observed between monocrystalline regions. An enlarged view obtained 
through an electron microscope is shown in FIG. 3D. The interior within a 
monocrystalline region presents such a monolayer hexagonal close-packed 
grating. Observation of color development dependent upon the size of 
particulate, as made with the sunlight as the light source, permits 
confirmation that, when the particulate size is of the same order as the 
wavelengths of visible light, diffraction color development is dependent 
upon the particle size and the crystal orientation, as 0.953 .mu.m (FIG. 
4A), 1.083 .mu.m (FIG. 4B), and 2.106 .mu.m (FIG. 4C). 
FIGS. 5A-5C compare the color development of the wing of Morpho butterfly 
and that of a monoparticulate film. The processes of color development 
with the same incidence of a monoparticulate film of 0.953 .mu.m 
polystyrene balls and Morpho SULKOWSKI (Morpho (Cytheritis) stoffeli 
stoffeli Le Moults & Real) are shown with different expansions. FIG. 5A 
illustrates real images of the Morpho butterfly: the upper image is a 
natural one, and the lower image is a monoparticulate film comprising a 
chromogenic film but into the shape of butterfly wing. The artificial wing 
was prepared by enlarging an actual size one by a computer. Color 
development shown in FIG. 5A was taken by irradiating a light from above 
to below at an angle of incidence of 60.degree.. When the incident 
direction is turned by 90.degree. with the same angle of incidence, color 
development of Morpho butterfly disappears. On the other hand, color 
development of the artificial wing comprising the monoparticulate film 
showed almost no change, i.e., there was no dead angle in the color 
development by the diffraction light. FIG. 5B is an enlarged view of a 
wing of a Morpho butterfly and a monoparticulate film as observed through 
an optical microscope: a crystal region of almost the same order as 100 
.mu.m scale of butterfly is observed. FIG. 5C is an enlarged view observed 
through an electron microscope: the beautiful shine of the Morpho 
butterfly comes from very regular linear diffraction gratings (FIG. 5C, 
upper stage). On the other hand, FIG. 5C reveals that the artificial 
monoparticulate film also forms very regular spot-like diffraction 
gratings, with a period substantially equal to that of Morpho butterfly. 
It is thus suggested that the reflection of light from a surface having a 
high-regularity texture has a high-luminance diffraction light, and the 
extent of color development thereof depends upon the grating period. Color 
development of an artificial monoparticulate film is however different 
from that of Morpho butterfly in two points. The first point is due to the 
difference in texture between the two cases in a color development region 
of the order of 100 .mu.m, as is clear from FIG. 5B. While the scale of 
the Morpho butterfly has perfectly uniform orientations of texture, the 
monoparticulate film has slightly varying crystal orientations, thus 
resulting, not in a single color development, but in a color zone having a 
range around a specific color. The two cases are similar to each other in 
that a shining diffraction color development is available. The second 
point is that color development of the artificial particulate film has no 
dead angle, surpassing nature. 
B. Preparation of monoparticulate film: 
The present inventors have already proposed the principle of preparation. 
More specifically, this method of preparation is as shown in a schematic 
drawing of the apparatus for preparation given in FIG. 6. That is, a glass 
substrate previously subjected to a hydrophilization treatment is immersed 
in a suspension containing particulates, and pulled up by means of a small 
pulley rotated by a step motor. The speed is adjusted by altering the gear 
ratio. Growth of the monoparticulate film is constantly observed through a 
horizontal type microscope while irradiating a light onto the film growth 
section. An enlarged image is converted by a CCD camera into a TV image, 
and the process of growth is monitored in a real time manner. For a 
typical apparatus, the cell has dimensions of 100.times.40.times.10 mm, 
and the pulley has a diameter of 2 cm. The microscope has a resolution of 
0.35 .mu.m, and the pulling speed is variable between 0.1 .mu.m/sec and 10 
.mu.m/sec. 
A monoparticulate film was actually prepared as follows. 
&lt;a&gt; Particulates: polystyrene particulates (properties as shown in Table 1) 
and 1 .mu.m silica particulates 
TABLE 1 
______________________________________ 
Diameter Polydispersity 
Density 
(nm) (nm) (g/cm.sup.3) 
Refractive index 
______________________________________ 
2106 .+-.17 1.053 1.580 
1096 .+-.7 1.057 1.592 
1083 .+-.10 1.058 1.594 
953 .+-.9 1.057 1.594 
814 .+-.23 1.065 1.565 
506 .+-.10 1.054 1.595 
479 .+-.5 1.054 1.595 
309 .+-.4 1.054 1.595 
144 .+-.2 1.065 1.565 
79 .+-.2 1.065 1.592 
______________________________________ 
&lt;b&gt; Solvent: For polystyrene balls, pure water, or an aqueous solution 
containing the following ingredients: 
a) 0.001 mol/l sodium dodecyl sulfate (SDS) 
b) 0.001 mol/l octanol 
c) 0.01 mg/ml ferritin 
For silica balls, 2,2,2-trifluoroethanol 
&lt;c&gt; Glass and treatment thereof: Slide glass (76.times.26.times.1 mm) was 
subjected to a hydrophilization treatment in the following procedure: 
immersing it in a chromic acid solution for a whole day and night, rinsing 
it with water, then immersing it in 0.1M SDS or ethanol for an hour, 
rinsing it with water and drying same for the SDS-dipped one, or directly 
drying the ethanol-dipped one. In order to prepare a uniform 
monoparticulate film, it suffices to immerse a glass plate in a suspension 
of particulates and then pull it up slowly. Conditions for film forming in 
this case are given by the following formula: 
##EQU1## 
where, V.sub.c is the crystal growing rate when forming a uniform 
monoparticulate film (i.e., the pulling speed of the glass plate); .beta. 
is a hydrodynamic coefficient (about 1); j.sub.e is an evaporation rate of 
water at crystal section with a length, l; d is a diameter of particulate; 
and .phi. is a volumic ratio of particulates in suspension. The above 
Formula 1! is in a sense a formula for film formation under ideal 
conditions. Actually, therefore, the pulling speed is controlled with due 
regard to various uneven surface conditions such as differences in 
wettability, presence of dust, and variations of the particle diameter. 
The process of the growth of the monoparticulate film is constantly 
monitored with a view to optimizing the pulling speed while watching the 
process of film growth. An actual example of the growth process of a 
monoparticulate film in the middle of monitoring is shown in FIG. 7A and 
FIG. 7B. FIG. 7A suggests that 0.814 .mu.m polystyrene particulates are 
forming a monoparticulate film comprising small regions having different 
orientations. FIG. 7B shows a monocrystal region formed with 0.953 .mu.m 
polystyrene particulates: a flow of particles running toward a crystalline 
film. Because of a high particle speed of 100 .mu.m/s, the particles in 
travel look obscure. Bright particles indicated by an arrow are admixed 
particles having a sufficiently large diameter of at least 0.953 .mu.m, 
which are at a standstill, since they are strongly pressed against the 
plate by a surface wetted with water. When using 1 .mu.m silica 
particulates, particles cannot be kept dispersed in a water suspension for 
a long period of time, because silica has a specific gravity of 1.4 to 
1.44, before start of precipitation. It is therefore impossible to form a 
satisfactory film through advection and accumulation of particles. By 
using trifluoroethanol as a solvent having a specific gravity larger than 
water, the large specific gravity permits satisfactory suspension of 
silica particles. FIG. 8A and FIG. 8B are enlarged photographs showing the 
process of growth of a silica monoparticulate film and formation of 
gratings. In the photomicrograph taken through a dark-field microscope 
shown in FIG. 8A, the black portion is a bare glass surface; the green 
portion is a silica monoparticulate film; and the yellow portion is a film 
wetted with silica suspension. These monitor photographs suggest that the 
principle of formation of a monoparticulate film is based on advection and 
accumulation of particles due to evaporation of trifluoroethanol and 
packing brought about by interparticle (lateral capillary) force in a 
liquid thin film, as in the case of polystyrene particulates as described 
above. The hexagonal close-packed film shown in the enlarged photograph of 
FIG. 8B suggests that the interparticle packing force exerts a strong 
effect. 
C. Adhesive stamp lithography: 
As a monoparticulate film is formed by the utilization of the flow of 
particles, it is assumed that particles in a suspension have originally a 
low adsorbing property relative to a substrate. After drying, therefore, 
the monoparticulate film can easily be stripped off from the substrate. By 
using this fact, it is possible to conduct an easy mechanical lithography. 
A stamp bearing a picture drawn with an adhesive (prepared by affixing an 
adhesive tape cut along the picture onto a flat glass surface) is 
prepared, pressed against the monoparticulate film, and particles are 
stripped off by adhesion. Then, a female picture remains on the 
monoparticulate film surface, and a male picture, on the stamp side while 
keeping the texture of the particulate film. This is the adhesive stamp 
lithography using the monoparticulate film. A two-side adhesive tape was 
affixed to a glass plate and thick letters LVMH were cut therefrom. With 
this as a stamp, a picture LVMH was formed by removing particles onto the 
monoparticulate film. FIG. 9A represents a female lithograph of a 0.953 
.mu.m monoparticulate film. FIG. 9B is a male lithograph of a 1.083 82 m 
monoparticulate film. FIG. 9C is a male lithograph of a 0.479 .mu.m 
monoparticulate film. FIG. 9D is an enlarged photograph of a character M 
shown in FIG. 9A, which shows clarity of the boundary of lithography. 
D. Intensification of color development and fixing of monoparticulate film: 
Fixing of a monoparticulate film and intensification of color development 
after preparation of a lithograph were found to be easily achievable by 
evaporation of silver or gold into a thickness of about 10 nm. This metal 
evaporation increases the refractive index of the particulate film and 
intensifies color development. Particles are covered with metal films by 
metal evaporation, thereby causing strong adherence with the substrate, 
and adhering strength is increased to an extent that the above-mentioned 
adhesive tape does not peel off. The adhering mechanism of particles is 
considered to be such that, in addition to the effect of metal film 
covering, direct adhesion of particles and the substrate is induced by 
local heating to a high temperature during evaporation. 
According to the present invention, as described above in detail, a 
high-luminance opal-like diffraction chromogenic film free from a dead 
angle is achieved. 
Also according to the present invention, it is possible to form a film 
through highly controlled advection and accumulation of a particulate 
monolayer film and to accomplish lithography, fixing and color development 
intensification.