Implicit induced force – significant color change art anti-counterfeiting technology upgrade
In nature, some animals are stimulated can change their color to hide from enemies and predators. This feature is mainly due to the animal body surface microstructure / nanostructure changes in response to external stimuli. Inspired by the researchers of this natural phenomenon, has developed a variety of materials may be (e.g., mechanical force, biomechanics actuation temperature, the chemical composition, electrical and magnetic) flexibly color changes according to an external stimulus. This unique structure means responsive coloring technical aspects may be well suited for sensors, robotics, smart windows and anti-counterfeiting. So far, most of the coloring material having the structure characteristics able to actively flexibly display various colors in external stimuli. However, achieve implicit – explicit color conversion behavior, especially in switching between a transparent state and a colored state, the more challenging and attractive. Based on this, Korea Ulsan University of Science and Technology Kyungjun Song and Taesung Kim common reported A printing by the inkjet patterning rigid SiO2(NP) and a flexible (PDMS) composite device thereof. Because of the similarity SiO2 NPs PDMS and optical characteristics of the device in the normal state is completely transparent. However, since the mechanical strength of the big difference between the two, under a compressive load, flexion instability generating device surface, generating a wrinkle or two dimensional array pattern. These then serve as a multi-dimensional array of grating, the display color can be switched according to the angle of observation on various structures.
SiO 2 -NPs / Construction
PDMS Hybrid Devices First, SiO2 NPs as an ink, a distance on a glass substrate 150 um patterned ink-jet printing. Each drop of ink after evaporation of a coffee ring formed nanostructure (FIG. 1) on a glass substrate. The PDMS is then poured into these nanostructures to patterned SiO2 NPs package to be peeled off after curing PDMS slab. Since PDMS and SiO2 NPs having very similar refractive indices (i.e. nSiO2 = 1.475, nPDMS = 1.43), SiO2-NP / PDMS composite device completely transparent appearance, even on a blue background and white print pattern can not appear. Further, SiO2-NP / PDMS composite device having a high degree of flexibility, allowing the external device is applied to various mechanical stimuli.
complex device structure discoloration mechanism under different external stimulus
SiO2-NPs / PDMS composite device in a normal state is completely transparent (FIG. 2a), on the one hand because the SiO2 and PDMS index value reflecting similarity. Partly because coffee periodic wrinkles around the ring patterns are completely absent in the normal state, so the device does not produce the grating structure, it will not interfere with the light. However, when the device is bent inwardly \”UNiST\” color image (FIG. 2b) logo will appear. This is because the inwardly compressive force due to bending induced by SiO2-NPs / PDMS layered composite surface along the y axis to generate a periodic wrinkles array which showed a one-dimensional diffraction grating, it is decomposed into spectral white light of different colors ( Fig. 2e). Thus, when the device is bent, the transparent SiO2-NPs / PDMS composite device exhibits iridescent color depending on the viewing angle. In contrast, when the device is bent in the opposite direction (outwardly), any color is not displayed (FIG. 2c). This may be due to the SiO2-NPs / PDMS layered composite surface with no wrinkles in the opposite array of external stimulation. Analysis found by thin, hard film on a soft thick substrate / surface layer (PDMS) / layers (SiO2-NPs) under compressive stress can assume a corrugated (FIG. 2d), forming a cyclical wrinkles / pattern array. Structure coloring mechanism
Hidden – Application of significant discoloration mechanism
the inward bending when the SiO2-NPs / PDMS composite device rotating 90 °, iridescent magically disappear (Figure 3a). This is because when bending along the y-axis as a wrinkle-dimensional structure (inwardly). Grating structure after the rotary diffract incident light beam into several different directions, Which is closely related to the beam spacing and the wavelength of light. If the viewing angle can be positioned no light beams may pass through one or more space / area outside, can lead to the observation of the transparency. This feature allows the device to display only a color within a specific angular range, thereby enhancing the confidentiality of information, in order to meet the needs of a particular situation. But when pressure is applied isotropic (2D) to the composite device, rotating 90 ° the color is still present (FIG. 3B), because it is generated from the one-dimensional parallel fold becomes a two-dimensional array structure divergent structure array. Based on this, the application of technology developed two covert anti-counterfeiting technology, under compression in different directions of bending, may exhibit printed smiley face sides (FIG. 3c), respectively; may be selectively hidden, transparent two-dimensional code pattern into distinct two-dimensional code pattern, and decodes the two-dimensional code by an ordinary mobile phone, directly to the predetermined site (FIG. 3d).
This link: https: //onlinelibrary.wiley.com/doi/10.1002/adma.202001467