Photonic crystal-based strain visualization film is promising for detecting the age-related deterioration of large man-made structures and public infrastructures. However, as the number of target structures increases, monitoring them all will become a major problem. We propose two solutions: (1) a portable solar-battery-powered automated monitoring station to monitor the color of photonic coatings, and (2) the application of real-time image analysis using mobile phones to record color changes. Both solutions make use of the power of small computers, while the former assists us with efficient data collection, and the latter helps non-experts to inspect structures without using expensive spectroscopes.
The portable monitoring station consists of a micro-computer connected to a 3G mobile network, a USB camera, and a solar battery system installed in a waterproof box. Photographs of the strain visualization film are taken once every hour and, at all other times, the computer disconnects the camera to save electricity. We placed four monitoring stations in the shade of a bridge or a tree and ran them continuously for more than a year.
The application displays a real-time image in which only the strain-free area of the film is extracted. As a result, the region under strain and the background appear in white. This software runs on many mobile computers with built-in cameras and the OSs including Android, iOS, Windows, and Linux. This is possible due to the versatility of the computer vision library we used, namely OpenCV, which is widely used in robotics and automatic car-driving.
We have been developing smart photonic coating for structural materials to visualize strain mapping on steel or aluminum and to detect cracks on concrete. In the technology, strain imaging sheet made of colloidal photonic crystal film coated on a polyethylene terephthalate sheet. The strain imaging sheets change structural color by mechanical deformation. Now we have been testing smart photonic coating for real concreate bridges One of the key issues is the durability of the sheet for long-term use at the outfield site. In outdoor exposure test and in laboratory accelerated exposure test, polystyrene particles in the colloidal photonic crystal film were damaged and lost the structural color. To protect the deteriorating, top coat layer containing ultraviolet absorber was effective to reduce the damage of the colloidal photonic crystal film.
Here we will propose the conceptual new idea of the inspection of concrete bridge using smart materials and mobile IoT system. We apply opal photonic crystal film to detect cracks on concrete infrastructures. High quality opal photonic crystal films were coated on black color PET sheet over 1000 cm2 area. The opal film sheet was cut and adhered to concrete or mortar test pieces by epoxy resin. In the tensile test, the structural color of the opal sheet was changed when the crack was formed. As a demonstration, we have installated the opal film sheet on the wall of the concrete bridge. Our final purpose is the color change will be recorded by portable CCD devices, and send to expert via IoT network.
We will present a simple and low cost method to visualize local strain distribution in deformed aluminum plates. In this
study, aluminum plates were coated with opal photonic crystal film with tunable structural color. The photonic crystal
films consist of a silicone elastomer that contains an array of submicron polystyrene colloidal particles. When the
aluminum sheets were stretched, the change in the spacing of the colloidal particles in the opal film alters the color of the
film. This approach could be useful as a new strain gauge having a visual indicator to detect mechanical deformation.
We report the application of a photonic crystal sensor for the components of a liquid mixture by a simple and easy
technique. A closely packed colloidal crystal film is made of arrayed 202-nm PS particles and PDMS elastomer is used
for filling the voids in the film. Bragg's diffraction in the visible wavelength region causes the formation of a structural
color in the colloidal crystal. The structural color varies according to the solvents depending on the swelling ability of the
PDMS elastomer. We can perform the quantitative analysis of the swelling phenomena by measuring Bragg's diffraction
peaks. The peak shifts as a function of the mixing ratio of the solvents, i.e., methanol, ethanol, and propanol. The peak
position is proportional to the solvent concentration. In the case of a water-ethanol system, there is little peak-shift up to
80 vol.% of ethanol concentration. Above this concentration, the rate of peak shift increases beyond 50 nm. A reflectance
spectrometer enables the detection of the components of ethanol liquids with the volume concentration of water ranging
from 0 to 10%. In this study, a commercially available optical fiber spectrometer detects the volume concentrations of
water of the order of 1 vol.% in ethanol. The photonic sensor has the potential to be used in a rapid analysis method that
employs a portable optical fiber spectrometer.
This paper describes an elastic colloidal crystal, soft opal, with tunable stop band and its potential application for mechanical strain sensing. A poly-dimethylsiloxane, PDMS, rubber sheet was coated with a thin layer of the soft opal. In the layer, polystyrene, PS, submicron particles were cubic-closely packed, CCP, and among the particles filled with PDMS elastomer. We design the array of CCP (111) planes selectively diffracts light in visible wavelength. The wavelength, i.e. photonic stop band, was reversibly tuned by tensile strain. As the PDMS sheet was stretched in horizontal direction, the film becomes compress in vertical direction. As a result, the lattice distance of CCP (111) planes decreased and the stop band shifted to shorter wavelengths. As released the mechanical strain on the PDMS sheet, the stop band completely returned to the initial wavelength. The PDMS sheet also displays reversible change of the structural color during elastic deformation. The potential application is a simple stress sensor enables visual judgment of the intensity of tensioning.
A new class of colloidal crystals, whose structural color can be tuned by changing lattice constants, was fabricated. They were composed of polystyrene (PS) submicron particles embedded in a silicone elastomer. The particles were self-assembled into a cubic close packing (ccp) structure, and the ccp (111) planes were parallel to the substrate. These ccp (111) planes produced the structural color of the crystal film by the Bragg's diffraction of incident light. The center-to-center distance between the planes, d was tuned by two approaches. One of the approaches involves the tuning of structural color by increasing d. A composite film fabricated with 202-nm PS particles exhibited a green color; its color changed to red due to the swelling of PDMS elastomer in solvents. On the other hand, a composite film coated on an elastic rubber sheet was stretched horizontally. Therefore, the lattice distance of ccp (111) planes decreased and the wavelength of reflected light reduced as a function of sheet elongation. In contrast, the structural color of the elastic rubber sheet was changed by applying mechanical stress. By tuning the structural color, composite films can be used for optical sensing, thereby avoiding special detector equipment.
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