Basic research on soft opal crystal films and control of their structural color

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Soft opal crystal is a colloidal crystal with flexible and soft mechanical property. The soft opal crystal consists of a composite material ordered array of mono-dispersed particles and among them infilling with elastic polymer. Here we report two results using thin film of soft opal crystal. 1. Swelling phenomena and basic research: A new easy and simple method to determine solubility parameter of unknown elastomer. We measured peak shift of Bragg's diffraction, △λ, at swollen conditions using about twenty solvents with given the solubility parameter, σ. Then the relationship between σ and △λ was plotted and gave us the maximum of △λ at σ max. This σ max is estimate the solubility parameter of the unknown elastomer. On reference data, we investigated the solubility parameter of same elastomer using the elongation, L/L0 of an elastomer block. The solubility parameter of elastomer is corresponding to σ at the maximum (L/L0). Both values are also almost same value. The measurement of △λ us in a few seconds. The new method is simple and easy to determine unknown of σ the elastomer. 2. Tunable structural color by elastic deformation: soft opal crystal coated on surface of rubber sheet. The particle array in colloidal crystal is changed by strain. This new material named "Photonic Rubber sheet" reversibly change its structural color from red to green by stretch-out. The relationship between strain and peak shifting enable apply for mechanical sensing. As distinguished result, the photonic rubber sheet was picked up in a textbook of Physics (Fundamental of Physics, 8th Ed., Wiley, 2007). Soft opal crystal is a good example due to Bragg's diffraction in visible wavelength and change of structural color.