Study On The Responsive Inverse Opal Photonic Crystals Based On Wettability

Responsive photonic crystals（RPCs）are widely used in information coding,smart display,biomedicine,chemical sensing,and soft actuators due to the stimulus-tunability of photonic bandgap.Although the regulation of liquid wetting on both photonic bandgap and light manipulation of RPCs have been widely reported,there are still some problems that need to be solved,such as reversible switching between single and double bandgap in inverse opal photonic crystals,light manipulation in the direction of the light source,and the dewetting behavior of bubbles on the photonic crystal in the gas-liquid system.In this paper,silica（SiO₂）opal templates were used to prepare wettability-controlled inverse opal photonic crystals on the front or front&back sides of the film by photocuring different kinds of acrylates and hydrofluoricacidetch.The above problems were investigated by using inverse opal or bilayer inverse opal photonic crystals via wetting response.Details are as follows:For the regulation of the photonic bandgap,three methods were proposed for regulating single-double bandgap.Bilayer inverse opal photonic crystals（PEPA film）with"Variable Layer"on the front side and"Fixed Layer"on the back side were prepared by using ethoxylated trimethylpropane triacrylate（ETPTA）,polyethylene glycol（200）diacrylate（PEGDA）and acrylic acid（AA）as filler materials.Based on the scattering of inverse opal,two methods of regulating single-double bandgap are established:1)Overturning the bilayer film,when the thickness of the"Variable Layer"reaches～10.26μm,the spectra show a single bandgap when observed from the"Variable Layer",and a double bandgap when observed from the"Fixed Layer".2)Liquid wetting response,using the wetting threshold（liquid contact angle equal to neck angle,～30°）,the front and back sides of the bilayer system were wetted with ethanol,respectively.Due to the refractive index matching,the scattering of the corresponding inverse opal could be removed,and the reversible switch between single and double bandgap and the reversible switch between front and back bandgap could be realized.The shape memory bilayer inverse opal photonic crystals（PEPAB film）were fabricated by using ETPTA,PEGDA,AA,and butyl methacrylate（BMA）as filler materials.The third method of regulating single-double bandgap was proposed.The pressure-induced deformation of inverse opal pores was used to increase the threshold to～73°,and the scattering can be relieved by water response.The switch from single bandgap to double bandgap and the conversion from monochromatic output to multicolor output were realized,which can be applied in the fields of security and information coding.For the light manipulation,inverse opal photonic crystal photo-switches（PEPAP film） were constructed by using ETPTA,PEGDA,AA,and tetra（3-mercaptopropionate）pentaerythritol ester（PETMP）as filler materials.The maximum diffraction wavelengths of the inverse opals with apertures of～284,～350,～395,and～441 nm are～440（blue）,～525（green）,～595（yellow）,and～635 nm（red）under the incident angle of 57.5°and detection angle of 47.5°,respectively,showing angle dependence.The diffracted light can be turned on and off periodically with a period of 60°as it rotates along the film plane.Combined with the scattering of the inverse opal,a bilayer photonic crystal photo-switch with different thicknesses of inverse opal was constructed.The rotational reproduction period of structure color is 30°/60°when observed from thinner/thicker layer.With the adjustment of p H to the wettability,the invisible pattern can be encoded and erased.After water immersion,the diffraction intensity can be weakened by～2/3,showing the pattern with the performance of rotating periodic display and hiding.A smart traffic warning board（PEPB film）was prepared by combining out-of-plane diffraction and wettability-controlled specular reflection.The output of the traffic signal was simulated.A warning module for landslide risk on rainy nights is established.The flickering speed limit board with discoloration was designed by using the periodic reproducibility and angle dependence of the diffraction.For the bubble dewetting on the surface of photonic crystal,using ETPTA,PEGDA,BMA,hydroxyethyl acrylate（HEA）,and active manganese dioxide（Mn O₂）as filler materials,the structural color sensor（PEPBH film）was prepared by loading Mn O₂ on the lower surface of inverse opal.Using oxygen generated by hydrogen peroxide decomposition catalyzed by Mn O₂ as the source of bubbles,the gas-liquid exchange in macroporous materials was revealed by structural color,and the bubble evolution was monitored in real-time.Due to the change of effective refractive index of inverse opal resulting from the gas-liquid exchange,the shift of the maximum reflection wavelength can reach～50 nm.Pressure-induced deformation of the inverse opal pores can enhance surface wettability,promote gas-liquid exchange,and increase the color contrast.The shift of the maximum reflection corresponding to～75 k Pa pressure reaches～65 nm.The deformed inverse opal pores can restore their morphology after ethanol wetting,showing recyclability.The corresponding 2D photonic crystals can be obtained by scraping the inverse opals.On the surface of the 2D photonic crystal,the bubble contact angle decreases from～150.6°to～141°,the adhesion increases from～142μN to～423μN,and the average bubble diameter increases from 120μm to 630μm after pressure-induced surface deformation,which can be used for bubble manipulation in liquid.The above properties can be further extended and applied to Mn O₂-bilayer inverse opal photonic crystal composite films.