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Broadband and pixelated camouflage in inflating chiral nematic liquid crystalline elastomers

Nature Materials volume 21, pages 41–46 (2022)Cite this article

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An Author Correction to this article was published on 27 September 2021

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Abstract

Living organisms such as fishes1, cephalopods2 and clams3 are cryptically coloured with a wide range of hues and patterns for camouflage, signalling or energy regulation. Despite extensive efforts to create colour-changing materials and devices4, it is challenging to achieve pixelated structural coloration with broadband spectral shifts in a compact space. Here, we describe pneumatically inflating thin membranes of main-chain chiral nematic liquid crystalline elastomers that have such properties. By taking advantage of the large elasticity anisotropy and Poisson’s ratio (>0.5) of these materials, we geometrically program the size and the layout of the encapsulated air channels to achieve colour shifting from near-infrared to ultraviolet wavelengths with less than 20% equi-biaxial transverse strain. Each channel can be individually controlled as a colour ‘pixel’ to match with surroundings, whether periodically or irregularly patterned. These soft materials may find uses in distinct applications such as cryptography, adaptive optics and soft robotics.

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Fig. 1: Programmable structural coloration of MCLCE membranes based on Poisson effect.
Fig. 2: Morphology of the stabilized MCLCE prepolymers and the MCLCE membrane.
Fig. 3: Elasticity anisotropy and enhanced coloration of the MCLCE membrane (M30,8).
Fig. 4: Pixelated coloration for displays and camouflages.

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Data availability

The authors declare that the data supporting the findings of this study are available within the text, including the Methods and the Extended Data. Source data are provided with this paper.

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Acknowledgements

We acknowledge donors of the American Chemical Society’s Petroleum Research Fund (no. 573238, S.Y.) and National Science Foundation through the University of Pennsylvania Materials Research Science and Engineering Center (DMR-1720530, S.Y.) for partial support of this research. We acknowledge use of SEM instruments and the Dual Source and Environmental X-ray Scattering facility supported by the National Science Foundation and Materials Research Science and Engineering Center (DMR-1720530) at the University of Pennsylvania and discussions with R. D. Kamien. The purchase of the Dual Source and Environmental X-ray Scattering facility was made possible by a National Science Foundation Major Research Instrumentation (MRI) grant (17-25969), an Army Research Office (ARO) Defense University Research Instrumentation Program (DURIP) grant (W911NF-17-1-0282) and the University of Pennsylvania.

Author information

Authors and Affiliations

  1. Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA, USA

    Se-Um Kim, Young-Joo Lee, Jiaqi Liu, Dae Seok Kim, Haihuan Wang & Shu Yang

  2. Department of Polymer Engineering, Pukyong National University, Busan, South Korea

    Dae Seok Kim

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Contributions

S.-U.K. and S.Y. conceived the research ideas; S.-U.K. and J.L. developed and prepared materials; S.-U.K., J.L., D.S.K. and H.W. conducted optical and X-ray characterizations; S.-U.K. and Y.-J.L. conducted the mechanical characterization; S.-U.K. and Y.-J.L. developed and characterized the coloration platforms; S.-U.K. and Y.-J.L. conducted the numerical modelling and the finite-element analysis; S.-U.K. and S.Y. wrote the manuscript; and S.Y. supervised the research. All authors read and commented on the manuscript.

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Correspondence to Shu Yang.

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The authors declare no competing interests.

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Peer review information Nature Materials thanks Jan Lagerwall, Oleg Lavrentovich and Yukikazu Takeoka for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Shrinkage of the MCLCE membrane after removal of the chiral nematic solvent.

(a) Transmittance spectra of the stabilized MCLCE prepolymers (M21,9, M33,8, and M50,10) at Stages I (before photo-polymerization) and II (after photo-polymerization and removal of the chiral nematic solvent). The anisotropic shrinkage of the MCLCE membrane in thickness results in the blue shift of the transmission spectra. (b) Photographs showing the sample appearances (M30,7) at Stages I and II, respectively. The reflection bandwidth is initially in the NIR range at Stage I and shifts to the visible range after removal of the chiral nematic solvent at Stage II. (c) The magnitude of the spectral shift as a function of the mass loading of the prepolymer (α). Here, λI and λII are the central wavelength of the reflection at Stages I and II, respectively.

Source data

Extended Data Fig. 2 Active structural coloration of the MCLCE membrane under the application of equi-biaxial transverse strains.

(a) Photographs of a MCLCE membrane produced from M30,7 under equi-biaxial transverse strains (εt = 0, 0.067, 0.123). The membrane was mounted on a custom-built biaxial strain gauge. (b) Reflectance spectra of a MCLCE membrane produced from M30,6 under equi-biaxial transverse strains (εt = 0, 0.029, 0.051, 0.074). (c-d) Transmittance spectra of a MCLCE membrane produced from M30,8 under equi-biaxial transverse strains (εt = 0, 0.025, 0.036) for left-handed (c) and right-handed (d) circular polarized incident light.

Source data

Extended Data Fig. 3 Strain distribution of the inflated supporting layer in coloration units.

(a-b) (a) The distribution of the maximum principal strain of the supporting layer, (a) aspect ratio (t/w) = 0.05 and (b) (t/w) = 0.25. (c) Principal strain as a function of 1/r at several t/w (0.001, 0.01, 0.02, and 0.25). Black and orange dashed lines represent the fitting of Eq. 1 and t/(2r), respectively. In (a-c), the width w is fixed as 2 mm. In (b), the supporting layer is compressed by the inflating pressure so that the strain distribution follows the thin-film bending model.

Source data

Extended Data Fig. 4 The trend of the strain distribution at the inflated supporting layer depending on the aspect ratio.

(a-b) The principal strain as a function of (a) the geometric coefficient w/(2r) and (b) the pressure. The prinical strain at the center of the top surface were examined for several aspect ratios via varing the thickness (t = 100 μm, 200 μm, 500 μm, and 1000 μm) and the width (w = 2.0 mm, 2.8 mm, 3.6 mm, and 5.0 mm). Labels represent the aspect ratios (t/w). Here, r denotes the bending radius measured at the middle plane in the thickness direction of the inflated supporting layer. In (b), when the supporting layer inflated above the critical deflection length (=w/2), the sign of the second derivatives (d2ε/dp2) is changed as shown in the inset for t/w = 0.02 (t = 100 μm and w = 5.0 mm).

Source data

Extended Data Fig. 5 The Layouts of the proposed coloration platforms.

(a) Three pixels with t/w = 0.04 (bottom), 0.06 (middle), and 0.1 (top) (w = 7.5, 5, and 3 mm at t = 0.3 mm, respectively), sharing the same air channel I shown in Fig. 4c and Supplementary Video 2. (b) Three pixels with t/w = 0.06 (bottom), 0.075 (middle), and 0.1 (top) (w = 5, 4, and 3 mm at t = 0.3 mm, respectively), sharing the same air channel I shown in Fig. 4d and Supplementary Video 3. (c) 28 pixels with t/w = 0.1 (w = 2 mm and t = 0.2 mm) and every 4 of pixels have shared channels (I1 ~ I7), which is shown at Fig. 4e and the Supplementary Video 4. (d) 7 × 5 pixels with t/w = 0.1 (w = 2 mm and t = 0.2 mm) and the alternating pixels are linked to two air channels (I1 and I2), which is shown in Fig. 4 f and Supplementary Video 5. (e) Several irregular pixels with variable t/w (0.04–0.1 for the left panel and 0.03–0.125 for the right panel) (t = 0.2 mm) that have a shard air channel I as shown in Fig. 4 g (left) and Supplementary Video 6.

Extended Data Fig. 6 Actuating sequence of the air channels to display 1-digit numbers in the seven-segment display.

In this coloration platform, 7 air channels (I1 ~ I7) are asynchronously manipulated by using independent air sources to display 1-digit numbers (1 by I1 and I5, 2 by I1, I2, I4, I6, and I7, 3 by I1, I2, I4, I5, and I6, 4 by I1, I3, I4, and I5, 5 by I2 ~ I6, 6 by I2 ~ I7, 7 by I1, I2, I3, and I5, 8 by I1 ~ I7, 9 by I1 ~ I6, and 0 by I1, I2, I3, I5, I6, and I7, respectively). The green-coloured pixels and air channels are in the actuated state and the others with white-coloured are in the un-actuated state.

Supplementary information

Supplementary Information

Supplementary Figs. 1–4.

Supplementary Data 1

Statistical source data for Supplementary Fig. 1.

Supplementary Data 2

Statistical source data for Supplementary Fig. 4.

Supplementary Video 1

Preliminary deformation of the MCLCE/PDMS bilayer. The thin MCLCE membrane conformally placed on the PDMS supporting layer shows a colour shift from R to B upon arbitrary out-of-plane deformation. Due to the large contrast in the thickness between the MCLCE membrane and PDMS supporting layer, any out-of-plane deformation is translated to the plane stress on the MCLCE membrane, providing the colour shift that simply depends on the bending radius of the bilayer system.

Supplementary Video 2

Simultaneous R, G and B coloration using a shared air inlet with different aspect ratios. By programming the aspect ratio, a spatial colour dispersion between coloration units is achieved using one shared air inlet for pneumatic actuation. Here, three coloration units with aspect ratios of 0.10, 0.06 and 0.04 increase their dispersion of colour as the pressure increases and finally exhibit R, G and B colours, respectively.

Supplementary Video 3

Spectra shift from NIR to visible to UV wavelengths. The platform, consisting of three linked coloration units with different aspect ratios (1.000, 0.075 and 0.060), initially are colourless, implying that the reflection wavelength is in the NIR wavelength, and show subsequent R, G and B colours as the pressure increases. When the pressure continues to increase, the coloration units of the 0.075 and 0.060 aspect ratios become colourless again as the reflection wavelength is shifted to UV wavelengths.

Supplementary Video 4

A seven-segment display configuring 28 coloration units and seven air inlets to asynchronously actuate every four linked coloration units performs a display of one-digit numbers.

Supplementary Video 5

Camouflage in the background of periodic patterns. This platform actuates alternating coloration units with an identical aspect ratio, 0.1 in a 7 × 5 array, to imitate the periodic background patterns (R, G, B and R–G chequer-board patterns).

Supplementary Video 6

Camouflage in the background of irregular patterns. Two platforms, configured in an array of coloration units with variable aspect ratios (0.04–0.10 for one platform and 0.030–0.125 for the other), show spatial colour dispersions to imitate irregularly coloured backgrounds.

Source data

Source Data Fig. 2

Statistical source data.

Source Data Fig. 3

Statistical source data.

Source Data Fig. 4

Statistical source data.

Source Data Extended Data Fig. 1

Statistical source data.

Source Data Extended Data Fig. 2

Statistical source data.

Source Data Extended Data Fig. 3

Statistical source data.

Source Data Extended Data Fig. 4

Statistical source data.

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Kim, SU., Lee, YJ., Liu, J. et al. Broadband and pixelated camouflage in inflating chiral nematic liquid crystalline elastomers. Nat. Mater. 21, 41–46 (2022). https://doi.org/10.1038/s41563-021-01075-3

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