Chameleon-inspired tunable multi-layered infrared-modulating system via stretchable liquid metal microdroplets in elastomer film

The study addresses how to dynamically regulate mid-infrared (7.5–14 µm) reflectivity using soft, stretchable materials for applications such as encryption, displays, camouflage, radiative cooling, human–machine interaction, and thermal management. Existing IR modulators often rely on 2D structures and have limited out-of-plane design flexibility. Inspired by the reversible brightness control in chameleon skin—where melanophore dispersion modulates the intensity (not wavelength) of reflected light—the authors propose a bio-inspired system in which liquid metal microdroplets embedded in an elastomer undergo reversible shape changes under strain to tune IR reflectance. The central hypothesis is that deformable eutectic gallium–indium (EGaIn) droplets in an Ecoflex matrix can serve as artificial “melanophores,” enabling large, reversible modulation of both total and specular IR reflectance through mechanical deformation, composition control, and temperature-triggered phase changes, and that multi-layer stacking can expand encoding/encryption capabilities beyond planar approaches.

Prior work shows metals provide high IR reflectance; phase-change materials enable temperature-dependent emissivity; and 2D nanomaterials offer component-dependent IR properties. However, many dynamic IR systems are 2D, limiting out-of-plane modulation. Specular reflectance modulation previously required engineered surface wrinkles on polymers. Cephalopod- and chameleon-inspired systems have informed adaptive reflectors and camouflage. Despite these advances, achieving high-performance, multi-modal modulation (total and specular reflectance), reconfigurability via strain/composition/temperature, and multilayer design remains challenging. The authors build on works demonstrating mechanically tunable IR properties, dynamic radiation modulators, and metal-layer-based reflectors, aiming to overcome constraints by leveraging deformable liquid metal inclusions and multilayer integration with evaporated metals.

• Materials: Eutectic Ga-In (EGaIn, 75.5 wt% Ga, 24.5 wt% In); low-melting-point (mp) alloys (mp 47 °C and 70 °C); Ecoflex 0030 elastomer.
• BLEE fabrication (bilayered Ecoflex/EGaIn film): Prepare a pure Ecoflex base layer (1:1 parts, cured at room temperature). Mechanically mix Ecoflex precursor and EGaIn at mass ratios 1:2, 1:4, 1:6, 1:8, 1:10 at 700 rpm for 5 min, coat atop the base, and cure at room temperature. Single-layer films were also made by coating the mixture directly on acrylic.
• Microstructure characterization: SEM and size distribution analysis for different mass ratios; X-ray microscopy (XRM) 3D reconstruction; optical microscopy to observe droplet-to-flake transformations under strain; TEM to assess oxide layer at EGaIn/Ecoflex interface.
• Surface topography: 3D laser scanning microscopy and roughness analysis before/after strain for various mass ratios.
• Mechanical testing: DMA to obtain stress–strain curves and Young’s modulus for pure Ecoflex and composites across mass ratios (0–400% strain). Cycling tests for durability (0–300% repeated cycles; performance checked at 1500% areal strain).
• Optical/IR characterization: FTIR with integrating sphere for total reflectance (7.5–14 µm); infrared imaging microscope for specular reflectance; FLIR IR camera imaging.
• IR camouflage testing (total reflectance mode): Place BLEE films (various mass ratios) on a 76 °C hot plate; image from above with IR camera at areal strains 0–1500%; compute apparent temperatures and correlate with measured reflectance spectra to calculate average reflectance and predicted apparent temperatures.
• IR camouflage testing (specular reflectance mode): Arrange a hot plate (IR source) and IR camera at specular geometry on either side of the sample; record IR images and temperatures across mass ratios and areal strains; measure specular and total reflectance spectra; compute average specular reflectance and its fraction of total reflectance.
• Size effect study: Fabricate films with nanodroplets versus microdroplets and compare strain-induced modulation; simulations to confirm size-dependent deformability.
• Polymer matrix variation: Replace Ecoflex with SEBS or PDMS to test generality and strain requirements.
• Low-mp alloy bilayers: Fabricate bilayer films using alloys with mp 47 °C or 70 °C by mixing alloy (kept liquid during mixing) with Ecoflex precursor; study reflectance/appearance at room temperature vs above mp and under strain (e.g., r-0%, r-1011%, m-1011% conditions). Perform cycling at 1011% areal strain with reheating.
• Multi-layered structures: Fabricate trilayer Ecoflex/EGaIn/Au films by masking and evaporating ~200 nm Au atop BLEE patterns to realize contact/overlap/partial-overlap designs. Assess IR patterns before/after stretching. Stack BLEE films of different mass ratios (e.g., 1:2, 1:6) with Au to create z-stacked patterns and programmable IR Morse coding across predetermined areal strains (0%, 178%, 334%, 525%, 751%).
• Demonstrations: IR ink painting (strain-tunable symbols/letters), mass-ratio patterned “flower” with normalized color scales across temperatures, visible/IR encoding using top/bottom sides (Ecoflex/EGaIn vs pure Ecoflex), temperature-programmed triangles using alloys with distinct mps, and laser-written patterns (808 nm) on 70 °C alloy films.

• Large, reversible IR modulation via strain:
  • Total reflectance change up to ~44.8% (from 29.8% to 74.6%) for BLEE; apparent temperature reduction of a 76 °C object by up to 34.5 °C at 1500% areal strain.
  • Specular reflectance change up to ~61.2% with maximum specular-to-total fraction ~95% (mass ratio 1:10 at 1500% strain); apparent temperature increase by specular reflection up to ~21.2 °C.
• Mechanism: Spherical EGaIn droplets deform into overlapping flakes under tensile areal strain, forming quasi-continuous metallic layers that enhance reflectance. Surface roughness changes are small; droplet deformation dominates specular modulation.
• Size dependence: Micron-scale droplets deform and enable modulation; hundred-nanometer droplets do not deform sufficiently, showing negligible strain-induced IR modulation (confirmed by simulations).
• Composition and strain as control knobs: Increasing EGaIn content and strain lowers apparent temperature (total reflectance mode) and raises it (specular mode), enabling programmable encoding/decoding.
• Temperature programming via alloy mp: For 70 °C alloy films, stretching above mp (m-1011%) yields higher reflectance than stretching at room temperature (r-1011%); IR images at 64 °C and 104 °C agree with calculations. Triangle patterns combining alloys (mp 16 °C, 47 °C, 70 °C) exhibit temperature-dependent IR color changes. Laser writing (808 nm) locally melts alloy to create patterns visible under subsequent strain and heating.
• Mechanical behavior and durability: Young’s modulus decreases at lower LM content (1:2–1:6) but increases at higher content (1:8–1:10) due to oxide-mediated interfacial stiffening. BLEE (1:10) retains average reflectance and IR performance after up to 2000 strain cycles; low-mp alloy films show stable reflectance after 4 thermal-strain cycles.
• Alternative matrices: Using SEBS (stiffer than Ecoflex) enables notable total reflectance changes (27.5%) at lower areal strain (137%), suggesting potential for electrically driven actuation.
• Multilayer encryption: Evaporated Au films have high initial reflectance that decreases with strain (opposite trend to BLEE). Integrating Au with BLEE in trilayers yields switchable patterns (contact/overlap/partial overlap) that change visibility upon stretching. Stacking BLEE films of different mass ratios with Au enables out-of-plane coding and programmable IR Morse code decryption at specific strains (0%, 178%, 334%, 525%, 751%).

The results validate the chameleon-inspired concept that modulating the intensity of reflected mid-IR radiation can be achieved by mechanically driven rearrangement of reflective elements—in this case, deformable liquid metal droplets in an elastomer matrix. By tuning droplet morphology via strain, the system transitions from a low-reflectance, scattering-dominated state to a high-reflectance, quasi-continuous metallic state, addressing both total and specular reflectance camouflage without requiring engineered surface wrinkles. The combination of control variables—strain, liquid metal concentration, and temperature (via alloy mp)—enables rich, programmable IR responses for encoding/decoding, painting/writing, and communication. Multilayer integration with evaporated Au extends functionality to out-of-plane patterning and increases information density and security relative to conventional 2D pixel-based approaches. The high modulation amplitudes, durability under thousands of cycles, and compatibility with different polymers underscore the relevance of the approach for IR camouflage, thermal management, and human–machine interfaces.

This work introduces a multi-modal, multilayer-capable IR-modulating platform using stretchable EGaIn microdroplets in elastomer films. It achieves among the highest reported changes in total (44.8%) and specular (61.2%) reflectance, large apparent temperature modulation (−34.5 °C in total mode; +21.2 °C in specular mode), and robust cycling stability. Programmability is demonstrated through strain, composition, and temperature (phase change) control, enabling IR encoding/decoding, painting/writing, and multilayer encryption (including Morse coding) by combining BLEE with evaporated Au. Future research could focus on reducing required strain via material and structure optimization, integrating electrical or pneumatic actuation, scaling manufacturing, refining droplet size distributions and interfacial chemistry for tailored mechanics/optics, and exploring applications in sensing, thermal regulation, soft robotics, and biomedicine.

• High areal strains (up to 1500%) are required for maximum modulation in Ecoflex-based BLEE; although SEBS reduces strain thresholds, further reduction would aid practical deployment.
• Nanodroplet composites do not exhibit strain-induced modulation, indicating a size window is necessary for effective droplet deformation.
• Evaporated metal films (e.g., Au) lose reflectance upon stretching due to fracture of the continuous film, which constrains their role in multilayer designs to pre-defined switching behaviors.
• Temperature-programmed effects require heating above alloy melting points, which may limit use scenarios and necessitate thermal management.
• Surface roughness changes are small and do not drive specular modulation; performance relies primarily on internal droplet morphology changes, which depend on composition, mixing, and strain history.