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

Dynamically regulating mid-infrared radiation (mid-IR: 7.5–14 µm) in response to external stimuli enables advanced technologies encompassing encryption, electrochromic displays, camouflage, radiative cooling, human–machine interaction and thermal management. Mechanical stimulated IR regulators can also be used in personal thermal management, radiative heat management, and finger motion sensing. Various materials are used in existing dynamic IR modulating systems to control IR properties. For example, metals exhibit high IR reflectance, phase-changing materials possess temperature-regulating IR emission, and two-dimensional (2D) nanomaterials show component-dependent IR emissivity. Most reported IR modulating materials are 2D structures, restricting IR modulation in the out-of-plane direction. Ideal IR-modulating materials should exhibit superior modulation performance, have multiple regulation mechanisms, and allow multi-layered design. Nature provides inspiration: melanophores in chameleon skin modulate the interaction between sunlight and xanthophores/iridophores through melanosome dispersion and aggregation. When melanosomes aggregate, the skin appears bright; when they disperse, the skin appears dark. This reversible process tunes the interaction between sunlight and iridophores, changing light intensity without altering wavelength. This mechanism is similar to an IR camera's representation of IR light intensity (7.5 to 14 µm). This tunable IR reflectance inspires the design of a reversible IR-modulating system.

Existing dynamic IR modulation systems utilize various materials to control IR properties. Metals offer high IR reflectance, phase-change materials regulate IR emission via temperature changes, and 2D nanomaterials exhibit component-dependent IR emissivity. However, most systems are limited to 2D structures, hindering out-of-plane IR modulation. The research draws inspiration from the chameleon's skin, where melanophores dynamically adjust the skin's brightness by altering the distribution of melanosomes. This biomimicry approach addresses the need for improved IR modulation performance, multiple regulatory mechanisms, and multi-layered material design.

The researchers developed a bio-inspired multi-layered structure using eutectic gallium-indium (EGaIn) droplets dispersed in silicone elastomer (Ecoflex). EGaIn droplets act as "melanophores", adjusting reflectance by shape-changing from spherical to flake form in response to strain. The Ecoflex matrix acts as a "hormone", driving EGaIn droplet deformation under mechanical strain. Before stretching, spherical EGaIn droplets and Ecoflex absorption result in low IR reflectance. After stretching, overlapping EGaIn flakes create high IR reflectance. Both total and specular reflectance-based IR camouflage were achieved. The study also explored programmable IR encoding/decoding via strain and liquid metal concentration adjustments. Different alloys with varying melting points enabled temperature-dependent IR painting/writing. A multi-layered structure was designed by integrating liquid metal-based IR modulating materials with an evaporated metallic film to enhance system performance. **Fabrication of BLEE:** Ecoflex precursor and EGaIn were mixed at different mass ratios (1:2, 1:4, 1:6, 1:8, 1:10), then cured. A bi-layered film was created with pure Ecoflex as the bottom layer (preventing leakage and providing a contrast IR response) and the Ecoflex/EGaIn mixture as the top layer. **Fabrication of bilayered Ecoflex/low mp alloy film:** Low melting point alloys were mixed with Ecoflex precursor while liquid, then cured on a pure Ecoflex layer. **Fabrication of trilayered Ecoflex/EGaIn/Au film:** A mask was used to create patterns on the bi-layered film before depositing a ~200 nm thick Au film via vacuum thermal evaporation. **Characterization:** SEM, optical microscopy, X-ray microscopy (XRM), 3D laser scanning microscopy (LSM), IR camera, dynamic mechanical analyzer (DMA), film thickness gauge, differential scanning calorimeter (DSC), FTIR spectroscopy, and infrared imaging microscopy were used to characterize the materials and their properties.

The study achieved significant advancements in infrared modulation technology. **Total Reflectance-Based IR Camouflage:** By varying Ecoflex/EGaIn mass ratios and areal strain (0–1500%), the researchers tuned IR reflectance. The apparent temperature decreased with increasing strain and EGaIn concentration. A total reflectance change of almost 44.8% (from 29.8% to 74.6%) was achieved, reducing the apparent temperature of a hot object (76 °C) by 34.5 °C. The average total reflectance calculated from measured spectra correlated well with experimentally measured apparent temperatures, confirming the link between total reflectance and apparent temperature. The mechanical properties of the BLEE films were also characterized; the Young’s modulus increased with higher liquid metal concentrations due to interfacial effects. The IR camouflage performance exhibited excellent reusability and stability even after 2000 cycles. **Specular Reflectance-Based IR Camouflage:** The researchers achieved specular reflectance-based IR camouflage, with a maximum temperature difference of 21.2 °C. Specular reflectance increased with areal strain, primarily due to EGaIn droplet deformation rather than changes in surface roughness. A specular reflectance change of nearly 61.2% was observed. **Multiple Modes of IR Regulation:** The researchers demonstrated three parameters that affect IR properties: areal strain, mass ratio, and temperature. Specific areal strains altered IR ink color, producing programmable IR patterns. Different mass ratios were assembled to form patterns with various false colors in the IR camera. The bottom layer (pure Ecoflex) of BLEE films also provided distinct IR reflectance compared with the top layer, enabling visible/IR encoding/decoding systems. The use of alloys with different melting points (47 °C and 70 °C) allowed for temperature-dependent IR painting/writing. **Multi-layered Structural Design:** The researchers integrated a high melting point IR-reflecting metallic film (Au) with the EGaIn-based IR modulating film to create a multi-layered structure. This improved the complexity of IR patterns. A multi-layered design using Au film and BLEE films with varying mass ratios demonstrated programmable IR Morse code generation and decryption.

The findings address the need for dynamic and tunable IR modulation. The bio-inspired design using liquid metal droplets offers superior performance compared to existing technologies. The ability to achieve both total and specular reflectance-based IR camouflage expands the applications of this technology. The multiple modes of IR regulation and the multi-layered structural design increase the complexity and functionality of IR systems. The combination of these features provides significant advancements in IR camouflage, encryption, and information display, with applications in defense, communication, and sensing technologies.

This research successfully developed a chameleon-inspired, tunable multi-layered infrared-modulating system using stretchable liquid metal microdroplets in an elastomer film. This system demonstrates superior IR modulation capabilities, including both total and specular reflectance-based camouflage, programmable encoding/decoding, and temperature-dependent IR painting/writing. The multi-layered design enhances the complexity and diversity of IR patterns. Future research could focus on exploring the integration of this technology into advanced devices and systems, further refining the control mechanisms, and investigating other bio-inspired designs for improved performance.

While the study demonstrates significant advancements, some limitations exist. The long-term stability of the material under repeated stretching and exposure to environmental factors should be further investigated. The scalability of the fabrication process for large-area applications needs to be addressed. The integration with existing IR systems and the development of user-friendly interfaces for control and operation require further research and development.