The capability of certain animals to change appearance serves multiple functions in physiological behaviors such as predation and self-preservation. This ability has fascinated humans for centuries and inspired the development of camouflage materials and devices. Currently, materials such as plasmonic nanodot arrays, responsive photonic crystals, and multilayer thermochromic liquid crystals have been designed to selectively reflect light in the visible spectrum to change its color and blend into the background; while electrochromic Devices, thermochromic materials, and mechanochromic systems can dynamically change their infrared emissivity in response to changes in ambient temperature. However, for all these optical manipulation techniques, a common challenge is the tuning range. In visible light, it is difficult to achieve fully tunable colors without chromatic aberration; and a large infrared tuning range is also difficult to achieve due to the limited response of materials to external fields. Furthermore, multiband camouflage covering both the visible and infrared spectrums is more difficult to achieve, since visual color tuning relies on frequency manipulation, while infrared spectral tuning relies on intensity changes.
Optofluidics utilizes light-fluid interactions to realize fluid flow or optical manipulation for different applications including light tuning and sensing. Microfluidics, as a camouflage and display technology, has two distinct advantages over existing electrochromic, mechanochromic, and thermochromic approaches. First, sufficient color switching can be achieved due to the high optical index contrast induced by fluid displacement. Second, the fluid contains solutes or particles as well as solvents, which provides the possibility of dual-function modulation for the needs of different spectral bands.
Recently, researchers from the School of Power and Mechanics of Wuhan University proposed a microfluidic strategy, using multilayer fluidic structures and dual-functional fluids to achieve dynamic camouflage in the visible and infrared bands, according to Memes Consulting. In addition, the study fabricated microfluidic devices in the form of textiles and demonstrated its ability to match leaves in different seasons in the full-band hyperspectral range, showing the application of programmable microfluidics in adaptive camouflage, broadband display and active thermal Potential for applications in management. The relevant research results were published in Microsystems & Nanoengineering under the title of "Programmable microfluidics for dynamic multiband camouflage".
As shown in Figure 1a, the programmable microfluidic device consists of a multilayer fluid with dye molecules and a solvent in a translucent state in the infrared band. In each layer of fluid, visible light is selectively absorbed through electronic excitation in the dye molecules, while infrared light is partially absorbed through molecular vibrations in the solvent. Through this discrete mechanism, the microfluidic device provides a way to manipulate visible and infrared light without interfering with each other.
This work tested the color-tunability of microfluidic thin-film devices with three primary colors: red, yellow, and blue (Fig. 2). For simplicity and ease of coding, arrays of (a, b, c) are used to represent the fluid states of the top, middle, and bottom layers of the device. Each letter has an optional value 0, 1, 2 or 3. Among them, 0 represents no liquid, 1, 2 and 3 represent red, yellow and blue liquid, respectively. As shown in Figure 2a, the (1, 0, 0), (0, 2, 0) and (0, 0, 3) states represent the original red, yellow and blue colors, respectively. Additionally, the brightness of primary colors can be adjusted by stacking identical fluids. In the states of (0, 0, 0), (1, 2, 0), (0, 2, 3), (1, 0, 3) and (1, 2, 3), white, orange , Green, Purple And Brown. Unlike conventional color mixing using dye mixtures, this approach exploits light subtraction at each layer of the device without dye mixing, allowing for easier reversible color manipulation. The measured reflectance spectra of the microfluidic thin film device in different states are shown in Fig. 2b. Based on these spectra, the researchers calculated the chromaticity of each color, and the results are shown in Figure 2c, covering a wide range of colors, and the obtained color range can be further expanded by adding microchannel layers.
Subsequently, the researchers investigated the infrared light manipulation performance of the microfluidic device. As shown in Figure 3a, at the same temperature of 60 °C, the infrared images of the microfluidic film under different filling states are obviously different. The emissivity of the film was obtained by adjusting the emissivity of the infrared camera until the film displayed the true temperature (60 °C). As shown in Figure 3c, the emissivity of the film increases from 0.42 to 0.90 with different filling states. To confirm the results, the researchers measured the reflectance spectra of the films. As shown in Fig. 3b, the shapes of the reflection spectra are almost the same, but the spectral intensities are different. By integrating the spectrally absorbed radiation relative to the wavelength of blackbody radiation, the researchers obtained the integrated emissivity of the film in the atmospheric transparency window from 7.5 μm to 14 μm. The results are in good agreement with the thermal imager measurements (Fig. 3c).
鉴于该微流控薄膜器件对可见光和红外光谱的可调谐性能，研究人员展示了该薄膜在可见光和红外伪装中的应用潜力。如图4a所示，将尺寸为2.5 cm × 2.5 cm的微流控薄膜贴附在温度为60℃的白色陶瓷加热器上。将加热器在人造背景（背景颜色依次从白色变为黄色、绿色和棕色）上移动，并使用数码相机和红外相机捕捉加热器的可见光和红外图像。如图4b、4c所示，加热器自适应地改变其颜色以匹配可见光和红外成像中的不同背景，实现了动态可见光和中红外伪装。而对于没有微流控薄膜的加热器，两台相机都清楚地捕捉到了移动的加热器。
In order to demonstrate the application potential of this microfluidic thin-film device, the researchers further fabricated the thin film into a textile form (Fig. 5a). The microfluidic textile is used for ground hyperspectral camouflage in the solar radiation spectral range from 400 nm to 2500 nm. As a common ground vegetation, natural leaves have complex optical properties, so this study uses natural leaves as the background. The study designed two layers of textiles to mimic the reflectance spectrum of leaves. The upper layer of colored water is used for visible color display, and the lower layer of gray water is used for reflection intensity modulation. White paper is placed on the bottom to enhance the reflection intensity. As shown in Fig. 5e, 5f, not only the color of the textile filled with the designed liquid is very similar to the green leaf, but also the reflection spectrum is similar. By changing the liquid inside the textile, the color and spectrum were immediately changed to those close to those of yellow leaves, suggesting the adaptability of the camouflage film to changes in seasons or locations in the natural environment.
In summary, this study develops a programmable microfluidic device with a multilayer fluidic structure and bifunctional fluid, which can dynamically display almost the entire visible light and emissivity in the presence of three primary color fluid inputs. Infrared spectral bands in the range 0.42 to 0.90. This capability makes this microfluidic device potentially useful in both visible and mid-infrared camouflage. In addition, the study fabricated microfluidic films in textile form for scaled-up applications. The microfluidic textile has a specially designed fluid and stack structure that can dynamically match the reflectance of the blade in the full spectral range including the visible to near-infrared bands. Considering its wide band and fine light modulation performance, this programmable microfluidic device may open up new avenues for smart optical surfaces across multi-band electromagnetic spectrum, and could be used in applications such as adaptive camouflage, broadband display and active thermal management. Has great potential.
---The essays is from MEMS
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