Abstract

Adaptive camouflage textiles with color changing and blending into the combat background (CB) and surrounding environments have been a great challenge for the color scientist. Defense professional urgently needs adaptive camouflage textiles for personal protection in extreme weather conditions and multidimensional CB environments. A technical approach of adaptive camouflage textiles can be formulated by using a novel combination of thermochromic colorant and liquid crystal. Absorption of heat can rapidly accelerate the thermal response of thermochromic liquid crystal (TLC) by changing molecular structure with thermo-color-light (TCL) mechanism of absorption and reflection of light at different wavelength. TLC shows chameleon performance of color tone which changes the light reflection of surface color; thus, target objects can be artificially confused by the replacement of chromatic appearance in multidimensional CB environments. TLC can be applied as deceiving mechanism and surface modification of textile substances with combination of dyes/pigment. TLC modified camouflage textiles have possibility of diverse applications in different weather of combat zone for defense actions and different CB environments. A single formulated camouflage textiles may be suited with different CB environments under TLC mechanism. Chameleon type of color tone in cooling and heating conditions of thermochromic changes automatically in both reversible and irreversible way. Therefore, the technical colorant combination has been preached for suitability of adaptation with surrounding CB. TLC treated textiles can be experimented with spectroscopic, microscopic, and photographic illumination. The applications of adaptive camouflage textiles are not only limited to military textiles, but also the principle of technologies have versatile applications for clothing of personal protection including fashionable garments production.

1. Introduction

Research and innovation on surveillance technology is being augmented hurriedly under the genuine growth of imaging technology in terms of hyperspectral and digital imaging. Advancement of surveillance technology is being used for both pessimistic and optimistic implementation as per needs of individual groups mainly implemented by special workers in defence. Oppositely researchers and scientists are also demanding to innovate anti-surveillance technology in different fields of engineering and developing chameleon camouflage textiles is one of them. Research and development have been found in literature review in anti-surveillance technology in terms of camouflage textiles but research in liquid crystal-based chameleon type camouflage textiles is still at the early stage. Chameleon camouflage textiles can change the color for adapting with surrounding environment in different temperature for pessimistic and optimistic implementation as per needs of human being. For example: snow/ice combat background (CB) needs low temperature adaptive camouflage and desert CB demands high temperature background camouflage. Toet ¹ reported that army professional needs multi-environment camouflage textiles for single mission of different CB. Neider et al. ² reported that spectral radiance depends strongly on the illumination incident of the target signature and the atmospheric conditions of CB environment.

2. Background of Technical Approach for Adaptive Coloration Technology

Adaptive camouflage has the numerous ways of concealment and disguise in the animal for natural adaptation against surrounding background ¹⁵². In nature, animal has adaptive capability by changing color in different environment. Cuttlefish, octopus, squid, and other cephalopods are the natural examples of adaptive camouflage for the protection of predators ^{3, 4, 48, 100}. Hanlon et al. highlighted that ¹⁵³ young cuttlefish use patterning primarily for concealment, such as general colour resemblance, disruptive coloration, obliterative shading, shadow elimination, disguise, and adaptive behaviours. Older cuttlefish also conceal themselves but increasingly use patterns for signalling, both interspecifically (warning or ‘deimatic’ displays) and interspecifically (sexual signalling). Nokelainen et al. ¹¹¹, Thayer et al. ¹⁴⁹, Stevens et al. ¹⁵¹ established the concepts of animal concealment from predators. Josef et al. ¹⁷⁶ remarked that octopuses have the ability of camouflage creation with surrounding. Stevenset et al. ¹⁷⁸ investigated that ghost crab (Ocypodecer atophthalmus), Singapore can change the color for adaptation with environment. Stuart-Fox et al. ¹⁵⁰ remarked the animal color changing pattern and adaptive camouflage in terms of thermoregulation. Rafael C. Duarte ¹⁰⁸ studied that some species, rhytms may facilitate thermoregulation and concealment. Merilaita, Lind, investigated that background is an important aspect for concealment and probability of detection by the predator ¹⁷⁵. Rosenholtz ¹⁵⁹ commented that shape and orientation of a surface impacts the object recognition, scene perception, and many visual-cognitive tasks. Cuthill et al. ¹⁷⁴ studied the bird disruptive and background matching coloration. Silbiger and Munguia ¹⁷⁶ experimented that color versus temperature changes of ucapugilator. Therefore, natural existence of adaptive camouflage may motivate the actual development of adaptive camouflage textiles, but the reality of such camouflage textiles still in earlier stage of research and development.

Thermochromic colorant may meet the performance requirements of chromatic materials and thermochromic liquid crystal (TLC) acts as deceiving mechanism of adaptive camouflaging under temperature changing against CB ⁴⁹. Chromatic materials change the color by the external stumuli like photochromic (light), thermochromic (heat), electrochromic (electricity), solvatochromic (liquid), carsolchromic (electron beam). Normally the molecular structure of thermochromic colorant is altered and color changes above room temperature but liquid crystal temperature changes (5-15)°C and cholesteric type liquid crystal is more suitable for thermochromism, and the pitch of the helical arrangement is responsible for temperature and reflection in wavelength ⁸⁵. Sudhakar and Gobi N ¹³⁰ proposed chameleon textiles in terms of pH changes, electric or magnetic field effects, mechanochromism, bond breaking/making, oxidation state changes. Polymer binder/resin and pigment for control of gloss, heavy metal pigment, lead, chromium, iron oxide, silica, polyurethane are the materials for camouflage textiles development ⁸⁸. Still now, thermochromic materials have limited application in camouflage textiles. Most thermochromic materials are organic having short lifetime and limited range of temperature in combination with liquid crystal. There is need of research for developing wide range of temperature and lifetime properties of thermochromic materials ^{87, 91}. Sudhakar and N. Gobi ⁸⁸ supported the opinion of Alvenmaa ⁸⁷ that chromophore with polymeric materials may enhance the stable field of color change process. Adaptive camouflage ⁴³ was proposed by color changing leuco dyes and advised for coupling color changing ability with high stimuli responsive study. Scott ⁵ claimed that CB environments (temperature, weather, etc.) are the prime question for augmentation of camouflage textiles in terms of adaptation. Joshi ⁶ also supported for making the chameleon type camouflage fabric by the implementation of light reflection engineering ⁷. Heating mechanism influences the thermochromic system for temperature responsive camouflaging ^{7, 8, 9, 20}. Conner ³² patented a camouflage textile with a material capable of different chromic states at different ambient light levels. The patent also observed the color changes of heat sensitive dyes in terms of both controllable (heat) and uncontrollable temperature (sunlight). Karpagam ⁶² experimented chemelyon type printing on cotton fabric with thermochromic colorant. CIE L^*, a^*, b^* were measured before and after heating with spectrophotometer. Out of textile engineering, researchers ⁵⁰ in mechanical system developed adaptive camouflage devices capable of producing black and white patterns for matching with its surrounding. Tianzhi et al. ¹³⁴ investigated electrical field-based color changing mechanism with invisible sensors. Ying Liet al. ¹³⁵ developed device for adaptive camouflage with background temperature. US army institute of environmental medicine (USAIEM) experimented the effect of temperature and wind chill on human body such as hot (32°C), pleasant (28°C), cool (22°C), very cool (16°C), cold (10°C), very cold (5°C), bitterly cold (0°C), fresh freezes (-5°C), exposed area of fresh freezes within one min (-24°C) ⁸¹. Temperature mechanism can be used for adaptive camouflage garments in desert (high temperature) camouflage, snow/iceland (low temperature) camouflage, common camouflage textiles in different season, and additionally for fashion textiles. This technology of adaptive camouflage textiles is not limited for garments implementation only, it has versatile applications in defense.

Ajeeb ⁴⁴ suggested higher heat transfer of wool and polyester than cotton. Hearle ⁸⁰ noted the thermal conductivity of different textile fibre when packing density 0.5m/cc. Cotton (71), wool (54), silk (50), PVC (160), cellulose acetate (230), nylon (250), polyester (140), polyethylene (340), polypropylene (120). Jassim and et al. ¹¹³ mentioned that thermal conductivity in (k)W/(m. K) of materials like acetone (0.16), acrylic (0.2) air, atmosphere (gas) (0.024), alcohol (0.17), carbon (1.7), cellulose, cotton, wood pulp and regenerated (0.23), cellulose acetate, molded, sheet (0.17-0.33), cellulose nitrate, celluloid (0.12-0.21), cotton (0.04), polyester (0.05), polypropylene (0.1- 0.22). Thermal conductivity of textile materials should be selected accurately for temperature responsive coloration of protection textiles.

Diverse applications [11-21,38] of thermochromic colorant has been investigated and successfully applied for commercialization like kitchen aprone and hand gloves, laboratory aprone, safety shoes, medical textiles, flexible display for clothing, transportation of sensitive goods, garments to detect disease and physiological conditions of the wearers, firefighting garments for the protection of extreme temperature, sports wears and building paints.

Liquid crystal and thermochromic colorants have versatile applications for the principle of thermochromism.

Hallcrest mentioned a brief applications of liquid crystal: a) general temperature indication/digital thermometers: room/household, refrigerator/freezer, infant bath/bottle, aquarium, hot/cold warning indicators b) battery testers and other voltage measuring devices c) temperature indicators for medical applications: forehead thermometers/fever indicators, hypothermia warning indicators, drug testing; temperature indicating labels for urine specimen authentication d) medical thermography: sub-cutaneous cancer detection, diagnosis of vascular diseases, placenta location, pharmacological tests, skin grafting and vein location, veterinary applications, chiropractic applications e) radiation detection: infrared, microwave, ultraviolet, ultrasonic, electromagnetic f) aesthetic: advertising and promotions, decoration, jewelry, badges, fabrics, clothing and other novelties g) ingredients in cosmetic formulations h) non-destructive testing/thermal mapping: surface and sub-surface flaw detection in metals, welded metals, bonded and other composite structures, fault/short location in electric components and electronic circuits i) aerospace and engineering research: heat transfer studies, flow visualization in fluids and on solid surfaces in airflows j) gas/liquid level indicators k) miscellaneous: chemical and gas detectors, pressure sensors, information displays. Novel applications for liquid crystal are continually under advancement ⁹⁴. Application of thermochromism of liquid crystal is not a new approach of thermal engineering. The application of liquid crystal for camouflaging on textiles substances is an innovative approach.

Thermochromism opined by Andy Town ⁴⁹, Aitken and et al. reported ¹⁵⁵, Eds H.R Mattila ⁸⁶, J.E Gilligan ²⁸ mentioned and summarized the thermochromic principle in Figure 1. A (green), B (red), C (yellow) are three schematic phases of colorant in three possible temperature ranges. Phase transition occurs under multi-phase heating mechanism in terms of geometrical and molecular structures.

Thermochromic materials [86,118-121,123,124,127,128] and color changing principle ¹²⁹ have been used thermal technology but limited for adaptive camouflaging on textile substances [86,118-121,123,124,127,128]. Organic compound shows the variation in crystal structure, stereoisomer, and molecular rearrangements. Liquid crystal shows different color at different temperature due to variation of reflection in wavelength from low temperature crystalline phase to high temperature isotropic liquid phase. Molecular rearrangement can create new chromophore for changing color like aid base, keto-enol. Inorganic thermochromism has found drawback in textile application that color often changes at high temperature ⁸⁶. Fujita et al. ¹⁵⁶ claimed that thermochromic microencapsulated pigment shows a color change temperature regulator. In the same way, Aitken et al. reported ¹⁵⁵ that phase transition, geometry and molecular structures influence the thermochromic system. Henry ¹⁶⁴ Conner patented coating materials to produce color changes in the camouflaging pattern include light and/or heat sensitive dyes and/or inks. Ramlow et al. ⁴⁵ recommended that there is still too much to be explored and discovered in the field of chromic textiles ⁴⁶. Cheng et al. ⁵³ supported that thermochromic materials are memory functions to the temperature. Thermochromic materials can be used for anti-counterfeiting technology. Geng et al. ⁵⁴ microencapsulated crystal violet lactone as thermochromic colorant and observed reversible thermochromic properties, improved thermochromism was also investigated when silver nano particle added for modification. In this investigation, microencapsulation was studied by microscopic analysis. Shibahashiet and et. al. ¹⁵⁷ opined and patented that the electron donating organic compound for microencapsulated thermochromism. Wilusz ⁸⁵ reported that Nanta from Toray Industries reported temperature sensitive fabric in 1988 with trade name SWAY by microencapsulation and resin coating. The microcapsule was made of glass and dye chromophore agent was electron acceptor and color neutralizer were alcohol showed color and decolor in terms of environment temperature. Phillips developed fabric changing color from white to blue with ultraviolet (UV) irradiation at 350-400 nm. Ladendal ²² investigated thermochromism of thermochromic leuco dyes and proposed an outcome of color changing with uncontrollable parameter (sun) in print design process. Bourque ²³ also studied temperature dependent color changes and experimented the tendency of thermochromism when color density increases with the increasing temperature. Lioyd ³¹ and Rao ²⁴ are also supported thermochromism who developed TLC ²⁵ coated fabric for biomedical application by using leuco dyes for extended temperature range. Vikovaet al. ²⁷ suggested the research scope of light fastness for color changeable thermochromic ink and thermochromic pigment ²⁹. Borque and researcher team ¹⁹³ experimented that alkyl chain lengths were poorly matched, the dye-developer interaction dominated in the solid state, and melt-lightened thermochromism was observed. Jin and other researchers ⁶⁰ experimented a heat stimulated luminous fibre at 30°C-20°C temperature by using heat sensitive pigment TF-G through wet spinning method. Heat stimulation was verified in naked eye by the researchers. Jinet al. ⁶¹ investigated a thermosensitive luminous fibre by thermochromic pigment. Researchers observed a rose red at room temperature and fibre sample showed colorless with blue light emission. In this study researchers claimed that phosphorescence of color is responsible for thermochromism. Potucket al. ⁶⁴ experimented physiological changes of skin temperature range between 33°C and 38°C with artificial microencapsulated leuco dyes named as thermochromic pigment and applied on nylon & spandex blended fabric. Thermochromism behaviours also studied by Martins ⁶³ at 10°C-60°C. Jassim et al. ¹¹³ found that thermochromism showed different colors like orange 30°C, light orange 32°C, light pink 36°C, colorless 41°C. Kulčar and his researchers group ¹¹² proved that the stability of decolorized state was examined at cooling for 10 hours. Ramigand et al. tested ⁶⁵ thermochromism by reflectance spectra and calorimetry data. Thus, smaller particle tends to create bluer shade and larger particle tends to create redder shade. Z. Ahmed et al. ⁵² reported that printed thermochromic ink on polyester and cotton blended fabric changes color from black to green in 10.8 s using 1.46 W DC power at 30°C temperature. Researchers recommended that the technology can be extended by choosing right formulation for specific purpose of thermochromism. Dean et al. ¹⁵⁴ discovered that FriXion erasable pens contain thermochromic inks that have colored low temperature forms and colorless high-temperature forms. Thermochromic coloration ^{40, 41, 42, 78} is not a new concept but thermal responsive and adaptive camouflage is a new approach. Kulcaret al. ⁹⁵ confirmed the reversibility of color change for leucodye-based TC ink. In 2019, thermochromic microcapsule by cationic dye was studied by Ma ⁶⁸. Ahmed and his team at IIT, Delhi, India ^{2, 26} experimented the thermochromic colorant on cotton fabric ¹⁰ and fabric shade was changed in term of heat. Bristetet al. ⁷⁰ patented that thermochromic materials produces a visual change of the visible (Vis) surface when an activation temperature of thermochromic materials is reached. Wan Zhang ⁷¹ examined the thermochromism performance on polyester fabric at 20°C, 45°C, 80°C with thermochromic leouco dye loaded silica nano capsule. The color reversely changed from dark blue (K/S =12) to light blue (K/S = 2) at 610 nm for more than 50 times. Zhang et al. ⁷² experimented temperature responsive thermochromic textiles between 5°C and 35°C. Jakovljevic and researcher team ⁷⁴ studied thermochromism based on leuco dyes and liquid crystal. Choudhury et al. ⁷⁵ remarked that ultraviolet temperature, p^H is the responsible for color changing of thermochromic colorant. Basnec and his researcher team ¹⁰¹ opined that temperature-dependent colour of thermochromic (TC) materials originates in phase transitions and changes in the geometry of the material when thermochromic composites were prepared using crystal violet lactone (CVL) dye, benzyl 4-hydroxybenzoate (B₄HB) developer and 1-dodecanol (DD) solvent and strongly suggested future study. Thermochromism exhibits with temperature versus color change ¹⁰³. Mary et al. ¹⁰² stated that temperature decreasing and wavelength increasing like 400 nm, 500 nm, 600 nm, 700 nm, and color change accordingly. Mary et al. ¹⁰² focused that neither thermochromic liquid crystals nor thermochromic organic dye mixtures can be applied directly for use in coloration: both require microencapsulation. Schubert et al. ¹¹⁴ focused and supported antireflection coating of four-layer (TiO₂/SiO₂)/nanoporous SiO₂ for the broadband and directional reflection. HULSEY and his team ¹¹⁷ designed temperature versus color change with leuco dye and the product was sold by color change corporation streamwood. Panak et al. ¹⁰⁸ experimented CIE lab ¹²⁶ thermochromism with crystal violet lactone as a colour former, bisphenol A as a developer and tetradecanol as a co-solvent and researchers suggested thermochromic system based on color change. Bašnec et al. ¹⁰⁹ also used lucodye-based composite for thermochromism. Vikova ¹¹⁶ proceeded kinetic model verification on color changeable textile sensor by the process of dyeing and printing. Serenet al. ⁷³ analyzed thermochromism of thermochromic pigment on leather finishing at 15°C and 31°C. The color measurement was studied by spectrophotometer. Candas et al. ⁶⁶ investigated thermochromism in leather finishing by UV absorber and cross linker. The evidence was supported by spectrophotometer and ageing test. Heet al. ⁵⁹ observed bisphenol A risk for thermochromic textile product and tested by high performance liquid chromatography (HPLC) method for bisphenol identification.

Dave and Patel ¹⁴⁰ mentioned that molecular diameter decreases when temperature raises under the investigations on 4-n Alkoxybiphenyl-4 Carboxylic acids and its 3-substituted derivatives. White ¹⁴¹ reviewed that the use of cholesteric liquid crystals (CLCs) as color changing optical materials. Liquid crystal has phase transition nature ^{96, 122}. Thermochromic liquid crystals have numerous applications ⁹⁴ for color changing but the implementation in adaptive camouflaging have not been explored. Ian sage ^{96, 122} reported the nature of phase transition of liquid crystal technology. Qi Hong ⁸⁴ investigated cholesteric liquid crystal and suggested that the number of pitches required for the stimulation of reflection. Yu Guan and other researchers ⁵⁵ used cholesteric liquid crystal 3-30 µm microcapsules as thermochromic materials with electrospinning method and the structure of microencapsulation was determined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images, core size was studied by TG. This study investigated the thermochromic property from red to blue chromatic hue. Gilligan et al. ²⁸ are critically proposed temperature versus color change properties ³³ of cholesteric liquid crystal for reversible, controllable, and reproducible. Aguirre ³⁹ illuminated the reversible color changing property of colloidal photonic crystal. Nadia et al. ¹⁹⁷ studied thermochromic liquid crystal for heat transfer. Choudhury et al. ⁵¹ commented that temperature can influence the reflection of light by liquid crystal due to having the structure of helices. Therefore, the wavelength of reflected light is altered causing progressive change in color spectrum. Seeboth and Lotzsch ⁷⁶ mentioned in their book that liquid crystal can flow like fluid. The length of helical pitch depends on the molecular structure and concentration of the chiral compound, phase type and its temperature. Therefore, pitch length influences the thermochromic properties. Choudhury et al. ⁵¹ remarked that the liquid crystral materials can be microencapsulated and microcapsules can be used like pigment on the fabric surface with resin binder. Christie et al. ⁷⁷ experimented thermochromic print based microencapsulated liquid crystal. The effect is most suitable over a black background. Treated nylon and lycra blended fabric was analyzed in terms of wavelength of reflected light, L* a^* b^* values, lightness, chroma and hue. Christie et al. ⁹¹ also remarked that leuco dye and liquid crystal for temperature sensitive environment using thermochromic color change by careful formulation with proposed technology of microencapsulation. Maclaren et al. ⁸³ used crystal violet lactone dyes for reversible thermochromic system. Similarly, Panak and other researchers ⁹³ supported the color change mechanism with CIE lab ⁴² by demonstrating thermochromic composite in formulation with crystal violate lactone as a color, bisphenol A as developer and tetradecanol as co-solvent.

Smith et al. ³⁵ patented the material coloration using particle scattering and Hemant Kumar et al. ³⁰ reported the feasibility of anti-reflective coating for thermochromism. Kumar et al. ³⁷ studied the thermochromism with vanadium pentoxide coating and suggested the possibility of chromium for thermochromism. Yan and other researchers ⁹⁸ investigated by x-ray diffraction (XRD) that the degree of crystallization of colored luminous fibre was decreased inorganic pigments when inorganic pigment and luminous materials were used. Xiao et al. ¹³² opined that the possibility of adaptive infrared camouflage which consists of a vanadium dioxide (VO₂) layer, with a negative differential thermal emissivity, coated on a graphene/carbon nanotube (CNT) thin film. Plan et.al ⁹⁹ remarked high refractive index of grapheme oxide. Zhu ¹³³ studied for high temperature thermal management in infrared camouflage by combining a silica aerogel for thermal insulation.

Krishna et al. ⁹⁷ studied that PCM dispersed with different types of nano particle for high thermal storage in terms of microencapsulation and researcher recommended the microencapsulation technique for high thermal storage with nano-PCM. Qu et al. ⁵⁸ reported that camouflage technique has not adequately explored the ability to continuously camouflage objects either temperature variations or wide observation angle. Researchers developed a thermal camouflage device by PCM Ge₂Sb₂Te₅ in 30°C to 50°C and 0° to 60° angle. Wu ⁶⁷ was studied simultaneous process of color changing and heat changing behavior after 100-time heating and cooling cycle by using spirolactine color former, phenolic hydroxyl compound as color developer, 1-hexadecanol as PCM and the microencapsulation was achieved by gelatin containing vinyl group and gum arabic to form glutaraldehyde for making stability and encapsulation. Sari et al. ⁷⁹ experimented with emulsion polymerization method for solid-liquid microencapsulated phase change material (PCM). The chemical and thermal characterization was done by SEM, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) where the DSC showed energy storage capacity and the TGA confirmed the thermal stability of microencapsulated PCM. The microencapsulated n-heptdecane with polymethylacrylate confirmed 5000 thermal cycles. Burkinshaw et al. ⁸² experimented with p^H sensitive spirolactone derived functional dyes and phase changes occurred during heating and cooling in terms of thermochromic effect. Wbendkowska ⁹⁰ studied that PCM has latent heat storage capability for thermal application.

3. Materials, Methods, and Experimental Design for Adaptive Camouflage Textiles

Materials, methods, analysis, and observations need a technical combination for adaptive camouflaging on textile substances against multidimensional background environments. The materials, methods, analysis, and observations are not established for adaptive camouflage textiles. Table 1 denotes the selection of suitable textile materials for thermochromism and adaptive camouflage textiles. Common textile materials have been mentioned by defense professional. Table 2 has been listed with thermochromic colorant for chromatic appearance with background environments; and TLC for deceiving mechanism (Figure 3) at different temperature. Table 3 demonstrates the method of textile treatment for thermochromism as camouflage coloration. Table 4 exhibits the assessment method of thermochromism, and analysis of textile treated samples. Table 5 has been mentioned with color changing parameters for background color matching at different temperature/weather conditions/background environments.

3.1. Textile Materials for Thermochromism

3.2. Materials for Thermochromism

3.3. Methods for Thermochromism

3.4. Methods of Thermochromism Analysis

3.5. Observations for Color Changing Parameters

4. Camouflage Identification Methods

Adaptive camouflage can be investigated by spectroscopic, microscopic, photographic illumination and human participants as evidence of different branches of research and developments. Wen et al. ¹³⁶ commented that target signature can be experimented by hyperspectral imagers and detection can be assessed in different distances ¹⁴⁶. Chang et al. ¹⁸⁹ experimented that camouflage target can be identified at different angle of target. Gupta ¹⁶³ remarked that spectral features can be detected the material properties of objects because of the emission, reflection, and absorption of light. Hyperspectral imaging can be used for photographic and spectral illumination in UV-Vis-infrared (IR) ranges ^{159, 160, 162, 172, 189}. Manolakis et.al ¹⁸¹ experimented on hyperspectral image analysis for the operation and performance of detectors. Akkaynak et al. ¹⁷⁰ suggested an image processing technique in perception science for color calibration from digital camera. Animal scientists are doing camouflage research with hyperspectral imaging technology. Pinto et al. ¹⁸² successfully experimented on frog skin reflectivity in hyperspectral imaging between 400-2500 nm and spectral reflectivity between 700-1100 nm. Chiaoet et al. ¹⁸⁰ investigated the color formation camouflage of cuttlefish with hyperspectral imaging technique. Akkaynak et al. ¹⁶⁶ studied in color change for flounder camouflage with hyperspectral imaging Hultgren et al. ¹⁶⁷ studied color change camouflage in marine isopod. Edelaar et al. ¹⁶⁸ studied color changing of grasshopper. Russell et al. ¹⁷³ experimented spectral camouflage of two crab species (Portunus sayi and Planes minutus) was assessed using hyperspectral imagery in 400-550 nm. Manolakis et al. ¹⁹¹ remarked that hyperspectral sensing can increase the detectability of pixel and subpixel size targets by exploiting finer detail in the spectral signatures of targets and natural backgrounds. Photographic illumination with digital camera and hyperspectral camera, and its image analysis have versatile applications ^{171, 179, 185, 186, 200, 201} including colored materials ¹⁸⁷ but limited for camouflaging of textile-based target signature. Muller and Muller ¹⁹⁶ studied that camouflage is a practicing issue of army operations in UV-Vis-IR spectrum. Reflection is a consideration of UV-Vis-IR ranges camouflaging ¹⁸³. Hamilton et al. ¹⁸⁴ performed liquid crystal analysis using CIE lab and image. Lin et.al confirmed that color analysis of camouflage can be investigated by CIE Lab color space in both specular ¹³⁷ and diffuse components ¹⁴⁸, its related calculation can be done by calorimetry ^{139, 145}. Volonakis et al. ¹⁴³ designed a human observer model for camouflage identification and its detection and recognition behavior was compared with human participants. Merilaita and other researchers ¹⁹⁸ remarked that camouflage target influences target feature, edge, surface, and objects. Ruxton GD and researcher team ¹⁹⁹ suggested that dorsum and ventral issues are responsible for background matching. Bhajantri et al. ¹⁹⁵ developed a model of camouflage defect identification based on texture feature. Stenet et. al. ¹⁹⁴ studied that background area selection for target detection influence the assessment of camouflage. Joe et al. ¹⁴⁴ studied the eye movement behaviors on camouflage pattern which may influence the precision surveillance of detection when digital camera is used by different personnel.

5. Research Limitations and Development Approach for Thermal Camouflage

Defense professionals are working at different weather and diverse CB. Adaptive camouflage textiles with surrounding CB are great challenge of different situations. Sometimes defense soldier needs to shift urban to desert, desert to urban, urban to forest, forest to urban, forest to desert and desert to forest in a single operation and/or multi operations. Thermochromic colorant had been studied in literature for color changing but still researchers had not been studied the techniques for the suitability of camouflage purpose for modern surveillance technology and limited research have been observed for the suitability of simultaneous thermochromic and liquid crystal based textiles for color changing. Limited research had been found on adaptive camouflage textiles in literature, although sensor based preliminary stage research on adaptive camouflage textiles had been seen. But limited research on adaptive textiles has been reported for the color changing behaviors of textile substances in terms of details investigations of pitch length changing and wavelength changing of liquid crystal under heating mechanism, but the reviewed mechanism of thermochromism and liquid crystal have been positively argued for the feasibility of pitch length changing and wavelength changing of liquid crystal, thus the technology can be studied for the augmentation of color changing and adaptive camouflage textiles with thermochromic colorant and liquid crystal. Furthermore, adding liquid crystal-based technology for the adaptive camouflage textiles may generate an invention of proposed technology in Vis and IR spectrum of camouflage technology for the concealment of modern surveillance technology including digital camera and/or hyperspectral camera.

6. Significant Approach for Adaptive Camouflage Textiles Technology

Burkinshaw et al. ¹³⁸ remarked that concealment from day light surveillance is a most established practice of camouflage technology in which different hues are employed to overcome the contrast of object and its surrounding. Thermochromism in color changing is already being used in textiles and leather engineering but this is a new approach for the application in adaptive camouflage textiles by the simulation of heat-light-camouflage mechanism in terms of liquid crystal technique. Researchers studied thermochromism in color changing for high temperature based applications by using metallic particle finishing but the technology has limitations for clothing based applications where needs a comfort range of temperature but reversibility of color, numbers of color changing, numbers of sensitive temperatures, tolerance levels of human body temperature like 0°C to 45°C range or other purpose in different temperature ranges, suitable application of colorants in terms of heat and illumination, accurate thermochromism materials like nanoparticles and/PCM for sustaining thermochromism in color changing are still a hidden issue. Review shows that color changing application has been found for maximum two colors having a limitation with a very minimum temperature difference for clothing, but still the technology has not been used for camouflage. So proposed technology will have a temperature dissimilarity in different wavelength and different reflection in wavelength in terms of camouflage with surrounding background. Application of liquid crystal is not only limited to develop digital technology ¹⁴², textile and material scientists are also using liquid crystal for improving different technical properties on textiles substances but liquid crystal applications for the purpose of camouflage textiles are still in development stage/new to the textile and material scientists. Thermochromism mechanism can change the surface illumination in wavelength and color formation of textile substances. Liquid crystal can circulate in different angle when altering molecular orientation is occurred. No detailed studies have been observed for the illumination of color in different molecular orientations and its proposed effect of color changing in different wavelength. Thus, there is so much feasibility of doing liquid crystal-based camouflage textiles which may have an impact in the research and development of anti-surveillance technology in terms of designing and developing camouflage textiles. Figure 2 shows the adaptation of adaptive camouflage textiles with multidimensional surrounding CB. CB environments are illustrated as CB-01, 02, 03 and observers are termed as predator-01, 02, 03, 04. Adaptive camouflage textiles will be a single textile-based target signature, will be able to match with CB 01, 02, 03 under temperature versus chromatic replacement.

7. Technical Approach of Liquid Crystal Mechanism for Adaptive Camouflage

Technical approach of adaptive mechanism ^{25, 77, 76, 94} of liquid crystal have been demonstrated and interpreted in Figure 3. Established Bragg’s law ^{104, 105, 106} has been used to signify the structure of color changing approach in liquid crystals molecules:

where λ is the wavelength of the illumination Vis-IR ranges, d is the spacing of the liquid crystal layer (path difference of wavelength), θ is the incident angle (the angle between incident ray and the scatter plane), and n is an integer. By putting the value of d, θ and n, we can get the value of λ and the changing value of λ identify the replacement of wavelength and changing of wavelength clarify the altering of chromatic appearance of target signature. Thermochromism mechanism in liquid crystal will change the value of distance, d, the distance will change the angle, θ; and wavelength λ will be changed which will create new reflection in specific temperature range and new color formation to the observer Figure 4 demonstrates the chromatic replacement principle by TLC mechanism. The order of molecular distance, angle of wavelength will be adjusted by temperature. Anne et al. ¹⁰² supported that the typical value of pitch length in liquid crystal phenomenon can be the order of wavelength can follow Bragg reflection theory. Therefore, color will change from black to red through orange, yellow, green, and blue, violet, and again black.

Reflectance spectrum or spectral signature shows the function of incident energy, typically natural illumination such as sunlight, that is reflected by multidimensional CB material as a function of the wavelength (λ) of the energy ¹⁸¹. Figure 5 signifies the proposed methodology for adaptive camouflage textiles formulation and design, applied by thermo-chromatic mechanism and coloration technique.

8. Conclusion

Liquid crystal shows precision of color change, but their color change range is limited. In contrast leuco dyes shows wider range of color but difficult to set with accuracy ¹²⁵. So, review can be strongly recommended for combined applications and formulations for thermochromic dyes and liquid crystal. Hypothesis of review undoubtedly states that there are enormous applications of thermochromism technology for color changing, liquid crystal mechanism for color changing and the combination of those techniques have been applied in different purposes for the development of color changing behaviors but the research and developments in the camouflage textiles applications have been limited. The technology of thermochromism and liquid crystal combination is not a new technology but the applications in camouflage and particularly in adaptive camouflage textiles are an original contribution in terms of leuco dyes, liquid crystal related to thermal engineering Adaptive camouflage for the surrounding CB and color matching is still a new technical approach for the implementation of defense professional. On the basis of review, it can be suggested that thermochromic liquid crystal can be implemented for future experimental study of adaptive camouflage, furthermore TLC based technology can be extended for the personal protection of different technical workers in extreme weather conditions including fashionable textile design although right formulation of liquid crystal may spawn a new invention in the area of thermal camouflage engineering and its related branches of textile technology. Crystals mechanism of birefringence may split the refracted ray into two rays in anisotropic media ¹¹⁵. The theory also can be applied for color changing of thermochromism and its feasibility with adaptive camouflage textiles. Proper percentage of liquid crystal (cholesteric liquid crystal, nematic liquid crystal and combination of both for color changing), appropriate selection of dyes (leuco dyes or other heat sensitive dyes), heating process for controlling and reversible process (conductive metallic yarn and/or nano particle, PCM), heating process for uncontrolling parameters (sun light) and a suitable process selection for textile coloration/finishing may generate expected color changing with surrounding and its right adaptation needs for the purpose of defense applications. Geometrical change will form an artificial wavelength and reflection on textile substances which is responsible for liquid crystal-based color changing process. Controlling geometrical change on temperature will create the expected color for adaptive camouflage textiles. Two reversible and irreversible color, multi-reversible and irreversible color, distance of temperatures and color changing, CB color and adaptive color can be developed by the controlling of rotation engineering and its pitch length of liquid crystal. A minimum percentage of liquid crystal needs to remain on the surface of textile substances for getting an outcome of pitch length changing, wavelength changing, color changing. Molecular size and distribution on the surface of textile substances will influence the surface modification in terms of heat changing, consequently wavelength changing and color changing accordingly. The amount of light scattered depends upon the size of molecules and the wavelength of light. Color change is occurred due to the absence of specific wavelengths of light reflected to the observer due to differences in molar absorptivity ^{31, 107}.

Nanoparticle/metallic-yarn/PCM/paraffin can be used for thermal storage in adaptive textiles for the camouflage formulation at different temperature. Research can be carried out by “Bisphenol A” free thermochromism process. Reflection can be changed artificially to the observer in UV-Vis-IR spectrum for making artificial confusion of target signature against surrounding CB environments. Color and temperature need to identify for thermochromism and color matching with surrounding CB. Color matching spectrophotometer can be applied for Vis camouflage, but the assessment of camouflage textiles with hyperspectral spectrometry and imaging is still new technology in review but animal scientists are also using hyperspectral camera for animal camouflage identification.

Declaration

Authors declare no conflict of interest to publish this article.

Acknowledgements

Author, Md. Anowar Hossain, acknowledges RMIT University and Australian government for funding through RTP stipend scholarship. Author acknowledges Professor Lijing Wang and Emeritus Professor Robert Shanks, School of Fashion and Textiles, RMIT University for their valuable comments and corrections.

References