Design and development of modular electrochromic device for active camouflaging

Abstract

Active camouflage refers to the dynamic change in color and/or texture of an organism/device to blend with the surroundings. Chameleons, octopuses, squid, etc. are some natural species that have this amazing ability. They are capable of adapting according to their habitat and survive. The present thesis aims to mimic their active camouflaging ability in an artificial system. It is a challenge to develop a general system considering the huge variations in color space. Therefore, we have limited ourselves to the greenish surroundings which represent a large domain of environments for surveillance applications. Active camouflaging has two major requirements; the first is to collect and quantify the information of surroundings, and the second is to make a device that can change its color using this information. A thorough literature survey reveals that algorithms have been developed to form digital patterns based on the collected information of surroundings. However, these are not utilized to change the patterns in an artificial physical system. In addition, novel materials are reported in the literature that are capable of dynamic changes in color and texture. High input power, complex design, and limited applicability are the main problems with these reported solutions. To address some of these issues, a modular electrochromic device (ECD) capable of changing color under an applied voltage is designed and developed in this work. ECD is a layered device comprising of electrochromic polymer (ECP), gel electrolyte (GE), and electrodes. Electrochromic polymer is responsible for color changing due to a jump in energy gap at different excited states under an applied potential. GE provides the ion and maintains a gap between the compliant electrodes. Repeated coloration-decoloration cycles, varying temperature, humidity, and mechanical loading of ECDs may induce cracks, cuts, and damage in GE. In such scenarios, ECD needs to be replaced. Therefore, it is essential to develop repairing strategies for GE to enhance the operational life of ECD. In addition, there are some other requirements of green vegetation that are essential for the development of active camouflage devices such as the multiple shades of green color in a single leaf, brown color of sand, mud, and dry leaves. To address these issues, the present thesis is divided into four objectives. The first objective is to induce self-healing behavior in the gel electrolyte (GE). GE comprises salt, solvent and polymer. Salt provides the ions, solvent gives the passage for ions and polymer solidifies the electrolyte. Polymethyl methacrylate (PMMA) is used as a polymer due to its excellent self-healing characteristics. The concentration of PMMA in GE is varied to optimize the composition of GE. The recovery in tensile strength and ionic conductivity of GE is found to be maximum for 15 wt% PMMA. Hence, GE with 15 wt% of PMMA is used to fabricate ECD. The effect of healing on the color contrast of ECD is also investigated. It is shown that the color contrast of ECD prepared with healed GE approaches that of original ECD with increasing healing time. Optimized weight percentage (15%) of PMMA in GE is used subsequently for the development of ECDs. After achieving the self-healing in GE, experiments are performed to achieve varying shades of green color and quantify the coloration of ECDs. Polyaniline (PANI) is used as an ECP which gives the light green, green, and blue colors at reduced, neutral and oxidized states respectively. Following Lambert’s law, PANI of different thicknesses (250 nm– 650 nm) is used in ECDs to modulate the shades of green in ECD. PANI is coated on ITO electrode by electropolymerization method using cyclic voltammetry (CV) technique. To vary the thickness, PANI is coated for 5, 10, 15, and 20 number of cycles. Comparative analysis is performed by digital images, spectroelectrochemical measurements, and spectral colorimetry. Results show that the minimum thickness (≥ 400 nm) of the ECP layer is required to get the noticeable color change in ECD. The color contrast of ECD ranges from 14 % to 80 % at both excited states and it increases with the increase in thickness of the ECP layer. ECDs get different shades of green and blue colors due to the variation in thickness. In the next part, an experimental investigation is carried out to achieve the shades of brown and green color in ECD. Dual ECDs are fabricated and tested using Poly (3-hexyl thiophene) (P3HT) as another electrochromic polymer with PANI. P3HT gives magenta and cyan colors at neutral and oxidized states respectively. Complementary dual ECD has P3HT and PANI ECP layers on the opposite electrodes while hybrid dual ECD has both the ECP layers on the same electrode. Comparative analysis revealed that the hybrid dual ECD has the ability to achieve green, brown, and blue colors at reduced, neutral, and oxidized states. To prepare the active camouflage device as a proof-of-concept, an Electrochromic Modular Architecture is designed and fabricated using four types of ECDs (3-single ECDs of different thickness of PANI and 1-hybrid dual ECD). To collect the surrounding information, a Pattern Detection Algorithm is developed which includes image acquisition, shadow removal, image segmentation, and image processing. A modular image is developed using surrounding information followed by the system integration of the Electrochromic Modular Architecture and Pattern Detection Algorithm. Modular image has the same number of modules arrangement as of electrochromic modular architecture. Then, a simulation is performed to get the best possible patterns. Digital Modular Architecture is prepared using the design principle of Electrochromic Modular Architecture and digital images of all four types of ECDs. Simulated Digital Modular Architecture gives the required input potentials of best-suited patterns. Using the simulation results, the potential is applied to the ECDs of Electrochromic Modular Architecture which gives different patterns for different surroundings. It is envisaged that the findings of the present study will be a significant step in designing the active camouflage layers (arrangements of devices) for the applications of surveillance, defense, and wildlife discoveries.