The development of mechanochromic, structurally colored materials is crucial for applications in visual communication and sensing. However, current fabrication methods often involve multiple steps, hindering scalability and economic viability. Existing techniques such as spin coating, 3D printing, lithography, and self-assembly approaches frequently require rigid substrates, lack mechanochromic properties, or are not suitable for elastomeric substrates. The challenge lies in creating a scalable, single-step fabrication technique that allows for precise control over nanoscale morphology and dynamically tunable structural color. Polydimethylsiloxane (PDMS), a biocompatible and optically transparent elastomer, is a popular substrate for flexible and deformable devices. However, integrating structural color components into PDMS typically involves complex chemical processes. Liquid gallium (Ga), a metal with plasmonic properties comparable to gold and silver, offers potential for flexible and mechanically reconfigurable devices. However, creating Ga nanostructures at the nanoscale has been challenging due to its high surface tension. This research aims to address these challenges by developing a novel, single-step fabrication technique for creating mechanochromic structural colors using Ga nanostructures embedded in a PDMS substrate.

The existing literature highlights the limitations of current methods for fabricating structurally colored materials with mechanochromic responsiveness. Many techniques, including spin coating, 3D printing, and lithography, are unsuitable for large-scale production or lack the required flexibility and mechanochromic properties. Bottom-up self-assembly approaches, while offering potential for nanoscale control, often involve numerous complex steps, making them economically unfeasible. The use of liquid metals, particularly gallium, has emerged as a promising area for flexible electronics, but the precise fabrication of sub-100 nm Ga nanostructures remains a significant hurdle due to its high surface tension. Existing methods for creating Ga nanoparticles typically require chemical methods and result in nanoparticles dispersed in a liquid medium, unsuitable for direct integration into flexible substrates. This study aims to overcome these limitations by proposing a novel one-step approach.

The fabrication process involves thermally evaporating gallium (Ga) onto PDMS substrates with varying oligomer content. The PDMS is prepared by mixing a base and curing agent at different ratios (PDMS 5 to PDMS 20), controlling the substrate softness and oligomer volume. The capillary interaction between the liquid-like oligomers and the deposited Ga leads to the formation of Ga nanodroplets embedded within the PDMS matrix. The process parameters, including substrate temperature and deposited Ga thickness, are carefully controlled to achieve desired color variations. The role of oligomers is investigated by comparing the results with samples where the oligomers have been removed using toluene treatment. Cross-sectional scanning electron microscopy (SEM) and high-angle annular dark-field (HAADF) imaging, along with energy-dispersive X-ray (EDAX) mapping, are used to analyze the Ga nanostructure morphology. A mathematical model, consisting of substrate (S), growth (G), and engulfing (E) equations, is developed to describe the fluidic interaction between the Ga and the PDMS oligomers, predicting the nanodroplet size and layer formation. Finite-difference time-domain (FDTD) simulations are performed to understand the optical properties of the Ga nanostructures and correlate them with the experimental observations. The mechanochromic response is characterized by applying uniaxial strain to the samples and measuring the resulting changes in reflectivity spectra. The stability of the fabricated structures against various environmental factors, including solvents, high temperatures, and mechanical stress, is also assessed.

The study demonstrates a single-step fabrication of tunable structural colors using Ga nanostructures embedded in PDMS. The process leverages the capillary interaction between liquid-like PDMS oligomers and liquid Ga, leading to the formation of multi-layered Ga nanodroplets with a narrow size distribution. The color is tunable by varying the PDMS ratio (oligomer content), resulting in a wide range of colors across the CIE chromaticity diagram. The presence of oligomers is critical for achieving the observed multi-layered structure and color variation; their removal results in a monolayer structure and significantly different color. A mathematical model is developed to describe the Ga nanodroplet formation, accurately predicting the experimental observations of droplet size and layer number. The mechanochromic behavior is demonstrated by applying uniaxial strain, resulting in a reversible blue shift of the reflectivity spectra and corresponding color change. The reversible color change is repeatable over 80,000 cycles, showcasing the robustness of the fabricated structures. FDTD simulations support experimental observations, showing that the color is determined by gap plasmon resonances between Ga nanodroplets, which are sensitive to changes in inter-particle spacing upon stretching. The fabricated structures show good stability against various environmental factors such as washing, exposure to solvents, and high temperatures. The application of the material is demonstrated through prototypes of flexible displays and sensors for curvature sensing, point stress detection, body part position monitoring, and real-time force mapping.

The findings demonstrate a significant advancement in the fabrication of mechanochromic structural colors. The single-step approach significantly improves the scalability and economic viability compared to existing multi-step methods. The use of Ga nanostructures embedded in PDMS offers a unique combination of plasmonic properties and mechanical flexibility. The developed mathematical model provides a valuable tool for understanding and controlling the nanostructure formation process. The wide range of achievable colors and the excellent mechanochromic response opens up exciting possibilities for applications in flexible displays, sensors, and soft robotics. The robust nature of the material against various environmental factors further enhances its potential for real-world applications.

This research successfully demonstrates a novel single-step fabrication technique for creating mechanochromic structural colors using Ga nanostructures embedded in PDMS. The method offers significant advantages in terms of scalability, cost-effectiveness, and tunability. The developed mathematical model provides a framework for understanding the nanostructure formation process. Future research directions include exploring different liquid metals and polymer matrices to expand the range of achievable colors and functionalities. Investigating the integration of these materials into complex devices and systems for various applications is also warranted.

While the fabricated structures show good stability, long-term stability studies under extreme environmental conditions are necessary. The current model focuses on uniaxial strain; further investigations are required to fully understand the behavior under more complex deformation modes. The influence of the native Ga oxide layer on the optical and mechanical properties requires more detailed study. The range of detectable strain might be limited by the resolution of the color detection system.