Hue-Resolved Strain Sensing in Mechanochromic Colloidal Photonic Crystal Films

Mechanochromic materials, particularly colloidal photonic crystals (CPCs) embedded in soft stretchable films, hold significant potential in applications requiring precise and real-time strain monitoring. These materials exhibit strain-induced color changes arising from alterations in lattice periodicity, yet conventional quantification methods like spectroscopy are often costly, complex, and challenging to scale. Here, an image-based quantification framework is presented that evaluates mechanochromic behavior with high precision by tracking hue shifts in optical images. CPC films are prepared from core-shell nanoparticles with acrylate shells of different glass transition temperatures, allowing systematic control of elastic modulus and composition-dependent chromatic responses. RGB image data are converted into the hue-saturation-value (HSV) color space, where the hue component serves as a robust optical readout of lattice compression, directly reflecting nanoscale spacing changes. The hue-strain relationship is reproducible, material-dependent, and consistent under multi-axial loading, thereby linking strain-induced color shifts to intrinsic material properties. Statistical filtering together with consistency across different brightness levels further ensures stability under variable imaging conditions. This framework establishes hue as a precise and scalable metric for mechanochromic quantification, broadening opportunities for cost-effective strain sensing in wearable devices, soft robotics, and structural diagnostics.