Visualization of strain rate based on mechanoluminescence tailing

Mechanoluminescence (ML) materials have the ability to emit light under mechanical force, allowing for visible mechanical force distribution and making them valuable for applications in smart textiles and wearable sensors. Detecting the force rate is crucial for applications, such as haptic feedback devices and mechanical monitoring, but visualizing the speed of strain has not been achieved yet. In this study, we have successfully visualized the strain rate by monitoring the ML tailing lengths of red, green, and blue (RGB) ML materials, including CaF 2 : Tm 3+ , CaF 2 : Eu 3+ , and CaF 2 : Tb 3+ phosphors. These materials exhibit varying ML tailing lengths at identical rotation speed of friction force, due to their distinct lifetime of the emitting centers. By incorporating these RGB ML phosphors into a composite film, the discrepancies in ML tailing lengths across different colors become visible, with these differences becoming more pronounced as the rotation speeds rise. Ultimately, the visualization of rotational speed in bearings was achieved successfully based on the vivid color changes in the ML tailing length at different speeds.