Recognition of microplastics suspended in seawater via refractive index by Mueller matrix polarimetry

Microplastics have become the marine pollution posing a human health risk, but they are difficult to be detected and recognized for different materials, irregular shapes, and broad size distributions. Microplastics' refractive index (RI) is related to the materials and can be characterized by the Mueller matrix. In this work, the particles are suspended in water and their Mueller matrices are measured by a particulate Mueller matrix polarimetry setup. Four kinds of spherical particles including microplastics are effectively classified by their Mueller matrices. Moreover, two kinds of common microplastics with broad size distributions, irregular shapes, and random orientations are also well recognized by the Mueller matrix. These results imply that RI plays a vital role in the recognition of microplastics suspended in water. By using the Mie theory and discrete dipole approximation simulation, the discussions explain in physics origin how RI affects Mueller matrix coupling with size and structure, and give some decoupling methods. Results in this work help advance future tools to in situ recognize the microplastics in seawater.

Automated summary

Microplastics, posing a human health risk as marine pollution, are difficult to detect and recognize due to their different materials, irregular shapes, and broad size distributions. This study successfully classifies four different materials of spherical particles, including microplastics, using their Mueller matrices. Furthermore, it also effectively recognizes two types of common microplastics with broad size distributions, irregular shapes, and random orientations through the Mueller matrix. These results highlight the importance of refractive index in the recognition of microplastics suspended in water and provide insights into the physics origin of how refractive index affects Mueller matrix coupling with size and structure.