Laser beam scattering for the detection of flat, curved, smooth, and rough microplastics in water

• Published in: Optical review (Tokyo, Japan) Vol. 27; no. 2; pp. 217 - 224
• Main Authors: Asamoah, Benjamin O., Amoani, James, Roussey, Matthieu, Peiponen, Kai-Erik
• Format: Journal Article
• Language: English
• Published: Tokyo Springer Japan 01-04-2020
• Subjects: Atomic; Espoo; Estonia; Finland and Tallinn; Lasers; Microwaves; Molecular; Optical and Plasma Physics; Optical Devices; Optics; Photonics; Physics; Physics and Astronomy; Quantum Optics; RF and Optical Engineering; Special Section: Regular Paper; The 13th Japan-Finland Joint Symposium on Optics in Engineering ((OIE'19); Microplastics; Surface roughness; Speckle contrast; Specular reflection; Laser speckle pattern
• ISSN: 1340-6000; 1349-9432
• DOI: 10.1007/s10043-020-00578-9

Microplastic (MP) pollution is alarming and poses an imminent threat to the environment with a direct impact on our health and that of fauna in natural water bodies. The understanding of light–MP interactions in water as well as the need for low-cost and robust optical sensors for the detection of MPs that appear everywhere is, therefore, necessary. We have demonstrated the use of a prototype optical sensor in the detection of flat and curved [from polyethylene terephthalate (PET) water bottle] pristine and rough MPs from commercial PET and low-density polyethylene plastics in water. The optical sensor utilizes a photodiode and charge-coupled device (CCD) camera to record simultaneously the specular reflection and the speckle pattern modified by the MPs. In this study, we have exploited the specular reflection in the detection of the pristine samples, whereas the speckle contrast, the normalized standard deviation of the speckle pattern intensity, is utilized in the qualitative estimation of the “effective” surface roughness of the MPs. With the sensor, one can, therefore, detect PET MPs with varying average surface roughness, Ra an indication of MP aging, embedded in water. The prototype can detect the effect of size, type, curvature, transparency, and the translucency of sunken and/or floating MPs in water based on the reflection, scattering, and the (edge) diffraction of light. However, the optical sensor is limited in the discrimination of MP concentration in water. Further modifications to the sensor are needed for its practical implementation in complex natural water bodies and wastewaters.