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Biofouling, particle morphology, and exposure reshape the threshold of microplastic incipient motion
Summary
Scientists found that when microplastics develop a slimy layer of bacteria and algae (called biofilm) in rivers and oceans, it changes how easily they get swept up and moved by water currents—similar to how a rougher surface affects grip. This matters because understanding where microplastics travel and settle helps researchers predict where these pollutants build up in waterways, which is a key step in figuring out how they might eventually reach our food and water supplies.
Microplastics (MPs) are pervasive pollutants in aquatic environments, and understanding their transport dynamics is critical for assessing environmental risks. This study represents a pioneering analysis of the threshold of MP incipient motion. By conceptualizing biofilm as an enhancement of surface roughness, we find that biofilm influences the incipient motion of MPs by altering the coefficient of static friction (μ).The dependence of the μ on the biofilm thickness is formulated empirically, and the associated increase in the critical Shields number (θ) for MPs is then determined. Furthermore, factors affecting the θ of MPs, such as particle shape, size, biofilm on the surface, and exposure conditions are systematically integrated into a semi-theoretical model by 45 incipient motion experiments and data extracted from published literature. The theoretical component is grounded in force balance analysis, while model coefficients are empirically calibrated using experimental results. Comparison with existing empirical models demonstrates that the proposed framework offers improved robustness and predictive capability for assessing the incipient motion of biofouled MPs.