The incipient motion threshold of microplastics (MPs), defined as the condition that is just sufficient to initiate MP movement, is key to assessing the transport and fate of MPs in water bodies, yet only a few studies have focused on its prediction. This study experimentally investigated the effects of bed roughness (smooth and rough beds) and MP properties (shapes, sizes, and densities) on the incipient velocity (U_i) and critical shear stress (τ_c) of exposed MPs in an open-channel flow. For a total of 19 types of MPs, U_i and τ_c were found to range from 0.06 to 0.21 m/s and 0.01 to 0.075 N/m², respectively. The commonly used thresholds for sediment transport, for example, critical shield parameter Θ_c and movability number Λ_c, were established for MPs based on τ_c. Based on the experimental data of the literature and this study, it was found that predictions of Θ_c and Λ_c for sediments do not apply to MPs. A new explicit formula for Λ_c was proposed for predicting the incipient motion of MPs by introducing the dimensionless particle diameter (d_*) and a new dimensionless parameter related to the particle size, density, and shape. The new formula has an absolute error of 12.3%, which is smaller than the existing formula for MPs (55.6%).

中文翻译：

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暴露微塑料在明渠流动中的初始运动

微塑料 (MPs) 的初始运动阈值定义为刚好足以启动 MP 运动的条件，是评估 MPs 在水体中的运输和命运的关键，但只有少数研究关注其预测。本研究通过实验研究了床层粗糙度（光滑和粗糙的床层）和 MP 特性（形状、尺寸和密度）对明渠中暴露 MPs的初始速度 ( U _i ) 和临界剪切应力 (τ _c ) 的影响流动。对于总共 19 种 MP，发现U _i和 τ _c的范围为 0.06 至 0.21 m/s 和 0.01 至 0.075 N/m ²， 分别。泥沙迁移的常用阈值，例如，临界盾构参数 Θ _c和可移动数 Λ _c，是基于 τ _c为 MPs 建立的。根据文献和本研究的实验数据，发现对沉积物 Θ _c和 Λ _c的预测不适用于 MP。通过引入无量纲粒径（d _*）和与粒径、密度和形状相关的新无量纲参数，提出了新的Λ _{c显式公式来预测MPs 的初始运动。}新公式的绝对误差为 12.3%，小于现有的国会议员公式（55.6%）。