Microplastics settle more quickly when near one another

Microplastics released into the environment are effectively permanent. Their interactions with organisms can have immense ecological implications, and the way they settle through bodies of water affects these interactions.

Seyfi et al. studied this settling behavior, focusing on how plastic microparticles behave near each other throughout the process.

“While many studies have examined particle settling inspired by other particulates, including natural sediments, microplastics can behave differently,” said author Shooka Karimpour. “We aim to uncover fundamental hydrodynamic mechanisms that are often oversimplified, but crucial for predicting how microplastics move, settle, and interact in water.”

By capturing the motions of spherical particles in a fluid tank, the researchers found the presence of nearby particles generally accelerates their settling behavior. The specifics of the interaction rely on particle properties like their initial distances, and even subtle interactions can alter the settling dynamics, but the same general trend remains consistent: When two particles are close to one another, the trailing particle speeds up to catch up to the leading particle.

“In practical terms and in terms of contaminant management, this means that accurately predicting settling in the presence of nearby particles is essential for some regions, like near a source such as a wastewater outlet, where simplified models — ignoring these interactions — might misrepresent how microplastics settle near these hotspots,” Karimpour said.

Microplastics can vary greatly in size, density, and shape — all of which affect how the particles accelerate. The researchers plan to expand their study to include other types of microplastics, like films and fibers, and to explore the nuanced effects of background flow. Karimpour emphasized the importance of establishing the physics of these particles to give policymakers a clear, dependable framework for understanding and managing environmental microplastics.

“Of course, particle interactions and settling are only one piece of the puzzle, and additional processes governing both near- and far-field transport have to be considered,” Karimpour said.

Source: “Settling and drafting of spheres in quiescent fluids in microplastic relevant regimes,” by Siamak Seyfi, Camillo De Castro, Wim S. J. Uijttewaal, and Shooka Karimpour, Physics of Fluids (2026). The article can be accessed at https://doi.org/10.1063/5.0334330 .