cosedimentation

Cosedimentation, or co-sedimentation, refers to the simultaneous settling of two or more components from a suspension under gravity or centrifugal force. The process depends on particle density, size, and shape, as well as interactions such as aggregation, flocculation, and the formation of larger complexes, which can alter settling velocities and sediment structure.

In practice, co-sedimentation is often used to infer interactions between components: if two species sediment together as part of a larger particle or complex, this can indicate association or binding. However, coincidental similarities in settling properties can also produce apparent co-sedimentation without direct interaction, so conclusions require careful controls and complementary evidence.

Applications span several fields. In sedimentology and soil science, co-sedimentation describes the concurrent deposition of mineral grains and organic matter. In environmental engineering and wastewater treatment, clarifiers rely on the co-settling of particulates and flocs to separate solids from water. In biochemistry and structural biology, co-sedimentation assays and analytical ultracentrifugation are used to study molecular interactions and complex formation, with observed co-sedimentation suggesting, but not proving, binding without additional corroborating data.

Common methods include batch settling tests to measure settling velocity and sediment rise, and ultracentrifugation techniques such as analytical ultracentrifugation or density-gradient centrifugation to resolve components by buoyant density. Interpretation should account for potential confounders, including differences in particle density, size, shape, and hydrodynamic effects, and ideally be supported by complementary approaches.