sedimenting

Sedimenting is the process by which solid particles suspended in a liquid settle downward under the influence of gravity, forming a sediment at the bottom of a container. In geology and environmental science, sedimentation describes the natural deposition of material from fluids in lakes, rivers, or oceans. In laboratory and industrial settings, sedimentation is used to clarify suspensions or to separate components.

The rate of sedimentation depends on particle size, density difference between particle and liquid, fluid viscosity, and temperature. For small, spherical particles in a Newtonian fluid under laminar conditions, terminal velocity is given by Stokes' law: v = 2/9 · r^2 · g · (ρ_p − ρ_f) / μ. In concentrated suspensions, interactions between particles cause hindered sedimentation, slowing rates compared with dilute systems.

Brownian motion can oppose sedimentation for very small particles, especially below about a micron. Methods to accelerate sedimentation include centrifugation and ultracentrifugation, which apply centrifugal forces greater than gravity. Separation by differential sedimentation or density-gradient sedimentation relies on size- or density-based differences.

Applications include clarification in water and wastewater treatment, preparation of biological samples (for example, separating cells or organelles), and materials science where sedimentation helps characterize powders and colloids. Sedimentation behavior can be quantified by sedimentation velocity experiments, often expressed in Svedberg units as a sedimentation coefficient.

In natural environments, sedimentation contributes to the formation of sedimentary rocks and to the vertical layering of deposits. Rates are influenced by flow, turbulence, particle aggregation, and chemical conditions that promote flocculation or dissolution.