sedimentation

The process of deposition of rock fragments suspended in water on to the floor of an ocean, sea, lake, or river floodplain. The unconsolidated sediment may become compacted, dewatered, and cemented together by processes collectively known as diagenesis, ultimately forming a sedimentary rock.

Sedimentation describes the motion of molecules in solutions or particles in suspensions in response to an external force such as gravity, centrifugal force or electric force. Sedimentation may pertain to objects of various sizes, ranging from suspensions of dust and pollen particles to cellular suspensions to solutions of single molecules such as proteins and peptides.

In a sedimentation experiment, the applied force accelerates the particles to a terminal velocity vterm at which the applied force is exactly canceled by an opposing drag force. In general, the drag force varies linearly with the terminal velocity, i.e., Fdrag = fvterm where f depends only on the properties of the particle and the surrounding fluid. Similarly, the applied force generally varies linearly with some coupling constant (denoted here as q) that depends only on the properties of the particle, Fapp = qEapp. Hence, it is generally possible to define a sedimentation coefficient that depends only on the properties of the particle and the surrounding fluid. The concentration of particles at the boundary is opposed by the diffusion of the particles.

The sedimentation of particles under gravity is described by the Mason-Weaver equation, which has a simple exact solution.

The sedimentation of particles under the centrifugal force is described by the Lamm equation, which likewise has an exact solution.

Particles with a charge or dipole moment can be sedimented by an electric field or electric field gradient, respectively. For electrophoresis, the sedimentation coefficient corresponds to the particle charge divided by its drag (the electrophoretic mobility). Similarly, for dielectrophoresis, the sedimentation coefficient equals the particle's electric dipole moment divided by its drag.