# Dictionary Definition

sedimentation n : the phenomenon of sediment or
gravel accumulating [syn: deposit, alluviation]

# User Contributed Dictionary

## English

### Noun

- The separation of a suspension of solid particles into a concentrated slurry and a supernatant liquid, either to concentrate the solid or to clarify the liquid

#### Derived terms

#### Translations

- French: sédimentation

# Extensive Definition

- This article refers to the scientific phenomenon of sedimentation. For sedimentation in the treatment of water and wastewater, see Sedimentation (water treatment).

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. Even small molecules
such as aspirin can be
sedimented, although it can be difficult to apply a sufficiently
strong force to produce significant sedimentation.

In a sedimentation experiment, the applied force
accelerates the particles to a terminal
velocity v_ at which the applied force is exactly canceled by
an opposing drag force. For small enough particles (low Reynolds
number), the drag force varies linearly with the terminal
velocity, i.e., F_ = f v_ (Stokes flow)
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, F_ = q E_. Hence,
it is generally possible to define a sedimentation
coefficient s \ \stackrel\ q/f that depends only on the
properties of the particle and the surrounding fluid. Thus,
measuring s can reveal underlying properties of the particle.

In many cases, the motion of the particles is
blocked by a hard boundary; the resulting accumulation of particles
at the boundary is called a sediment. 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
coefficient s in this case equals m_/f, where m_ is the buoyant
mass.

The sedimentation of particles under the centrifugal
force is described by the Lamm
equation, which likewise has an exact solution. The
sedimentation coefficient s also equals m_/f, where m_ is the
buoyant mass. However, the Lamm equation differs from the
Mason-Weaver equation because the centrifugal force depends on
radius from the origin of rotation, whereas gravity is presumed
constant. The Lamm equation also has extra terms, since it pertains
to sector-shaped
cells, whereas the Mason-Weaver equation pertains to box-shaped cells (i.e., cells
whose walls are aligned with the three
Cartesian axes).

Particles with a charge or dipole moment can be
sedimented by an electric
field or electric
field gradient, respectively. These processes are called
electrophoresis
and dielectrophoresis,
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.

### See also

sedimentation in German: Sedimentation

sedimentation in French: Sédimentation

sedimentation in Indonesian: Sedimentasi

sedimentation in Italian: Sedimentazione

sedimentation in Japanese: 堆積

sedimentation in Polish: Sedymentacja

sedimentation in Swedish: Sedimentation

sedimentation in Turkish:
Çökelme