Sedimentation coefficient

The sedimentation coefficient s of a particle is used to characterize its behaviour in sedimentation processes, notably centrifugation. It is defined as the ratio of a particle's sedimentation velocity to the acceleration that is applied to it (causing the sedimentation).

The sedimentation speed ${\displaystyle v_{t}}$ (in ms⁻¹) is also known as the terminal velocity. It is constant because the force applied to a particle by gravity or by a centrifuge (measuring typically in multiples of tens of thousands of gravities in an ultracentrifuge) is cancelled by the viscous resistance of the medium (normally water) through which the particle is moving. The applied acceleration a (in m/s²) can be either the gravitational acceleration g, or more commonly the centrifugal acceleration ${\displaystyle \omega ^{2}r}$. In the latter case, ${\displaystyle \omega }$ is the angular velocity of the rotor and r is the distance of a particle to the rotor axis (radius).

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