Sedimentation coefficient

The sedimentation coefficient (s) of a particle characterizes its sedimentation during centrifugation. It is defined as the ratio of a particle's sedimentation velocity to the applied acceleration causing the sedimentation. The sedimentation coefficient (s) of a particle characterizes its sedimentation during centrifugation. It is defined as the ratio of a particle's sedimentation velocity to the applied acceleration causing the sedimentation. The sedimentation speed v t {displaystyle v_{t}} (in m/s) is also the terminal velocity. It is constant because the force applied to a particle by gravity or by a centrifuge (typically in multiples of tens of thousands of gravities in an ultracentrifuge) is balanced by the viscous resistance of the fluid (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 ω 2 r {displaystyle omega ^{2}r} . In the latter case, ω {displaystyle omega } is the angular velocity of the rotor and r is the distance of a particle to the rotor axis (radius). The viscous resistance is given by Stokes' law: 6πηr0v, where η is the viscosity of the medium, r0 is the radius of the particle and v is the velocity of the particle. Stokes' law applies only for large spheres in an infinite amount of fluid. The centrifugal force is given by the equation: mrω2, where r is the distance of the particle from the axis of rotation. When the two opposite viscous and centrifugal forces balance, the particle moves at constant (terminal) velocity. The terminal velocity is given by the equation: