When suspending liquid containing small particles let stand don't move, because the role of the gravitational field of suspended particles sinking gradually. The heavier particles, the sinking faster, whereas density than the liquid of small particles will rise. Particles moving under the gravitational field is associated with the size, shape and density of particles, and is related to the strength of the gravitational field and the viscosity of the liquid. Like red blood particles, the size of a micron in diameter, in normally under the action of gravity to observe their settlement process.
In addition, the material in the medium in the sedimentation was accompanied by a diffusion phenomenon. Diffusion is unconditional absolute. Is inversely proportional to the diffusion and the quality of the material, the smaller the particle diffusion is more serious. And settlement is relative, conditional and subject to external forces to movement. Settlement is proportional to the mass, the bigger the particle sedimentation. In less than a few microns particles such as viruses or protein, they come colloid or half colloid state in solution, using only the force of gravity is impossible of sedimentation process was observed. The slower because the smaller the particle sedimentation, and diffusion phenomenon is more serious. So need to strong centrifugal force generated by the centrifuge to force these particles sedimentation movement overcome diffusion.
Centrifugal is strong centrifugal force generated from the centrifuge rotor high-speed rotation, speed up the settlement of particles in the liquid, the samples in different subsidence coefficient and buoyant density of the material.