When suspended solids containing small particles are stationary, the suspended particles gradually sink due to the gravitational field. The heavier the particles, the quicker the sink, and the other particles that are smaller than the liquid will float. The velocity of particles moving in the gravitational field is related to the size, morphology and density of particles, and is related to the intensity of gravity and the viscosity of liquids. Particles like red blood cells, in diameter of micron, can be observed under normal gravity.
In addition, the diffusion phenomenon is accompanied by the deposition of substances in the medium. Diffusion is unconditional absolute. The diffusion is inversely proportional to the mass of the material, and the smaller the particles spread. And settlement is relative, conditional, to be subjected to external forces to exercise. The settlement is proportional to the weight of the object, and the larger the particles settling faster. Particles such as viruses or proteins that are smaller than a few microns are colloidal or semi-colloidal in the solution, and it is impossible to observe the subsidence process using gravity alone. Because the smaller the particles settling slower, the more serious the diffusion phenomenon. So it is necessary to use centrifugal force to produce powerful centrifugal forces, which can force these particles to overcome the diffusion and generate subsidence motion.
Centrifugal is the use of centrifugal rotor high-speed rotation of the powerful centrifugal force, speeding up the sedimentation of the particles in the liquid, the different sedimentation coefficient and buoyancy density of material separated from the sample.