When the suspension containing fine particles is left stationary, the suspended particles gradually sink due to the action of the gravitational field. The heavier the particles, the faster the sinking, whereas the particles with a lower density than the liquid will float. The rate at which particles move under the gravitational field is related to the size, morphology, and density of the particles, and is related to the strength of the gravitational field and the viscosity of the liquid. Particles of the size of red blood cells, a few micrometers in diameter, can be observed under normal gravity.
In addition, the substance is accompanied by a diffusion phenomenon when it settles in the medium. Diffusion is unconditional and absolute. The diffusion is inversely proportional to the mass of the material, and the smaller the particle, the more severe the diffusion. The settlement is relative, and if it is conditional, it must be subjected to external forces to move. The settlement is proportional to the weight of the object, and the larger the particle, the faster the sedimentation. For particles smaller than a few micrometers such as viruses or proteins, they are in a colloidal or semi-colloidal state in solution, and it is impossible to observe the sedimentation process by gravity alone. The smaller the particles, the slower the sedimentation, and the more severe the diffusion phenomenon. Therefore, it is necessary to use a centrifugal machine to generate a strong centrifugal force in order to force these particles to overcome the diffusion to produce a settling motion.