When the suspension containing fine particles stands still, the suspended particles gradually sink due to the action of the gravitational field. The heavier the particle, the faster it sinks, whereas the particle with less density than the liquid will float up.

The speed at which particles move in a gravitational field is related to the size, shape, and density of the particles, as well as to the strength of the gravitational field and the viscosity of the liquid. Particles the size of red blood cells, with a diameter of several micrometers, can be observed to settle under normal gravity.

In addition, when the substance settles in the medium, it is accompanied by the phenomenon of diffusion. Diffusion is unconditional and absolute. Diffusion is inversely proportional to the mass of the substance, and the smaller the particles, the more serious the diffusion. The subsidence is relative and conditional, and it needs to be subjected to external force to move.

The sedimentation is proportional to the weight of the object, the larger the particle, the faster the sedimentation. For particles smaller than a few microns, 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 only by gravity. Because the smaller the particles, the slower the sedimentation, and the more serious the diffusion phenomenon.

Therefore, it is necessary to use a hydraulically driven centrifuge to generate a strong centrifugal force to force these particles to overcome the diffusion and produce a sedimentation movement.

Hydraulic-driven centrifugation is to use the powerful centrifugal force generated by the high-speed rotation of the centrifuge rotor to accelerate the sedimentation speed of particles in the liquid, and separate substances with different sedimentation coefficients and buoyancy densities in the sample.