Centrifugation utilizes the immense centrifugal force generated by the high-speed rotation of a centrifuge rotor to accelerate the sedimentation rate of particles in a liquid, separating substances with different sedimentation coefficients and buoyant densities in the sample. Therefore, a centrifuge is needed to generate a strong centrifugal force to compel these particles to overcome diffusion and undergo sedimentation.
  
When a suspension containing fine particles is left undisturbed, the particles gradually settle due to gravity. Heavier particles settle faster, while particles less dense than the liquid will float. The speed of particle movement under gravity depends on the particle size, shape, and density, as well as the strength of the gravitational field and the viscosity of the liquid. Particles the size of red blood cells, with diameters of several micrometers, can be observed to sediment under normal gravity.
  
Furthermore, sedimentation of substances in a medium is accompanied by diffusion. Diffusion is inversely proportional to the mass of the substance; smaller particles diffuse more severely. Sedimentation is relative and conditional, requiring an external force for movement. Sedimentation is directly proportional to the weight of the object; larger particles sediment faster. For particles smaller than a few micrometers, such as viruses or proteins, which exist in a colloidal or semi-colloidal state in solution, sedimentation cannot be observed using gravity alone. This is because smaller particles sediment slower, while diffusion becomes more significant. Therefore, a centrifuge is needed to generate a strong centrifugal force to compel these particles to overcome diffusion and undergo sedimentation.
  
Centrifugation techniques are widely used in bioscience, particularly in biochemistry and molecular biology research, with every biochemistry and molecular biology laboratory equipped with various types of centrifuges. Centrifugation is mainly used for the separation and preparation of various biological samples. In high-speed rotation, the immense centrifugal force acting on the biological sample suspension causes the suspended tiny particles (organelles, biological macromolecule precipitates, etc.) to sediment at a certain speed, thus separating them from the solution. The sedimentation rate depends on the mass, size, and density of the particles.
  
Basic Principle:
  
When a particle (biological macromolecule or organelle) is subjected to centrifugal force during high-speed rotation, this centrifugal force "F" is defined by the following formula:
  
F = mω²r a — particle rotational acceleration, m — effective mass of the sedimenting particle, ω — angular velocity of the particle rotation, r — rotational radius of the particle (cm).
  
Centrifugal force is usually expressed as a multiple of Earth's gravity, thus called relative centrifugal force "RCF." Or it can be expressed as a number multiplied by "g," for example, 25000 × g, indicating a relative centrifugal force of 25000. Relative centrifugal force refers to the multiple of Earth's gravity that the centrifugal force acting on the particle in the centrifugal field is equivalent to. The unit is gravitational acceleration "g" (980 cm/sec²), and the "RCF" relative centrifugal force can be calculated using the following formula:
  
RCF = 1.119 × 10⁻⁵ × (rpm)²r
  
(rpm — revolutions per minute)
  
From the above formula, as long as the rotation radius r is given, RCF and rpm can be converted to each other. However, due to differences in the shape and structure of the rotor, the distance between each point from the top to the bottom of the centrifuge tube of each centrifuge and the axis of rotation is different. Therefore, the average radius "rav" is used in the calculation:
  
rav = (rmin + rmax) / 2
  
Generally, low-speed centrifugation is often expressed in terms of rotational speed "rpm," while high-speed centrifugation is expressed in "g." When calculating the relative centrifugal force of a particle, attention should be paid to the distance "r" between the centrifuge tube and the center of the axis of rotation, that is, the centrifugal force experienced by the sedimenting particle at different positions in the centrifuge tube is different. Therefore, when reporting ultracentrifugation conditions, the multiple of Earth's gravity "×g" is usually used instead of the revolutions per minute "rpm," because it can truly reflect the centrifugal force and its dynamic changes at different positions in the centrifuge tube. The centrifugal force data in scientific literature usually refers to its average value (RCFav), which is the centrifugal force at the midpoint of the centrifuge tube.