We define centrifugation as the pro

We define centrifugation as the process of rotating a sample to generate a centrifugal force that operates on the particles and liquid. In contrast, sedimentation is the movement of particles through a liquid under the influence of an external field, such as a gravitational field, centrifugal field, electric field, etc. In the laboratory or clinic, centrifugation is a convenient method to produce sedimentation. Most importantly, because particles may sediment at different velocities, they may be separable from each other during a sedimentation process.Sedimentation occurs in a gravity or centrifugal field because the gravity or centrifugal force that moves a particle is balanced by the buoyancy force and drag force on the particle. The buoyancy and gravity forces are constant, and their imbalance creates a net acceleration on the particle; the acceleration creates movement, and this movement creates a drag force that increases as the particle velocity increases. A centrifugal force is slightly different in that it increases with rotational velocity and distance from the axis of rotation; but at constant rotation over a short distance centrifugal force can be considered constant. In either case, within a fraction of a second the particle reaches a velocity at which its gravity (or centrifugal accelerating force) is exactly balanced by the drag and buoyancy forces. That velocity remains constant (or increases slowly during centrifugation) until the particle hits an immobile interface (wall or packed particles), it enters a fluid of the same or greater density, or the separation driving force is stopped. The very small particles in blood sediment in the Stokes flow regime, as they have Reynolds numbers less than 0.1. (Re=ρfνsDpμ, where Dp is the particle diameter, ρf is the fluid density, and µ is the fluid viscosity.) In theory, for rigid spheres in Newtonian fluids under Stokes flow, the sedimentation velocity νs is given byνs=D2p(ρp−ρf)(Rω2+g)18μ(1)where ρp is the particle density, R is the rotational radius, g is the rotational angular velocity, and g is the gravitational constant.53 In practice, the actual sedimentation velocity is often less because of interparticle interactions (collisions) and nonspherical shapes. For blood and bacteria, the velocities are slower than predicted by Eq. (1) because RBCs and most enteric bacteria are not spherical, blood is not a Newtonian fluid, and the particles interact with each other. Nevertheless, Eq. (1) gives a fairly good first-order estimate of sedimentation velocities, as the correction factors for spheroids are not large,53 and blood plasma is nearly Newtonian at the low shear rates generated during sedimentation.43Centrifugation processes have been used for more than a century to separate blood into its various components (cells, platelets, and plasma). In such cases, the centrifugation time is sufficiently long that the particles sediment until they are separated into layers according to their density. We will refer to this as isopycnic (equal density) or equilibrium centrifugation in which all dynamic sedimentation movement has stopped either because there is no more density difference to drive particles to move, or because the particles have come to an impenetrable barrier. At the end of the process, all the particles will be separated sequentially according to their density differences.Referring to Table 2, we see that isopycnic centrifugation of septic blood would not separate the bacteria from the red cells, as the range of bacterial density overlaps the range of RBC density. But Table 2 also shows that the sedimentation velocities of the various blood components are different— and in some cases very different—from bacteria. Therefore, the principles of sedimentation velocity may be used to separate bacteria. Specifically, the nominal νs of WBCs is about 96 times faster than E. coli, and that of RBCs is 30 times faster. In the time that it take RBCs to move to the end of a centrifuge tube, the bacteria have only moved about 1/30 of that distance, and only about 3.3% of randomly dispersed bacteria will have encountered the end of the tube (ignoring all other cells in the way). Quickly collecting the plasma (which will have been clarified of RBCs and WBCs) could theoretically recover about 97% of the bacteria. However, the actual amount will be less because a fraction of bacteria may become trapped in the cell pack. Another deficiency is that there will also be many platelets in this plasma because their sedimentation velocity is similar to bacteria, but at least the RBCs and WBCs will be separated from these smaller components.