Centrifugation is a process used to separate or concentrate materials suspended in a liquid medium.
Two particles of different masses will settle in a tube at different rates in response to gravity. Therefore, a given centrifuge may use multiple rotor sizes to give flexibilty in choosing centrifugation conditions. The rotor is a generally a dense metal which dissipates heat quickly, and is of sufficient mass that it generates momentum, i.e., once its spinning it requires little energy to keep it going. Centrifuges generally work under vacuum and are refrigerated to reduce heating caused by frictional forces as the rotor spins. Rotors are usually stored in refrigeration units to keep tham at or near the operating temperature.