What is the main difference between horizontal and fixed-angle centrifuge rotors?

• A. Horizontal rotors operate at lower speeds but provide better packing; Fixed-angle rotors spin faster but do not improve packing
• B. Horizontal rotors spin faster but packing is worse
• C. Fixed-angle rotors have lower speeds and better packing
• D. Horizontal rotors and fixed-angle have identical behavior

Answer: (A)

The difference comes from how the rotor design shapes sedimentation and pellet formation. In a horizontal (swinging-bucket) rotor, the tubes swing to a near-horizontal position during spin, creating a longer sedimentation path and a gentler, more uniform stacking of material at the bottom. This lets you achieve effective separation at comparatively lower speeds and tends to produce a pellet that is well packed and easier to re-suspend. In a fixed-angle rotor, the tubes are fixed at a slant, so particles settle along a shorter path toward the inner wall, allowing higher centrifugal forces to be reached quickly. However, simply cranking up speed does not inherently improve how the pellet is packed, and the pellets tend to be less uniformly packed than with horizontal rotors. Therefore, the main practical difference is that horizontal rotors operate at lower speeds and yield better pellet packing, while fixed-angle rotors can run faster but do not improve packing.