Which centrifuge is used when rapid centrifugation of solutions containing small particles is needed?

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The main idea is that how the tube is oriented in the rotor affects how quickly small particles pellet during centrifugation. In a fixed-angle rotor, the tubes are held at a constant angle to the axis, so particles sediment toward the bottom at an oblique direction and reach the pellet with a shorter travel distance. This geometry allows you to apply substantial centrifugal force and obtain rapid pelleting of small particles, with a compact pellet and less tendency for re-suspension.

In contrast, a horizontal-head (swinging-bucket) rotor has tubes that swing to a horizontal position during spinning, so the sedimentation path is longer and the pellet forms more slowly; diffusion and re-suspension can be more of a factor, making it less ideal for the fastest pelleting of small particles.

A cytocentrifuge is specialized for preparing cytology slides by concentrating cells onto a slide, but for the general goal of rapid centrifugation of solutions containing small particles, the fixed-angle design is the most efficient option.

So, the fixed-angle-head centrifuge is the best choice for rapid pelleting of small-particle suspensions.