Ferrofluid rotary seals are standard commercial devices that utilize magnetic fluids to maintain a hermetic seal around a rotating shaft while sustaining a pressure differential, making citation unnecessary.
The claim describes a well-established engineering application of ferrofluids where magnetic fluid is held in place by a magnetic field to create a vacuum-tight seal around a rotating drive shaft. This is a matter of common technical knowledge and standard industrial practice rather than a novel research finding requiring a citation. None of the retrieved papers specifically address ferrofluid vacuum seals, so the entry is evaluated as common knowledge.