  This letter proposes a scheme for transporting nanoparticles immersed in a
fluid, relying on quantum vacuum fluctuations. The mechanism lies in the
inhomogeneity-induced lateral Casimir force between a nanoparticle and a
gradient metasurface, and the relaxation of the conventional
Dzyaloshinski\v{i}-Lifshitz-Pitaevski\v{i} constraint, which allows quantum
levitation for a broader class of material configurations. The velocity for a
nanosphere levitated above a grating is calculated and can be up to a few
microns per minute. The Born approximation gives general expressions for the
Casimir energy which reveal size-selective transport. For any given
metasurface, a certain particle-metasurface separation exists where the
transport velocity peaks, forming a "Casimir passage". The sign and strength of
the Casimir interactions can be tuned by the shapes of liquid-air menisci,
potentially allowing real-time control of an otherwise passive force, and
enabling interesting on-off or directional switching of the transport process.
