  In the fluid transport of particles, it is generally expected that heavy
particles carried by a laminar fluid flow moving downward will also move
downward. We establish a theory to show, however, that particles can be
dynamically levitated and lifted by interacting vortices in such flows, thereby
moving against gravity and the asymptotic direction of the flow, even when they
are orders of magnitude denser than the fluid. The particle levitation is
rigorously demonstrated for potential flows and supported by simulations for
viscous flows. We suggest that this counterintuitive effect has potential
implications for the air-transport of water droplets and the lifting of
sediments in water.
