  Quantum-critical states of diverse strongly correlated systems are predicted
to feature universal collision-dominated transport resembling that of viscous
fluids. However, investigation of these phenomena has been hampered by the lack
of known macroscopic signatures of the hydrodynamic regime at criticality. Here
we identify vorticity as such a signature and link it with an easily verifiable
striking macroscopic transport behavior. Produced by the viscous flow,
vorticity can drive electric current against an applied field, resulting in a
negative nonlocal voltage. We argue that the latter may play the same role for
the viscous regime as zero electrical resistance does for superconductivity.
Besides offering a diagnostic of viscous transport which distinguishes it from
ohmic currents, the sign-changing electrical response affords a robust tool for
directly measuring the viscosity-to-resistivity ratio. The strongly interacting
electron-hole plasma in high-mobility graphene provides a bridge between
quantum-criticality and the wealth of fluid mechanics phenomena.
