  Recent progress in fabricating high-quality conductors with small densities
of defects has initiated the studies of the viscous electron fluid and has
motivated the search for the evidences of the hydrodynamic regime of electron
transport. In this work we come up with the spin imaging technique allowing us
to attest to the emergence of electron hydrodynamic flows. Based on numerical
calculations we demonstrate that the injected electron spin density is
inhomogeneous across the channel when the viscous electron fluid forms the
Poiseuille flow. We also argue that the Hanle curves at different positions
across the channel acquire relative phase shifts resulting from the variation
of the electron drift velocity in inhomogeneous hydrodynamic flows. The studied
effects can be employed to evidence and study the viscous electron fluid
non-invasively.
