  Collective modes in two-dimensional electron fluids show an interesting
response to a background carrier flow. Surface plasmons propagating on top of a
flowing Fermi liquid acquire a non-reciprocal character manifest in a $\pm k$
asymmetry of mode dispersion. The nonreciprocity arises due to Fermi surface
polarization by the flow. The flow-induced interactions between quasiparticles
make collective modes of the system uniquely sensitive to subtle "motional"
Fermi-liquid effects. The flow-induced Doppler-type frequency shift of plasmon
resonances, arising due to electron interactions, can strongly deviate from the
classical value. This opens a possibility to directly probe motional
Fermi-liquid effects in plasmonic near-field imaging experiments.
