  Two-dimensional electron systems offer an appealing platform to explore
long-lived excitations arising due to collinear carrier scattering enabled by
phase-space constraints at the Fermi surface. Recently it was found that these
effects can boost excitation lifetimes over the fundamental bound set by
Landau's Fermi-liquid theory by a factor as large as $(T_F/T)^\alpha$ with
$\alpha\approx 2$. Long-lived degrees of freedom possess the capability to
amplify the response to weak perturbations, producing lasting collective memory
effects. This leads to non-Newtonian hydrodynamics in 2D electron fluids driven
by multiple viscous modes with scale-dependent viscosity. We describe these
modes as Fermi surface modulations of odd parity evolving in space and time,
and discuss their implications for experimental studies of electron
hydrodynamics.
