  Fluid dynamics is one of the cornerstones of modern physics and has recently
found applications in the transport of electrons in solids. In most solids
electron transport is dominated by extrinsic factors, such as sample geometry
and scattering from impurities. However in the hydrodynamic regime Coulomb
interactions transform the electron motion from independent particles to the
collective motion of a viscous `electron fluid'. The fluid viscosity is an
intrinsic property of the electron system, determined solely by the
electron-electron interactions. Resolving the universal intrinsic viscosity is
challenging, as it only affects the resistance through interactions with the
sample boundaries, whose roughness is not only unknown but also varies from
device to device. Here we eliminate all unknown parameters by fabricating
samples with smooth sidewalls to achieve the perfect slip boundary condition,
which has been elusive both in molecular fluids and electronic systems. We
engineer the device geometry to create viscous dissipation and reveal the true
intrinsic hydrodynamic properties of a 2D system. We observe a clear transition
from ballistic to hydrodynamic electron motion, driven by both temperature and
magnetic field. We directly measure the viscosity and electron-electron
scattering lifetime (the Fermi quasiparticle lifetime) over a wide temperature
range without fitting parameters, and show they have a strong dependence on
electron density that cannot be explained by conventional theories based on the
Random Phase Approximation.
