  The Navier slip condition describes the motion of a liquid, relative to a
neighboring solid surface, with its characteristic Navier slip length being a
constitutive property of the solid-liquid interface. Measurement of this slip
length is complicated by its small magnitude, expected in the nanometer range
based on molecular simulations. Here, we report an experimental technique that
interrogates the Navier slip length on individual nanoparticles immersed in
liquid, with sub-nanometer precision. Proof-of-principle experiments on
individual, citrate-stabilized, gold nanoparticles in water give a constant
slip length of 2.7$\pm$0.6 nm (95% C.I.) - independent of particle size.
Achieving this feature of size independence is central to any measurement of
this constitutive property, which is facilitated through the use of individual
particles of varying radii. This demonstration motivates studies that can now
validate the wealth of existing molecular simulation data on slip.
