  The goal of this work is to make progress in the domain of near-wall
velocimetry. The technique we use is based on the tracking of nanoparticles in
an evanescent field, close to a wall, a technique called TIRF (Total Internal
Reflection Fluorescence)-based velocimetry. At variance with the methods
developed in the literature, we permanently keep track of the light emitted by
each particle during the time the measurements of their positions ('altitudes')
and speeds are performed. By performing the Langevin simulation, we quantified
effect of biases such as Brownian motion, heterogeneities induced by the walls,
statistical biases, photo bleaching, polydispersivity and limited depth of
field. Using this method, we obtained slip length on hydrophilic surfaces of 1$
\pm $5 nm for sucrose solution, and 9$ \pm $10 nm for water; On hydrophobic
surface, 32$ \pm $5 nm for sucrose solution, and 55$ \pm $9 nm for water. The
errors (based on 95% confidence intervals) are significantly smaller than the
state-of-the-art, but more importantly, the method demonstrates for the first
time a capacity to measure slippage with a satisfactory accuracy, while
providing a local information on the flow structure with a nanometric
resolution. Our study confirms the discrepancy already pointed out in the
literature between numerical and experimental slip length estimates. With the
progress conveyed by the present work, TIRF based technique with continuous
tracking can be considered as a quantitative method for investigating flow
properties close to walls, providing both global and local information on the
flow.
