Near-wall nanovelocimetry based on total internal reflection fluorescence with continuous tracking

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. The particles are filmed continuously, with no time gap between two frames, so that no information on their trajectories is lost. A number of biases affect the measurements: Brownian motion, heterogeneities induced by the walls, statistical biases, photobleaching, polydispersivity and limited depth of field. Their impacts are quantified by carrying out Langevin stochastic simulations, in a way similar to Guasto & Breuer (Exp. Fluids, vol. 47, 2009, pp. 1059-1066). By using parameters calibrated separately or known, we obtain satisfactory agreement between experiments and simulations, concerning the intensity density distributions, velocity fluctuation distributions and the slopes of the linear velocity profiles. Slip lengths measurements, taken as benchmarks for analysing the performances of the technique, are carried out by extrapolating the corrected velocity profiles down to the origin along with determining the wall position with an unprecedented accuracy. For hydrophilic surfaces, we obtain 1 +/- 5 nm for the slip length in sucrose solutions and 9 +/- 10 nm in water, and for hydrophobic surfaces, 32 +/- 5 nm for sucrose solutions and 55 +/- 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 spatial precision and velocity errors of a few per cent. 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.