Effects of surface forces and non-uniform out-of-plane illumination on the accuracy of nPIV velocimetry

The accuracy of nano particle image velocimetry (nPIV) measurement obtained from a traditional cross-correlation data reduction method is investigated. nPIV uses evanescent wave illumination of particle tracers in the wall–fluid interface where illumination intensity decays exponentially with distance normal to the wall. Therefore, in a shear flow slower particles near the wall appear 'brighter' and 'bigger' than faster particles further away. Furthermore, the presence of hindered Brownian motion combined with hydrodynamic drag, electrostatic, van der Waals and buoyancy forces affects particle displacement and distribution in the measurement region. The effect of all these factors on the calculated velocity using nPIV images is explored using Monte Carlo simulations of particle movement in this region. Fluid velocimetry results are considered for different shear rates of G ≅ O(103 s−1), inter-acquisition time of Δt ≅ O(10−3 s) and typical experimental surface forces. The results show that these factors can lead to an underestimation of the measured averaged velocity in this region that is in contrast with the previously reported bias when using particle tracking velocimetry techniques. An experimental nPIV measurement of Poiseuille flow in a micro channel is presented to show this difference and implementation of the results of numerical simulation. (paper)