Multilayer nano-particle image velocimetry (MnPIV) in microscale Poiseuille flows

Multilayer nano-particle image velocimetry (MnPIV) uses the exponential intensity decay of evanescent-wave illumination with wall-normal distance to obtain near-wall velocity data at different distances from the wall z. Such data can then be used to determine velocity gradients within the first 400 nm next to the wall, and hence wall shear stress. In this paper, the technique is applied to measure near-wall velocities for the incompressible, steady and fully developed Poiseuille flow of four working fluids, namely water and sodium tetraborate solutions at three different molar concentrations (1, 10 and 20 mM), seeded with 100 nm fluorescent colloidal tracers through trapezoidal microchannels with nominal cross-sectional dimensions of 41 mu m x 469 mu m for pressure gradients up to 0.8 bar m(-1). In all cases, the flow Reynolds number based on mean velocity and hydraulic diameter is O(10(-1)). The tracer images were divided into three layers based on their intensity and the mean velocity of each layer was obtained with a particle tracking method. The distribution of the tracers over z is highly non-uniform, with almost no particles within a particle diameter of the wall, and the distribution varies with the fluid. When corrected for this non-uniform tracer distribution, the MnPIV results are in good agreement with the classic analytical solution for two-dimensional Poiseuille flow based on the measured pressure gradients and channel dimension, and the velocity gradients obtained from the MnPIV results are on average within 5% of the analytical solution.