Multilayer Nano-Particle Image Velocimetry in Microscale Poiseuille Flows

In multilayer nano-particle image velocimetry (mnPIV), fluorescent colloidal tracers are illuminated by evanescent waves with an intensity that decays exponentially along $z$, or the direction normal to the wall. Multilayer nPIV exploits the non-uniform nature of this illumination, binning the tracers in ``standard'' evanescent wave PIV images into a few sub-images at different $z$ based upon tracer image intensities. These sub-images are then processed to extract the velocity components parallel to the wall at distinct $z$-locations within about 400 nm of the wall. Although the feasibility of this technique has already been demonstrated using synthetic images of plane Couette flow [Li \textit{et al.} (2006) \textit{Exp Fluids}, \textbf{41}, 185], we present here results from experimental images. Velocity profiles obtained from three sub-images in Poiseuille flow through rectangular 40 $\mu $m $\times $ 300 $\mu $m microchannels will be presented for pressure gradients up to about 1 Bar/m. The two mnPIV points farthest from the wall are used to estimate velocity gradients (and hence wall shear stresses). The accuracy of the mnPIV velocity gradient results is discussed.