(402f) The Effect of Tracer Properties On Interfacial Particle-Image Velocimetry Results in Microscale Flows

Surface, or interfacial, effects can be significant over the small spatial scales typical of micro- and nanoscale transport. To date, there are few experimental techniques that can quantify interfacial transport with a resolution fine enough to resolve flows at these spatial scales. This talk describes a particle-image velocimetry (PIV) technique, multilayer nano-PIV (MnPIV), which exploits the exponentially decaying intensity of evanescent-wave illumination to obtain velocities at different distances from the fluid-solid interface within 400 nm of the wall.

Previous experimental studies have demonstrated that velocities obtained with 100 nm diameter fluorescent polystyrene tracers in steady and creeping Poiseuille flow give velocity gradients, after correcting for the nonuniform distribution of tracers, that are within 6% on average of analytical predictions. Nevertheless, the velocities from this PIV technique and for all tracer-based velocimetry techniques, for that matter, are, strictly speaking, tracer velocities, which are then assumed to be identical to the fluid velocities.

Experiments were performed with smaller (40 nm diameter) tracers, to both reduce electrostatic effects and improve the spatial resolution of these data. Results are presented on the effect of tracer size on near-wall particle and velocity distributions in Poiseuille flow, as well as in electroosmotic flow where the effect of particle electrophoresis is non-negligible, though microchannels with a minimum cross-sectional dimension of about 40 um.