Keywords: Interfacial particle-image velocimetry, colloidal particles, microfluidics, electrokinetically driven flows Abstract: Interfacial effects are important in many cases for microscale transport. One of the few experimental techniques that can resolve interfacial transport with sub-micron spatial resolution is evanescent wave-based, or nano-, particle-image velocimetry (PIV), which determines fluid velocities over the first 500 nm next to the wall from the displacements of 100-500 nm neutrally buoyant tracers. The wall-normal spatial resolution of nano-PIV is further improved by 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 about 500 nm of the wall. In agreement with DLVO theory, the distribution of the colloidal tracers within a particle diameter of the wall measured by MnPIV is highly nonnuniform due to repulsive electric double layer interactions and van der Waals effects. Nevertheless, the MnPIV results for steady creeping Poiseuille flow are in good agreement with analytical predictions once the velocities have been corrected for this nonuniform distribution. This talk describes velocity and Brownian diffusion coefficient measurements obtained from tracers with diameters ranging from 100 nm to 500 nm for Poiseuille flow through hydrophilic and hydrophobically coated fused-silica microchannels and for electrokinetically driven flows through fused-silica microchannels with a minimum cross-sectional dimension of about 40 microns. The near-wall particle distributions for 500 nm tracers are shown to vary with electric field for the electrokinetically driven flows, due presumably to electrophoresis and induced charge electroosmosis.