Nonuniform particle distributions in near-wall particle-image velocimetry

Multilayer nano-particle image velocimetry ({\mbox MnPIV}) uses fluorescent colloidal tracers illuminated by evanescent waves to visualize the flow within the first 500~nm next to the wall. Because the evanescent-wave intensity decays exponentially with wall-normal distance {\it z}, the {\it z-} position of each tracer particle can be correlated to the intensity of its image, assuming that the particle image and illumination intensities behave in a similar fashion. Recent experimental calibrations suggests that the z-position of 100~nm fluorescent polystyrene spheres can be determined with an accuracy of about 20~nm [Li \& Yoda (2008) {\it Meas. Sci. Technol.} {\bf 19}, 075402]. Near-wall particle distributions were obtained as a function of {\it z} for the Poiseuille flow of monovalent electrolyte solutions at various pH and ionic strengths through bare hydrophilic and coated hydrophobic fused- quartz microchannels with similar nominally rectangular cross- sections. The tracers were then divided into three sub-layers, each containing about $1/3$ of the particles, based on the measured particle distribution, and the average velocities in each layer were placed at the average {\it z-}position sampled by the particles in that layer. The effect of pH and wall properties on the near-wall particle distributions and the resultant {\mbox MnPIV} data is discussed.