Abstract
The ''nano-particle image velocimetry'' tech- nique uses evanescent-wave illumination generated by total internal reflection at the wall to excite colloidal neutrally buoyant fluorescent tracer particles. The dis- placement of these particles over time as they are con- vected by the flow then gives the flow velocity components tangential to the wall. Since the extent of the illumination region normal to the wall is comparable to the particle diameter, a major source of error in this technique is particle ''mismatch'' within a pair of images due to Brownian diffusion causing a particle to move in to or out of the illuminated region. The ''brightness'' (proportional to the amount of imaged fluorescence) and size of indi- vidual particle images in nPIV data are discussed. A sequence of artificial nPIV images are generated for a known uniform velocity field with the particles subject to hindered Brownian diffusion. The velocity fields calculated from these artificial images are compared with the known velocity field to determine the effect of Brownian diffu- sion-induced particle mismatch on nPIV accuracy. A similar analysis is carried out for experimental nPIV images. The results provide design guidance for experi- mental measurements using the nPIV technique.
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