Measurement bias in evanescent wave nano-velocimetry due to tracer size variations

Evanescent wave nano-velocimetry is an imaging tool for studying biophysical transport and nano-scale fluid mechanics via tracking the 2D or 3D motion of fluorescent nanoparticles within several hundred nanometers of a fluid--solid interface. Because the information on the third dimension is encoded in the tracer intensity, variation in the tracer size, which is proportional to the particle intensity, can lead to significant measurement uncertainty. In this work, we consider the measurement bias introduced by tracer size polydispersity on the fluid velocimetry measurements. We present a general theoretical model to account for tracer size variation, interaction potentials between tracers and the solid substrate (e.g., electrostatic, van der Waals forces) and analysis of image data. Computational results are provided for typical experimental conditions, and the implications for nano-velocimetry accuracy are discussed. We find that increased tracer particle size variation and the detectability limit of tracer intensity increase measurement bias by up to 10%, which should be accounted for in experimental measurements.