Particle velocity field measurements in a near-wall flow using evanescent wave illumination

Evanescent waves from the total internal reflection of a 488 nm argon-ion laser beam at a glass–water interface were used to measure velocity fields in creeping rotating Couette flow within 380 nm of the stationary solid surface. Images of fluorescent 300 and 500 nm diameter polystyrene and silica particles suspended in water recorded at 30 Hz were processed using cross-correlation particle image velocimetry to determine the two in-plane velocity components with an in-plane spatial resolution of 40×40 µm over a 200 µm (h)×150 µm (v) field of view. The results are in reasonable agreement with the exact solution for the corresponding single-phase Stokesian flow. These data are, to our knowledge, the first velocity field measurements with this small out-of-plane spatial resolution (in all cases less than 380 nm), and the first such measurements in this interfacial or near-wall region. This paper describes the novel experimental diagnostic technique used to obtain these results.