High-speed quantum dot tracking and velocimetry using evanescent wave illumination

Total internal reflection velocimetry (TIRV) is applied to measure the dynamics of 17 nm diameter, colloidal quantum dot (QD) tracer particles within 200 nm of a microchannel wall at shear rates in excess of 20,000 s⁻¹. QDs are quickly developing into viable tracer particles for measuring microscale fluid dynamics. However, the low emission intensities of QDs usually require long exposure and inter-frame times, which limit velocity resolution and compromise accuracy (due to their fast diffusion as a consequence of a small diameter). In this study, a two-stage, high-speed image intensifier and camera were integrated into an evanescent wave microscopy imaging system. This provided the necessary temporal resolution to image the fast diffusion dynamics of QDs in real-time (up to 10,000 fps), which allowed individual particles to be tracked continuously for extended periods of time. In addition to examining the trajectories of individual particles, ensemble-averaged tracking measurements reveal near-wall velocity distributions in high-speed microchannel flows (Re ∼ 10), where velocities on the order of 5 mm/s are measured within 200 nm of the microchannel wall. This data provides a robust confirmation of recent results demonstrating diffusion-induced bias error for near-wall velocimetry.