Evanescent Wave-Based Flow Diagnostics

Miniaturized flow systems have been developed for various applications, including integrated chemical analyses and thermal management of microelectronics. Understanding interfacial transport is important in designing and optimizing such flow systems, since surface effects become significant due to the large surface areas and small volumes at these length scales. Recently, various near-wall flow diagnostic techniques have been developed based on evanescent-wave illumination. Since evanescent waves only illuminate the fluid in the region over the first few hundred nanometers next to the wall, these techniques have much better spatial resolution than conventional methods based on epifluorescence microscopy. This paper presents recent advances in evanescent wave-based flow diagnostics using fluorescent tracers, including evanescent-wave particle velocimetry applied to flows driven by both pressure and voltage gradients and evanescent-wave fluorescence, which has been used to measure near-wall liquid temperature and pH fields, as well as the surface charge, or wall ζ-potential, distributions.