Evanescent Wave Illumination

Fig. 9 Excitation geometry for evanescent wave illumination of surface bound molecules. Sketch on the left show the smaller fiber at the periphery of the connector. Measured near-field intensity distribution shows the guided capillary modes...

In micro- and nanoscale fluid mechanics, measurements of mass transport and fluid velocity are used to probe fundamental physical phenomena and evaluate the performance of microfluidic devices. Evanescent wave illumination has been combined with several other diagnostic techniques to make such measurements within a few hundred nanometers of fluid—solid interfaces with a resolution as small as several nanometers. Laser Doppler velocimetry has been applied to measure single-point tracer particle velocities in the boundary layer of a fluid within 1 pm of a wall. By seeding fluid with fluorescent dye, total internal reflection fluorescence recovery after photobleaching (FRAP) has been used to measure near-wall diffusion coefficients and velocity (for a summary of early applications, see Zettner and Yoda [2]). 

On the other hand, evanescent wave illumination provides illumination to a fluid layer thinner than the DOF alone, allowing for measurements closer to liquid-solid interfaces. Secondly, since only a thin layer of fluid is illuminated, there is virtually no background noise from out-of-plane excitation, resulting in a very high SNR (Fig. 1). Finally, the exponential decay of the evanescent wave intensity away from the interface is well known. Studies have shown that the intensities of fluorescent tracer particles excited by an evanescent wave closely follow the local illumination intensity [7]. Thus, through a careful calibration. 

The two common types of illumination schemes for creating evanescent wave illumination are prism-based systems and objective-based systems. There are several configurations for each, both of which have their own advantages and specific shortcomings [1]. 

Typically in fluid mechanics, a wealth of information about any given flow system can be fotmd from the velocity field, which is often visualized and quantified through the use of tracer particles. As mentioned above, micro-PIV and micro-PTV are both well-established tools for extracting quantitative information from microscale fluid systems [3], However, special care must be taken in applying these techniques to very nearwall flows with evanescent wave illumination. Both techniques require imaging the instantaneous positimis of tracer particles seeded in the flow at two different instances in time to infer fluid velocities. 

Zettner CM, Yoda M (2003) Particle velocity field measurements in a near-wall flow using evanescent wave illumination. Exp Fluid 34 115-121... 

Jin S, Huang P, Park J, Yoo JY, Breuer KS (2004) Near-surface velocimetry using evanescence wave illumination. Exp Fluid 37 825-833... 

Given the small size of the observation region generated using evanescent wave illumination, it is desirable to minimize the size of the tracer particles for better resolution, and the use of semiconductor nanocrystals or quantum dots (QDs) has been demonstrated. QDs are single fluorophores with fluorescence lifetimes similar to tradi-... 

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