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Image-Based Photonic Techniques for Microfluidics
Published in Sushanta K. Mitra, Suman Chakraborty, Fabrication, Implementation, and Applications, 2016
David S. Nobes, Mona Abdolrazaghi, Sushanta K. Mitra
Molecular tagging velocimetry (MTV) (Lempert et al., 1995; Gendrich et al., 1997) uses caged fluorescent molecules premixed in the flow instead of seed particles to trace the motion of the fluid. The use of fluorescent molecules eliminates particle-induced disturbances from the measurement. Fluorescence of the dye, known as caged fluorescein or photoactivated fluorophore (PAF), is prevented because of the chemical attachment of a caging group. An ultraviolet laser known as the write laser is used to irreversibly remove the caging group from the dye and tag a group of molecules. This laser is used to write a pattern into the flow that has the dye homogeneously premixed in it. Exposure of the uncaged dye to an appropriate wavelength laser causes these uncaged molecules to fluoresce. The uncaged molecule is invisible until exposure to the fluorescing laser and is termed the read laser. The temporal evolution of this fluorescing dye with the flow is captured onto a camera for further image processing to determine flow velocity. A correlation-based approach is used to determine the motion of the fluid pattern to derive velocity.
The Challenge to Measure Single-phase Convective Heat Transfer Coefficients in Microchannels
Published in Heat Transfer Engineering, 2019
Molecular Tagging Velocimetry (MTV) can be defined as a whole field optical technique that relies on molecules that can be turned into long-lifetime tracers upon excitation by photons of appropriate wavelength (see Koochesfahani and Nocera [27]). This technique uses passive molecular tracers for the velocity determination of liquid and gas flows in channels. It is possible to distinguish two cases: the molecules used as tracers (typically acetone or diacetyl) are added to the working fluid [27];the molecules used as tracers are generated within the working fluid as the product of a laser excitation of the molecules of the working fluid (i.e., NO molecules can be obtained by exciting a N2O flow [28]).