Molecular tagging velocimetry by photobleaching of rhodamine 6G: towards high spatio-temporal resolution
Open AccessMolecular tagging velocimetry (MTV) is a non-intrusive optical diagnostic well suited to measure velocity gradients. However, in liquids, the technique has suffered from limited temporal and spatial resolutions and the need for specialized lasers.Here, an inexpensive rhodamine 6G dye is the molecular tracer in a two-laser write/read configuration. A frequency-tripled, pulsed, Nd:YAG laser photobleaches the dye efficiently. The read signal is a laser-induced fluorescence of the untagged dye by a green (frequency-doubled Nd:YLF) laser, leading to images of a dark pattern against a bright background. The write process is prompt (order of nanoseconds) enabling very short read times; it is also permanent. Additionally, very fine patterns (tens of micrometers) are generated by Talbot-effect structured illumination.With those improvements, MTV by photobleaching reaches unprecedented spatio-temporal resolutions and becomes a very flexible scheme to study a broad range of liquid flows.Here, a proof of concept is presented in a turbulent channel flow at Reτ=800. 100 to 300 μm-spaced, 25 μm-wide, MTV lines are tagged at up to 2 kHz and interrogated at 10 kHz. The mean wall shear stress obtained with MTV is within the uncertainty error of the manometer measurements. Moreover, the time-resolved high-spatial resolution data gives access to the small-scale turbulent structures near the wall.
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